["Oxygenation of the Aerobic atmosphere respiration Nitrogen Photosynthesis Complex cells fixers with a nucleus Sex Decomposers Males and BACTERIA females AND Fungi Multicellular life ARCHAEA First invaders of land, creating the first soils Animals Algae and plants Corals and anemones Body-plan Worms experimentation Invertebrates CELLULAR LIFE, DNA, Living Mollusks AND THE PROCESS OF communities NATURAL SELECTION become more complex Arthropods Vertebrates (fish) Backbone Invasion of land Reptiles Coal More living Birds spaces and lifestyles Shelled egg Limbed Living vertebrates (amphibians) communities on land become Mass Conifers Seeds and Centipedes, First more complex extinction Flowering plants pollen millipedes, forests Flying insects, and Longer and Mammals insects more complex spiders food webs","TIMELINES The earliest known animal embryos and cnidarians (relatives of jellyfish and STORY OF LIFE anemones) are fossilized, 635 MYA. Life is over 4 billion years old, first emerging when Earth was only one-tenth of its present age. But even Rifting of supercontinents, at the very beginning, life, although microscopic, was 650\u00a0MYA, creates Iapetus Ocean and already the most complex thing in the known universe. may have triggered Ediacaran and Cambrian explosions of evolution. Sponges, the first animals, evolve 750 MYA, DNA evidence suggests. The Earth\u2019s violent birth left a planet where life was not only possible, PLANTS AND GREEN ALGAE ANIMALS but perhaps inevitable. As the land cooled and permanent oceans formed, the first protocells emerged\u2014probably deep under water Plants appear 934 MYA, 1 BYA around chemically-rich fissures in the young ocean floor. Within a according to modern few million years, these protocells had become microbes\u2014and for DNA evidence. \u201cExperimental\u201d Ediacaran anim billions of years after that, the world belonged only to them. They evolved ways of getting energy either from sunlight or by eating 3 MULTICELLULAR LIFE other microbes, laying the foundations for the rest of life\u2019s diversity. The oldest fossil of a multicellular organism\u00a0is 1.2 billion years old The biggest, most complex of life-forms\u2014multicellular life\u2014 and belongs to a seaweed, evolved only in the last billion years of Earth\u2019s history. These are Bangiomorpha. The fossil is the organisms that evolved into the familiar plants and animals complete with a possible stalklike of recent times. It was then that life could emerge from the \u201chold-fast\u201d and reproductive microscopic and fill the oceans and land with greenery organs. This is also the earliest and fast-moving creatures. complex organism (eukaryote) that can be attributed to a group still alive today\u2014the red algae. Bacteria 1 ORIGIN OF LIFE 1.5CBhYclAoe,rlaElowsllupiotniklotwathaocsinrtocpsygflolrloaecaompctlonmieptpkmtseeleslipneauk(glorxeneerrrlxociagggeunyahlpdntls.sia),sn1smip.m6sliaBtlY-A. als, including Charnia (above), appear, 550 MYA and archaea A carbon trace in Australian rocks 4.1 billion years split from their old could be life\u2019s oldest \u201csignature.\\\" DNA 2 BYA Mitochondria\u2014the energy- common ancestor, evidence (from living organisms) leads to a slightly producing factories of complex LUCA, 4.2 BYA, earlier estimate of life\u2019s beginnings and predicts cells\u2014evolve, 2 BYA. according to that all organisms alive today can trace their DNA evidence ancestry back to a hypothetical microbe called from organisms LUCA\u2014our Last Universal Common Ancestor. alive today. 4 BYA oftfohaDrecacAotNmcmttgeoAmyrrhopeglpdeeiveeesinnsx,doogufecnt3c.lenatloelc8roseucB.,msYhoAa,deearnFossil stromatolite EARTH laonfadtlcihBcvieaoncrdtgdaietbrneiagacottifeoenotvrviirDihaacd.geidNoe3l.neeAnol5aomcearfnS5lsioentitBeirudeoYsfsno,m3At.ladipt1mfbsfre8aoyefs.otBsovYbsoriiAlal,idcmteteeesdr,ial Cwiothmpalenxuccleelluss MICROBES evolve 2.73 BYA aDccNorAdeivnigdteonce. First permanent oceans The Late Heavy 2 COMPLEX CELLS possibly form, 4.4 bya, Bombardment\u2014 Eukaryotes have complex providing the first a peak in space impacts cells with a nucleus and habitat for life. 4.1\u20133.9 BYA\u2014possibly include plants, animals, fungi, strips away the and many microbes. Traces atmosphere and kills 3 BYA of steroid-like substances\u00a0\u2013 all early life. unique to eukaryotes\u2014have ThOexcGoyormgfeepwEanltaieatrtthteihoos\u2019nsxthyagtemefnloo,so2pd.h4ineBrgYeA.eviden2o.rc9egBaoYSfnAio,ieoclapnsrrllmroaiyavtlcniitfhdde.ieer,n been found in rocks 2.4 stSiltlrfoomrmaitnogliotnes\u2014 FbooaTshftcosrthlatiaiorlfaebgcesrtfiaeoiitoplansalinhlitato-s,ohlrsmaf2eutenao.srt7adnhfshia,yBeroa2cYnwfxvAe.it6eriy,htsiBgsehtnYtettAdoairnc.iraxcatcyaetegetdeedn. billion years old, but direct evidence comes from the coltoondiaeys\u2019sofAbuasctrtaelriiaan\u2014caoraestline. fossil Diskagma\u2014a possible fungus\u2014from 2.2 BYA. 100 THRESHOLD 5","THERE ARE 4 MILLION DIFFERENT KINDS OF earltyhmPperarifimpmmMormseaaessttasieeellsrDresevevlfaecoilrsraolwsuakrtitdetniaiopmi5puni6pest,,eiiMcn4naaYc7rGpAluiMse.nudYrlmiAen.agonfyAp2e5smevyoal.ve 0 MYA ANIMALS AND PLANTS IN THE WORLD\u2014 rceapupetstvileeoersAlsou,esstaxiatloteluoinrnrwosco,iitfdnaiolngainmdrtghopleaefaarmdnecgixdtean,pmmbo6losi5marasdrMauiisvnlrYs.eseA,, 4 MILLION DIFFERENT SOLUTIONS TO THE Darwinius PROBLEMS OF STAYING ALIVE. David Attenborough, natural history broadcaster, 1926\u2013 Life on Earth, 1979 600 MYA Ediacaran fossil, possibly a jellyfish 100 MYA covoerokosnolnainad. Twoawseprrinogviadbeodvbeyththeemfwirsatspthlaenetsnwigimthatsictr,ofunnggturasnlikspeoPrrtottiosstauxeiste4s2. 0 MYA, plaanAntFasrdlc,poMhipnwaecoeealnfurrritdnu1si2cgentc0guhMsiaY,A. sucGhr aosunCd AsrhhcoahfwvlaiegsehdothteFpa1votte5esblr0soyiirpMlxdeYsdA. Trpahrcaoevveoesfevfmoo1asl6vmse5imldMfafuYulrAsr. afnolaudsnensAHvdiildii;nadzeoiieekmnldndota:cieufMlesiicettfrhdykralotnlachomorkyktwwshu5.non3apmy0fiossMindhsYhgsAaiooraowenf plaSnetesdd-biveearrsiinfyg Bonelike tissue is 190 MYA. fossilized 510 MYA: dentine in a fish. Experimental animal body plans fossilized 505 MYA in Canada's Burgess Shale. 500 MDYinANSsfepApocohrsteosarsivevvleieseizdvomieoendfnlovvla4csaeAds7enl4ie0difrske8dbuMrfep3rtogleiYlapralgaaMiAnrazlteenmna,haYdssitntiAim.hniltslot.iogsa,psnwlase\u2014nndlideavin,idin4nvag2atfe8onorncsMneslaYiaslAlnv.dit\u2014al 200 MYA fospsrielpJ4salae3wcr6voseMdfdieYrArasmt,sianfiash. ev3iF8do0esnsMicYleAtroafenfedsorpaerrsoetvsip.rdeeserved sFhorioseswwfpsoeitslsirMslsreeoielw,baimpozrmateeretidamlseveykre3,anyir1n3lat-h21gelpai8MbksfbrirMeYdeiaAtY,eat.Aeto,ssf.destcheantbdwraainlilntsccsth:araum2tserec6aae\u2014t0mctuhtMrame,elYrla,aiAfazlmeF,sneaso.udhitstsuoasscriwulelalsscraygdrenijvaoewdhdriseonegnroaita,lsdnaEstuartrrhe,lp2eiet3ais1lgteeMsfo.oYsAfs,il atnhAcMeedisreaitrnMlomoermpbmsiltaak2iaslmi2lipals5emnr, uosMcausdyYl,csAnueh2.ocv1aodtsi0loovnMentfYerAov.omlves lavvenFerdrtotes3esb7ibrl5ratatMteetYes)rA.aasrpheooidnwv(slaitdhimanbtged PearnmmdioTapnrhleapenteGhtrasironpedea7,ct02iDep5sye0airnMtcgteYhnAket.ileolsnfadkllnotofrwitlhonebaitneismaanl d ccfyrpaFrcenlooeescvsooeoiflnpmslwdpeaalretneytetdlrsesa,ythn3hahd.va7eie2briltbiMafeYtecAs-,oamned 300 MYA 400 MYA Fossil cones show that plants 305 MYA have evolved structures that protect their seeds. STORY OF LIFE 101","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS LIFE\u2019S INGREDIENTS FORM Earth\u2019s crust is made of dozens of chemical elements, but only some\u2014 Atmosphere was heavy including carbon, hydrogen, oxygen, and nitrogen\u2014are the stuff of living with carbon dioxide, so things. Their atoms lock together into complex molecules and it was this kind of chemical assembly that precipitated the origin of life. atmospheric pressure was higher than today, Earth has an iron core surrounded by mostly hydrogen joined to other elements, making allowing water to stay silicon-based rocks. Carbon is comparatively methane (CH4) and ammonia (NH3). In scarce, but all known life is carbon-based. 1953, American chemists Stanley Miller and liquid way above Both silicon and carbon atoms bond Harold Urey simulated early Earth in the lab its modern-day prolifically with others, but while silicon\u2019s with electrical sparks to imitate lightning. boiling point affinity is mainly with oxygen (making up They showed that with enough heat and the silicon dioxide that dominates Earth\u2019s energy, the chemicals in Earth\u2019s atmosphere Clouds of water rocks), carbon is versatile. It bonds with could make simple organic molecules\u2014 droplets would other elements, such as hydrogen, life-giving, carbon-based chemicals. nitrogen, and phosphorus. have filled the EVEN BIGGER MOLECULES sky, as they Complex life needs complex molecules. But life needed more\u2014proteins, which are do today Earth\u2014with its rocks still cooling in the long chains of amino acids, and DNA. wake of its violent birth, and liquid water Today, pools rich with protein would be condensing into the first oceans\u2014provided cleared by hungry organisms. But early just the right conditions for them to form. Earth was energized by warmth and full of minerals that acted as catalysts, boosting Earth\u2019s first atmosphere was thick with specific chemical reactions. Giant molecules unbreathable gases, such as carbon dioxide, could persist long enough to get trapped in hydrogen, nitrogen, and water vapor\u2014 membranes\u2014precursors of the first cells. but these were sources of life\u2019s elements. In a world without oxygen gas to react with it, \u25b6 Recipe for life SIMPLE INGREDIENTS \u25bc Life wrapped up Small molecules of The chemicals that created life less than half a dozen Oxygen Nitrogen needed compartments, where they atoms were abundant could become concentrated. These on newly-formed Carbon may have been provided by oily PHOSPHOLIPID Earth. They reacted molecules called phospholipids to form bigger organic Hydrogen (present in all cell membranes Water-loving molecules with a today), which naturally aggregate phosphate head \u201cskeleton\u201d of bonded Water Ammonia Methane into membranes in water. When carbon atoms\u2014which, they form spheres, they can trap Water-hating in turn, linked to form the life-giving chemicals inside. tail long-chain molecules. SIMPLE ORGANIC MOLECULES Range of different amino acids Water-loving heads point outward, LARGE ORGANIC MOLECULE toward water Liquid water\u2014in Water-hating tails which the first life Groups specific to each point inward kind of amino acid (in formed\u2014would full color) stick out Water-loving heads have first persisted also point toward the watery space inside as oceans at some time between MEMBRANE FORMING A SPHERE 4.4 and 4.2 BYA Standard amino acid repeating units, in faded colors, form the backbone Chain of amino acids\u2014the beginnings of a protein 102 THRESHOLD 5","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES \u25c0 Hellish origins Earth in the Hadean eon (4.6\u20134.0 BYA) was unrecognizable. Exposed land was unprotected from the sun\u2019s burning radiation and would remain lifeless for another billion years\u2014by which time erosion had started to make the first soils. Conditions suitable for life may have existed, however, in the deep ocean or shallow seas. Dry land was formed by crater rims made by asteroid impacts, not by tectonic movements, which create mountains today Lava could fill craters as asteroid impacts triggered volcanic activity Cooling seas, cut off from the violence elsewhere, may have provided the conditions needed for newly-formed complex organic molecules to persist Earth\u2019s crust was mostly too hot and unstable to nurture life. The greater internal heat of the young Earth, and the frequent impacts from space, caused volcanic activity on an unimaginable scale Volcanoes spewed minerals that may have acted as catalysts, helping to drive the formation of bigger organic molecules at calmer locations LIFE\u2019S INGREDIENTS FORM 103","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS THE GENETIC CODE A living organism is the most precisely ordered thing in the known universe. The assembly and upkeep of a living body need direction and control. The entire operation is guided by self-replicating molecules of nucleic acid (DNA and its ancestors) that were present at the dawn of life. Until the discovery of DNA\u2019s precise shape acids, possibly a type called RNA, were \u25b6 Reading the code in 1953, it was a mystery how life-forms probably capable of boosting their own In a living cell\u2019s nucleus, DNA\u2019s double helix is passed on genetic information to the next replication reactions. Their chains could have unzipped so that genes can be used to make generation. Once revealed, the double- acted as templates guiding the assembly RNA,\u00a0and then protein. Here, a strand of RNA stranded structure of DNA hinted at how of new parallel chains. Copying from a is being built by matching bases (chemical information was inherited whenever one template is also used by DNA in living components), copying the sequence. This RNA cell splits into two. In the next years, organisms today, but it happens only when strand will go on to make a specific protein useful experiments confirmed not only that the two chains of the double helix separate to the life-form. The sequence of RNA bases is the code for a specific sequence of chemical components that makes just the right protein. DNA IS LIKE A COMPUTER PROGRAM BUT FAR, FAR MORE The rungs that connect ADVANCED THAN ANY SOFTWARE EVER CREATED. DNA\u2019s backbones are chemical components called nucleobases, Bill Gates, technology pioneer and philanthropist, 1955\u2013 or bases for short. Each base is a DNA carried units of heredity (called genes), in preparation for cell splitting. Otherwise, unit of digital information but also that it exerted its influence in an one chain is fixed to another like the sides of astonishingly intricate way. a ladder. The copying results in two double Bases colored yellow helices, each with identical information are adenine. There are three INFORMATION CARRIERS destined for a new daughter cell. In this way, other types: guanine (green), DNA is a giant long-chain molecule\u2014just genetic information is copied and inherited. cytosine (blue), and thymine like protein, cellulose, and many other (orange). Each binds only to biological molecules. But whereas cellulose USING THE INFORMATION is a monotonous fiber of identical subunits, DNA cannot carry out any tasks alone. It one other type of base those of DNA\u2014and protein\u2014come in instructs other molecules\u2014the proteins\u2014to \u25bc They were different kinds. Different subunits follow in do the work of maintaining and developing simpler times... an information-carrying sequence\u2014just as a living organism. A single DNA molecule Today, DNA needs letters form a word. DNA belongs to a class of carries hundreds of sections\u2014genes\u2014each protein to replicate and long-chain information-carriers called nucleic carrying instructions to make a specific RNA to make protein acids. The sugars and other components of protein. In a living cell, sections of DNA are to carry out all its other their structure would have been among life\u2019s continually being unwound and wound\u2014as functions. At the origin primordial ingredients. The first nucleic genes are exposed for protein manufacture. of life, there was no such complexity. The earliest replicating molecules, possibly RNA, had the ability both to carry data and multiply unaided. RNA chain Cycle repeats A second, identical, DNA chain folds into a binds to the specific pattern of specific shape with reaction- bases on its partner, forming the famous double helix structure boosting properties Reaction- Active site Replication boosting part boosts the reaction replication is called the More RNA active site reaction chains formed, each with an active site","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES Repeating units of nucleic acids (DNA or RNA), made of sugar, phosphate, and a base, are provided in a steady supply by the cell. These are RNA subunits DNA\u2019s backbone is a string of sugars and phosphates RNA subunits bind to the specific pattern of bases on the DNA template Order of bases carries the genetic code DNA\u2019s code is being read for This unzipped portion of making RNA. This particular RNA the DNA molecule exposes its will turn the instructions into a sequence of bases. The unzipping particular protein molecule is controlled by a living cell\u2019s molecular machinery RNA contains uracil base (purple) instead \u25c0 Discovering DNA of DNA\u2019s thymine In 1953 in Cambridge University, scientists RNA backbone has made a breakthrough. a different sugar than American biologist DNA James Watson (far left) and British DNA IS AMONG THE LONGEST OF biologist Francis Crick MOLECULES\u2014CHAINS IN HUMANS deduced that DNA ARE UP TO 3.3IN (8.4CM) LONG AND (deoxyribonucleic acid) CONTAIN 249 MILLION BASE PAIRS had a regular double- helical shape and THE GENETIC CODE 105 properties that would allow it to pass on genetic information.","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS LIFE BEGINS RNA (ancestor of DNA) Reaction boosted controls everything by enzyme to Life arose from nonliving matter by processes inside protocell make organic food of gathering complexity. As self-replicating molecules mixed with catalysts\u2014substances that drive chemical Carbon dioxide, reactions\u2014self-assembly snowballed into the first dissolved in sea water, cells: organisms with familiar characteristics of life. seeps into protocell Carbon dioxide All life consists of cells with the chemicals of life contained inside \u25b2 Hot habitat Enzyme made by the More complex a membrane. A living organism is continually dynamic, resisting As water emerges from a deep-sea protocell incorporates protocell collapse into disorder and death. How such a system emerged from vent, encrusting minerals build up the nonliving Earth is a mystery, but scientists apply what they know \u201cchimneys,\u201d some of which appear to the original mineral, Protocells disperse about biochemistry and conditions on early Earth to deduce what smoke with dark iron sulphide. These but its complex into the surrounding might have happened. The transition demands a special setting, and habitats support bizarre life-forms shape makes it a sea water conditions may have been just right around 4 BYA. today\u2014entirely dependent on the better catalyst chemical energy in the effluent. Chimney formed PAID TO EAT A FREE MEAL Minerals from encrustations Deep-sea volcanic vents were rich in chemicals and were warm, \u25b6 The origin of life of minerals spewing but not so hot as to break apart big molecules. Billions of years ago, A chemical reaction boosted by Minerals from the vent they were also a safe haven from bombarding asteroids and fierce minerals inside a deep-sea chimney, solar rays. Vents today get encrusted with metal sulfides as the water and contained inside a membrane, cools. These minerals boost, or catalyze, reactions\u2014some of which may have been the basis for the first convert carbon dioxide into acetate. Acetate has a pivotal position life\u2014a \u201cprotocell.\u201d More complex in the metabolism of all life today. What is more, one sort of acetate- protocells later started to make their forming reaction can even generate energy. This combination of own catalysts, which drove their food manufacture and payment in energy\u2014all trapped within reactions. To begin with, these the catalytic encrustation\u2014could have been a \u201chatchery\u201d for life. catalysts may have been RNA. But eventually, protocells developed TODAY\u2019S DNA, STRUNG THROUGH protein catalysts called enzymes. ALL THE CELLS OF THE EARTH, IS... RNA (and eventually DNA) took AN EXTENSION AND ELABORATION over the job of controlling OF THE FIRST MOLECULE. the entire assembly. Lewis Thomas, physician, writer, and educator, 1913\u20131993 Organic food, such as sugar or acetate, produced ESCAPING THE CHIMNEYS Carbon dioxide seeps into protocell The first \u201cprotocells\u201d formed when oily membranes encapsulated chemicals that were generated in the chimneys. Sea water helped Energy released protocells disperse from the chimneys, and the catalytic minerals when carbon in it helped maintain their primitive metabolism. dioxide forms organic food The versatility of the element carbon\u2014which forms the skeleton of acetate\u2014means that its atoms can assemble into a wide range Minerals of molecules. Some of the molecules generated by mineral-catalysis catalyze (boost) may have developed catalytic abilities of their own\u2014and could even drive their own assembly. It is possible that these molecules the reaction may have been related to RNA\u2014a material found in all cells today. RNA\u2014or molecules like it\u2014marked the emergence Chimney of biological information, too. Such molecules could control of a how cells maintained the emerging qualities of life. deep-sea vent Protocell LIFE BEGINS GRADUALLY AS PROTOCELLS 106","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES Raw materials Larger protocell with Protocell, becoming surrounding the protocell more cell membrane large and unwieldy, and other materials are incorporated into it breaks into two RNA (the genetic RNA copies itself Daughter material) stays the before the cell splits protocells same\u2014it carries the Protocell Protocell same information Protocell Each daughter before growth after growth protocell contains some of its \u25b2 Growth \u25b2 Reproduction splitting parent\u2019s RNA As protocells acquired and made more organic molecules, these became incorporated into their structure\u2014allowing The biggest protocells would have been them to grow. Membranes became more expansive, but kept the same two-molecule-thick structure that is common unstable. Splitting into two provided a to all cell membranes to this day. way of perpetuating themselves. RNA, which had begun habitually copying itself, may have ended up in both daughter cells. Organic food Channel protein \u25c0 All the attributes of life absorbed into Protocells developed the abilities cell through a we recognize today as defining life. special channel Not only did they grow and multiply, they also moved (for example by Food RNA, pumping molecules through their containing membranes), and they could sense their surroundings. They could take genes in nutrition, and they had a primitive Enzyme metabolism that built molecules and also broke molecules down Molerecauclet-iobnuilding Large to release energy\u2014a process molecule known as respiration. Finally, they got rid of waste Enzyme Cell\u2019s metabolism by excretion. builds larger molecules The beginning of sensitivity in Mdoolewcnulree-abcrteiaokning- living organisms is represented by Carbon receptor molecules dioxide on the cell\u2019s surface Energy released by Receptor molecule \u201crespiration\u201d reaction detects chemical from cell\u2019s breaking down environment food molecule Receptor Waste product molecule of respiration reaction is carbon dioxide Waste product (carbon dioxide) excreted by cell PROTOCELLS ACQUIRE FULL CHARACTERISTICS OF LIFE","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS HOW LIFE EVOLVES Even at the dawn of life, the process of evolution was under way. Life was changing, and at the root of every novelty was mutation\u2014imperfections in DNA\u2019s copy-making process. The mistakes produced variety, and on a changeable planet some variations succeeded, while others failed. All organisms change during their lifetime. beneficial mutations are selected\u2014they But a grander scale of change, at the level of proliferate and pass on their \u201cgood genes\u201d to populations, happens through generations. at least some of their offspring. Those with When an organism reproduces, it copies its mutations that harm their survival or ability entire DNA, which ranges from under a to reproduce will diminish and may die out. million to many billions of digital \u201cbits\u201d of information. The enterprise represents a The changing environment, and a monumental turnover of molecular data. life-form\u2019s habitat and survival strategy Even with natural system-checks in place, within it, determine whether its mutations are copying errors, called mutations, happen. helpful or harmful. Deep-sea fish have big eyes and glowing devices that allow them to EVOLUTION HAS NO LONG-TERM GOAL. THERE IS NO LONG-DISTANCE TARGET, NO FINAL PERFECTION TO SERVE AS A CRITERION FOR SELECTION. Richard Dawkins, evolutionary biologist, 1941\u2013 Mutation produces the raw material of hunt in the dark, while desert cacti have NEW SPECIES variation. Some mutations have scarcely any water stores defended with spines. Cactus effect, but others can abort development, spines and luminous lures need genetic Although some mutations can produce while a few are beneficial. diversity to appear, but it is the environment sudden, distinct novelties, evolutionary that selects them for the right places. Chance change is generally slow and gradual. SELECTION BY THE ENVIRONMENT can play a role in spreading mutations, Selection typically works on sets of genes While mutation is haphazard, evolution is especially in small populations, but only that work together to control broad features far from random. The mutations are subject natural selection can explain adaptation\u2014 such as size or shape. But living diversity is to a selection process. Life-forms with the fitting of an organism to its environment. not continuous\u2014it occurs in discrete units called species. New species arise when \u25b6 Reaching the limit two populations can no longer interbreed. A few microbes, that They cannot exchange genes, and their today stand out as evolutionary paths drift apart. This bright colors at the divergence might happen across an edges of hot acidic emerging barrier\u2014such as a river or pools, are a testament mountain range. But mutations themselves, of the extent to which such as those involving whole chromosomes, genetic variation and especially among plants, can prevent adaptation allows life interbreeding and isolate populations. to live in extremes. There are millions of species living today, but all\u2014including countless more that lived in the past\u2014are products of evolutionary change shaped by the environment. 108 THRESHOLD 5","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES \u25b6 Selection by predators Ancestral Predator\u2014an A living organism\u2019s environment includes katydid agent of selection all the other organisms in it, particularly predators, which influence its evolution. Mutation happens Cricketlike leaf-eating insects called during reproduction katydids are colored by gene mutations. Ears are elaborately sculpted by Pink or orange variants show up against Eaten Orange mutations, the need to receive faint echoes vegetation and are eaten, leaving Eaten like the pink ones, camouflaged green ones to are selected by at specific frequencies coming predominate as they breed. predator from prey or obstacles Camouflaged Unfavorable Fur insulates the katydid hidden mutations continue tiny body, helping to from predator to occur, but are maintain its high eaten more often temperature and fast metabolism Face is shaped by evolution into a dish that helps Camouflaged katydids to broadcast a beam of sound emanating from prevail in the population the nostrils. Bats use this to detect obstacles and prey by their echoes One clawed digit\u2014the thumb\u2014has been left free of the wing by natural selection, because bats need it for grooming and for gripping cave roofs. Bats without a free thumb would soon be selected out of the population Skin of the wing membrane has become Digits 2\u20135, the fingers, have ultralight and almost hairless, making it been vastly lengthened by aerodynamic and extra-sensitive\u2014so natural selection the bat can detect faint turbulence and adjust wing shape for efficient flying or HOW LIFE EVOLVES 109 even to catch prey \u25b2 Shaped by evolution Fossil mammals older than the earliest bat had plain faces and ordinary, weightbearing forelimbs. During bat evolution, then, selection must have favored the drastic elongation of their fingers, allowing them to support wings. In horseshoe bats, it also favored the extraordinary modification of their faces, which improved their echolocation (their natural sonar).","\u25b6 Gal\u00e1pagos finches When Darwin learned that all of these Gal\u00e1pagos bird specimens were finches, despite their different bill shapes and sizes, he began to suspect that they had diversified from a single, shared ancestor. L ife changes over the course of EARLY CLUES been created in their current form by God. thousands, and millions, of years. Every species on Earth had always been From one form of life another will arise, Philosophers of antiquity had anticipated there, and they could not be changed. Fossils modified in some way by the environment evolutionary thought: some considered the could be explained away as animals that had in which it lives. The second form of life is possibility that all life could be ranked in died during the Great Flood. Scientists who more adapted to survive in its environment, a hierarchy\u2014with humans at the top. compared the anatomy of various animals and it retains some aspects of its previous saw plenty of parallels between species. form. This is evolution by natural selection, In the 17th and 18th centuries, western These similarities supported the idea of an and we can track its progress through the naturalists explored the world and filled affinity between certain groups of animals. fossil record. museums with fossils. Those that named For instance, African baboons were these extinct animals did so from a religious undoubtedly closer to Asian macaques than point of view. Animals were assumed to have they were to diminutive South American marmosets. Likewise, chimpanzees seemed BIG IDEAS close to humans. What did this closeness mean, if anything? HISTORY OF EVOLUTION ALTERNATIVE WORLDVIEW For Charles Darwin\u2014born into a reverent Some have called it the biggest idea of all time: that everything that has society\u2014these anatomical affinities caught ever lived on Earth\u2014dodos and diatoms, cabbages and kings\u2014has his attention. He was recommended for a descended from a single common ancestor. The possibility of life\u2019s five-year voyage aboard the HMS Beagle. evolution occupied some of the greatest minds, but it took one gentleman\u2019s During his journey, he collected specimens lifetime pursuit of \u201cthe species problem\u201d to explain how it could happen. from across the globe. Darwin pondered on the unexpected regional similarities in his specimens. Similarities between species that lived thousands of miles away from each other seemed to go against the idea of a single, spontaneous Creation event. Animals on 110 THRESHOLD 5","HISTORY WARNS US... THAT IT IS THE CUSTOMARY FATE OF NEW TRUTHS TO BEGIN AS HERESIES... Thomas Henry Huxley, biologist, 1825\u20131895 the Gal\u00e1pagos Islands resembled those in time, but how exactly did these changes were more likely to express themselves than nearby South America, and the unusual occur? The popular view was that others. When this generation was interbred, wildlife in Australia seemed to belong to hereditary qualities blended from two the result were a group of peas with mixed a different Creation altogether. Upon parents\u2014akin to mixing paints of different colors, indicating that traits could also Darwin\u2019s return to England, ornithologist colors. No one knew if these qualities skip generations. John Gould examined his collection of physically existed. In reality, this blending Gal\u00e1pagos birds. Darwin assumed they led to a dilution of varieties, not the Mendel\u2019s discoveries not only augmented belonged to multiple families, but Gould emergence of new ones, and so was Darwin\u2019s, despite each having no knowledge showed how they were in fact species of not a sufficient explanation. of the other\u2019s work, but also debunked closely related finches within one family. popular rival theories\u2014such as Darwin\u2019s experiences were persuading CHARLES DARWIN WAITED \u201cLamarckism.\u201d The French naturalist him that not only were these new species 23 YEARS BEFORE PRESENTING Jean-Baptiste Lamarck had proposed that modified from a former generalized HIS IDEAS TO THE PUBLIC, DUE features acquired through life, such as larger species, but perhaps that was the case with and stronger muscles, could be transmitted all forms of life\u2014that there is one common TO THEIR CONTROVERSY to offspring. Mendelism was finally ancestor for all. Darwin ruminated on rediscovered in 1900 and more scientists his theory that evolution happened by The breakthrough came from an unlikely began thinking about evolution with genetic infinitesimally small changes over many, source: an Augustinian friar in Austria. In inheritance in mind. With genetics as the many years and animals with traits that the 1860s, Gregor Mendel\u2019s experiments in exciting new discipline of natural science, aided survival were more likely to breed breeding different varieties of pea plants it became clear that new varieties of genes and pass these \u201cfavorable\u201d characteristics on allowed him to deduce that inheritance was arise by a process of spontaneous mutation. to the next generation. Natural selection acts upon these varieties by choosing, and keeping, the most useful. In 1858, English naturalist Alfred Russell By the 1940s German-American biologist Wallace wrote to Darwin with the same idea. A year later, Darwin published his EVOLUTION COULD... BE DISPROVED IF... A SINGLE FOSSIL ideas in a book, his famous On the Origin of TURNED UP IN THE WRONG DATE ORDER. EVOLUTION HAS Species in 1859, which caused a stir in the PASSED THIS TEST WITH FLYING COLORS. scientific community. He faced outrage, since it essentially challenged Biblical Richard Dawkins, biologist, 1941\u2013 Creation as fact. Nevertheless, Darwin\u2019s theories gained respectable supporters, due to particles, later called genes. Sexual Ernst Mayr showed that if populations including the English naturalist Thomas reproduction remixed genes to produce fragmented, evolution could take different Henry Huxley, a friend of Darwin\u2019s who unique combinations, some of which may courses away from a single ancestor\u2014and championed his cause in the scientific express themselves in later generations. This create new species. community. Within a few years, evolution explained two mysteries: the appearance by natural selection was being lauded in of characteristics that skip generations, and Fossils record evolution in progress: fish textbooks. In his Principles of Biology, the the perpetuation of characteristics that fins morphing into amphibious limbs, limbs philosopher Herbert Spencer coined an aided survival (natural selection). When he into wings, mammalian limbs back into expression that became synonymous with bred yellow and green peas together, Mendel finlike flippers, and so on. Today, DNA Darwin\u2019s ideas: \u201csurvival of the fittest.\u201d saw that the next generation of peas were analysis proves beyond doubt that even uniformly yellow. Therefore, some traits the lowliest and loftiest life-forms share A UNIFIED THEORY the same origins. Darwin\u2019s On the Origin of Species was exhaustive in its catalog of evidence, but the mystery of inheritance remained. Darwin understood that life changed over HISTORY OF EVOLUTION 111","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS MICROBES APPEAR Bacteria have been around far longer than any other kind of organism. They were the first to photosynthesize, the first to consume food\u2014and are still the only living things capable of making their food in the absence of light. Billions of years ago, they were pioneers of both oceans and land. Bacteria are the simplest cellular organisms, \u25bc Bacillus Plasmid\u2014one Main genome\u2014a long, but also by far the most abundant and The shapes of bacteria vary from spherical of many short twisted, closed loop of DNA, widespread. They are far smaller than the to spiral-shaped, but this rod-shaped type, loops of DNA cells of plants and animals\u2014most are about called a bacillus, is very common. It shows a containing a few thousand one-tenth the size of a human skin cell. range of features present in some modern genes, loosely bound to the They are called prokaryotic (\u201cpro\u201d meaning bacteria. Most early bacteria would before, and \u201ckaryon\u201d meaning kernel), not have had the outer capsule center of the cell because their cells lack the dense nucleus layer, nor the hairlike pili. that contains DNA in more complex cells. Bacteria seem uniform in structure, but this belies remarkable chemical diversity. In 1977, biologists recognized some kinds of prokaryotes as an entirely new life-form, BACTERIA ARE SO WIDESPREAD, SOME LIVE 2 MILES (3KM) DEEP IN EARTH\u2019S CRUST, LIVING ON ENERGY FROM RADIOACTIVE URANIUM \u25bc Bacteria called archaea. These archaea\u2014mostly repelling substances, Some of these bacteria inside animals living in hostile environments, such as salt so-called antibiotics, as invaded soils and became Many food-eating lakes or hot acidic pools\u2014had unique, they competed for food and critical for other life by recycling bacteria live inside the ether-based membranes unlike any other space. Bacteria, therefore, have layers elements such as nitrogen. Others\u2014the guts of animals\u2014such living thing. Some performed bizarre of defenses. Outside their thin cell cyanobacteria\u2014evolved photosynthesis, as these on the lining of chemical processes, spewing out methane. membrane, which is common to all life, making food from sunlight, and were the a human colon. Most they have a tough cell wall, and most types first organisms to pour oxygen into the maintain a cooperative BANKS OF DEFENSES also have a second membrane that helps atmosphere. But as microbial communities relationship with their Early bacterial evolution happened in a stop antibiotics from penetrating\u2014and evolved to be more complex, many became host by exchanging world teeming with other microbes\u2014and still today, bacteria with a wall sandwiched food-eaters\u2014absorbing nourishment from nutrients \u2013in humans, many of these early life-forms produced between inner and outer membranes their surroundings. It was bacteria like they are essential to are most resistant to antibiotics. these that\u2014billions of years later\u2014would digestion. But a few invade the dead and living bodies of plants cause disease. CHEMICAL DIVERSITY and animals, becoming decomposers or Bacterial nutrition spans the full range disease-causing parasites. of types seen in plants and animals\u2014and more besides. Many have retained the food-making capability of life\u2019s earliest ancestors, deriving energy from minerals. 112 THRESHOLD 5","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES Last Universal Common \u25c0 Tree of life Ancestor, or \u201cLUCA\u201d This tree shows the branching relationships among all forms of life, according to DNA BACTERIA ARCHAEA analysis. The analysis suggests that all cellular life alive today has a common origin\u2014it evolved just FUNGI once, from an unknown ancestor dubbed \u201cLUCA,\u201d and that it has three main branches, LAND PLANTS ANIMALS or domains: bacteria, archaea, and eukaryotes. EUKARYOTES KEY Bacteria are prokaryotes\u2014all simple, single-celled microbes. Archaea are prokaryotes, like bacteria. They resemble bacteria, but at a chemical level they are utterly different, and only distantly related. Eukaryotes are much more complex (see pp.118\u2013 19), but most branches are also microbes. The plants, animals, and fungi are just small twigs within the eukaryote limb of the tree of life. Cell membrane lines the Cell wall is a rigid casing made inside of the cell wall of murein\u2014a tough substance unique to bacteria Capsule is a thick Food stores are Ribosome is a tiny molecular machine coating of gelatinous found throughout that helps to turn DNA instructions material surrounding the cell into specific proteins some bacteria Flagellar machinery drives the rotating flagellum Cytoplasm is the fluid material filling the cell and containing a thick soup of proteins and other substances Pilus is a hairlike attachment made Flagellum\u2014a whiplike appendage from protein. Bacteria use their pili that powers the bacterium to attach to surfaces or to interact through fluid. Many types of with other cells bacteria lack a flagellum MICROBES APPEAR 113","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS LIFE DISCOVERS SUNLIGHT Life needs energy, and the first living things drew it from minerals and made their food in the darkness of the deep ocean. Those that followed found energy in other places\u2014and, as ancestors of plants and animals, they captured sunlight in the shallows or ate food made by other cells. Every living thing\u2014from a microbe to the The most self-sufficient strategy for nutrition \u25b2 Energy from sunlight tallest tree\u2014consumes energy that changes is to make food, such as sugar, fat, and small molecules into big ones, pumps protein, from nonfood materials. Carbon A thin mat of cyanobacteria on a living stromatolite life-giving matter into cells, and resists dioxide in air or dissolved in water provides decay. The immediate energy source for the carbon and some of the oxygen. Water uses green chlorophyll to trap sunlight. The energy this is food. Energy-rich substances, such can provide the hydrogen\u2014and minerals as sugars and fats, go through a kind of such as nitrates, phosphates, and sulfates is used to make organic food from carbon dioxide controlled combustion inside cells\u2014in deliver nitrogen, phosphorus, and sulfur. the same way that chemical fuel can be Today the world is covered in plants that and water, and oxygen bubbles off as a byproduct. burned to power any machine. But instead use the sun\u2019s energy to do just that\u2014but the of ignition, cells use molecular catalysts full scope of food-making life is far greater. food in darkness: sunlight contains much (called enzymes) to tease the energy from more energy than minerals. These microbes their nutritive fuel in a safe and manageable MAKING FOOD therefore thrived as they basked in coastal way. The process is called respiration. Plants are not the only food producers. The seas. They reorganized and reinvented most self-sufficient organisms of all can live chemical processes, changing energy-giving \u25b6 Predator in miniature without light and survive on nothing but reactions into new ones that used solar Amoebas get food by water dosed with minerals. These life-forms\u2014 radiation. They did it with pigments, such as engulfing smaller all of them bacteria or archaea\u2014can extract chlorophyll, that absorbed and trapped the organisms, such as energy from chemical processes involving light energy. The first photosynthesizers algae, and breaking these minerals\u2014and use it to manufacture converted carbon dioxide to sugar by adding them down using their food. Organisms that perform this the hydrogen from hydrogen sulfides. digestive enzymes. It chemical nutrition were among the first Scientists know this due to the yellow means amoebas can live life-forms to thrive in the deep, mineral-rich deposits of sulfur this process left behind in in darkness but need oceans. Some are now the unseen recyclers of rock. But a later refinement to photosynthesis prey to stay alive. nature, their mineral-changing abilities helped life-forms get hydrogen from water helping to return the nitrogen in dead plants instead. The substance left over this time\u2014 and animals to other living things. oxygen\u2014eventually filled the atmosphere (see pp.116\u201317), and later helped cells burn A significant shift in the abilities of prehistoric microbes came when they invaded sunlit shallow waters. These new bacteria used sunlight to make food\u2014in the process of photosynthesis. They could only get nourishment in daylight\u2014but the reward for doing so far outweighed that of making BY BLENDING WATER AND MINERALS FROM BELOW WITH SUNLIGHT AND CARBON DIOXIDE FROM ABOVE, GREEN PLANTS LINK THE EARTH TO THE SKY. Fritjof Capra, physicist, 1939\u2013 114 THRESHOLD 5","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES their food in respiration more efficiently. acquired dissolved food\u2014such as sugars\u2014 Marine algae \u25bc Where photosynthesis is happening These pioneers were probably like today\u2019s simply by absorbing it from the vicinity. are concentrated in Photosynthesis is the principal food-making cyanobacteria. They grew into sticky films of Decomposers, such as fungi, still get seasonally recycled, process for modern life. Plants and algae are cells that trapped sediment. Over thousands nourishment this way\u2014producing digestive nutrient-rich waters the producers of food chains that support of years, these colonies formed rocky mounds juices to break down any organic materials far from the equator animals on land and in oceans. called stromatolites (\u201cstroma,\u201d bed; \u201clithos,\u201d that are close by so they become more rock). Stromatolites still live in a few warm absorbable. Active hunting, in which one or near to coasts Tropical rain forests have especially coastal seas, where extra-salty conditions organism eats and digests another, became high productivity on land suppress grazing animals\u2014but they are an obvious next step, and complex cells, abundant in the fossil record. such as amoebas, evolved the means to KEY Vegetation density on land engulf tinier organisms. It was the Chlorophyll density in the ocean CONSUMING FOOD appearance of this predatory behavior that As soon as some life-forms started producing marked the start of microscopic food chains. Minimum Maximum Minimum Maximum food, the opportunity for a shortcut existed. Instead of being producers, organisms could Today, producers and consumers are evolve a new strategy\u2014they could eat food linked by the transfer of energy along bigger produced by others. These organisms food chains. Ocean and land life starts with abandoned food-making and became the solar-powered algae and plants that consumers\u2014collecting their nourishment now provide almost all of the world\u2019s food. in ready-made form. Those that consume Herbivores and predators are voracious in the organic food in this way are represented scale of their consumption, while all these by animals, fungi, and a whole range of living things are, in turn, dependent on the microbes. The earliest food-eaters probably fungi and bacteria that\u2014in their various ways\u2014recycle dead matter. LIFE DISCOVERS SUNLIGHT 115","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS OXYGEN Early atmosphere was composed FILLS THE AIR of relatively unreactive gases, such as nitrogen and carbon dioxide, and Nearly two and a half billion years ago, Earth\u2019s air the sky was red underwent a dramatic change: it became oxygenated. This momentous event was caused by new kinds Sunlight filtered into of microbes, and it was incredibly important for shallow ocean the future of all life. Most life-forms These microbes, bubbling away in the ocean\u2019s sunlit shallows, lived on the sea produced oxygen, and ensured that the organisms that followed floor and did not would never be the same again. photosynthesize Oxygen is a remarkable element. It causes fire, which turns Some microbes organic material to cinder\u2014but it is also a component of complex moved to shallower molecules, such as DNA. Most living things need it to breathe and waters and began stay alive. Today, oxygen gas makes up about one-fifth of the to photosynthesize atmosphere\u2019s chemical composition, but for the first half of Earth\u2019s history, there was practically no gaseous oxygen at all. Instead, all Life probably originated in the deep ocean, beyond the reach oxygen lay chemically bound in water and rocks. Photosynthesizing of sunlight. As early life-forms dispersed into new habitats, microbes were the first organisms to release oxygen by splitting it those in the sunlit shallows found a new source of energy for away from water as they made their food (see pp.114\u201315). making food: light energy from the sun. POISON TURNED PROFIT Oxygen was released Carbon dioxide-rich Early life was so unaccustomed to growing levels of oxygen that into the air from an atmosphere slowly the response was cataclysmic. The same oxygen that can corrode became richer in oxygen metal to rust wreaked havoc on the delicate machinery of cells oxygen-saturated ocean ill-protected from it. Much of early life, having evolved in habitats devoid of oxygen, died in the new poisonous oxygen onslaught. Bands of iron oxide A few microbes had the means to survive\u2014they had enzymes that formed deeper layers locked the oxygen away inside their molecules where it could do no as new sediment layers damage. But one kind of life-form went a stage further by exploiting were laid down above the fact that oxidation can be productive as well as destructive. For hundreds of millions of years, oxygen produced by photosynthesis was soaked up The eagerness with which oxygen reacts means that oxidation by the ocean\u2019s iron and laid down in rusty bands that today comprise an important part releases energy. So much energy is released during combustion that of the world\u2019s reserves of iron ore. When the ocean\u2019s dissolved iron ran out around the reaction grows hot. For billions of years, cells had been honing 2.4 BYA, oxygen saturated the water, then started to fill the atmosphere. ways of capturing energy to drive the processes of life. The presence of oxygen opened up a new avenue of metabolism\u2014 116 THRESHOLD 5 aerobic respiration\u2014by reacting oxygen with organic molecules (see pp.102\u201303) and harnessing the energy that was released. It was such an efficient mechanism for creating energy that within another billion years, virtually all life on Earth was breathing oxygen. Layer of chert Rich iron layer \u25c0 Bands of evidence Excavation of rocks dating back to before the Great Oxygenation Event reveals bands of red iron ore. They formed in the seas in which oxygen was being released.","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES Stromatolites were formed by layers Taillike flagellum \u25c0 Solar-powered microbe of minerals laid down by microbes used to swim The photosynthesizing microbes hungry for the sun\u2019s energy evolved pigments, colored green, that absorbed the sun\u2019s energy. They Oxygen was released harness this energy to make organic into the ocean by food, such as sugars, in the process photosynthesizing of photosynthesis (see pp.114\u201315). microbes Green pigment packed onto membranes inside microbe Cell wall surrounds simple cell body Bacterial Iron oxide formed a red, IT IS THIS CONDITION THAT MAY HAVE mats formed rusty layer on the seabed SET THE ENVIRONMENTAL STAGE AND ULTIMATELY THE CLOCK FOR Microbes in shallow seas began photosynthesizing between 3.8 and 3.2 BYA. They THE ADVANCE OF... ANIMALS. formed colonies, collecting as bacterial mats and building stromatolites. By extracting hydrogen from water, they released oxygen, but it did not escape into the atmosphere. Timothy Lyons, biogeochemistry professor, c.1960\u2013 It reacted with the ocean\u2019s dissolved iron, turning it to iron oxide on the seabed. New organisms Atmosphere was evolved that can use oxygen-rich, and the sky was blue oxygen for energy Earlier Stromatolites microbes fossilized, turning to rock died off Stromatolites died off After 2.4 bya, the ocean\u2019s water was full of oxygen and the New microbes evolved and could now use oxygen to extract more energy atmosphere was oxygen-rich. Since organisms had evolved in habitats from food and went on to be the dominant life-forms in the new oxygen- low in oxygen, these new conditions poisoned most of them. Only a few rich habitat. A few oxygen-hating microbes persisted where oxygen could had the means to detoxify the oxygen, and so could survive. not reach them\u2014such as in thick muds. OXYGEN FILLS THE AIR 117","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS COMPLEX CELLS EVOLVE In a world 2.7 BYA, teeming with microbes, life found a way to move forward. Simple bacteria were joined by bigger cells to form microscopic cooperatives, merging and collaborating to form complex new cells. Such cells would become the living units of plants and animals. The abilities of bacteria are limited by their larger body. Some, notably chloroplasts and Eukaryotes expanded more than simple structure. Although they can perform mitochondria, are reminiscent of some bacteria ever could. Some used their chemical tricks impossible in more complex free-living bacteria. It suggests they came to photosynthesizing chloroplasts to become life, they are restricted in how they move be when microbes in prehistoric communities algae and plants. The food-eaters became and socialize. Greater possibilities opened engulfed smaller cells for food, but instead of amoebas, fungi, and animals. A few, such up when bigger microbes swallowed smaller eating them, held them captive, preserving as Euglena, could even switch between ones\u2014and kept them alive inside them. their life processes. In this way, some photosynthesis in sunshine and absorbing photosynthetic bacteria of yesterday became food in darkness. But it was cell-to-cell CELLULAR COMPARTMENTS the chloroplasts of today. And mitochondria, interaction that continued to be the Plant and animal cells are eukaryotic (\u201ceu,\u201d which respire using oxygen, came from driving force in escalating complexity\u2014 true; \u201ckaryon,\u201d kernel), meaning they have oxygen-breathing bacteria. Even the nucleus so, in time, eukaryotes evolved into the a central compartment called the nucleus. may have begun like this, although little largest and most elaborate organisms This, together with many other membrane- remains to hint at its probable archaea on the planet. bound chambers, distinguishes these complex ancestors. In each case, the prisoners were cells from bacteria. The chambers are called \u201ccultivated\u201d and passed down whenever their Golgi body is a cluster Ridged surface, or organelles, because their uses in a cell are hosts reproduced. Over millions of years, host of sacs that help refine pellicle, is tough enough comparable to the functions of organs in a and organelle became entirely codependent. and sort proteins and to protect the organism other cellular products but flexible enough to Photosynthetic membranes, Chloroplast creates sugar by let Euglena engulf prey arranged just like those in photosynthesis. The ancestors of chloroplasts were probably ancient cyanobacteria alive today, are cyanobacteria. Like mitochondria, packed with chlorophyll, chloroplasts have their own DNA, with around 100 genes which absorbs light energy Outer membrane is Outer membrane is in permeable to organic three layers\u2014a relic of molecules needed for the chloroplast\u2019s origin aerobic respiration as a cyanobacterium inside a host cell CHLOROPLAST MITOCHONDRION Mitochondrion is rod-shaped like a Food particles Endoplasmic reticulum is a stack Nucleus contains DNA inside bacillus and is a descendant of free- engulfed by the cell of membranous envelopes. Most its double membrane. Nuclei Inner membrane folded living bacteria. It even retains some are digested inside are studded with protein-making may have originated as free- to fit in lots of enzymes that DNA, containing more than 30 genes, granules called ribosomes; others living archaea\u2014microbes that perform aerobic respiration\u2014 from its time as a free-living organism by enzymes survived in hot acidic pools the process that uses oxygen help make oily substances to break down food for energy 118 THRESHOLD 5","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES Neither animal nor plant A marvel of microscopic intricacy, Euglena is a single-celled pond dweller that can photosynthesize like a plant, but can also eat food like an animal. Its whiplike flagellum helps it to move into sunlight or toward nourishment in darkness. Contractile vacuole squeezes out excess water from Euglena to keep its body fluids in balance. This allowed its ancestors to move from the salty ocean to fresh water Light-sensitive bulge at base of flagellum tells Euglena the direction of light Main flagellum beats, triggered by the bulged light receptor at its base, so that the cell moves toward light Stigma, or eye-spot\u2014a patch of orange pigment that casts a shadow on Euglena\u2019s light-sensitive bulge COMPLEX CELLS EVOLVE 119","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS SEX MIXES GENES BEFORE GENE TRANSFER Mistakes in the copying of genetic material, known as mutations, create new genes and characteristics\u2014but it is the sexual behavior of life that Main genome on mixes them up, creating unique individuals. Sexuality is a basic property a single looped of all known life and it is likely that it emerged very early on in evolution. chromosome DNA is also on shorter loops called plasmids Some organisms reproduce without sex, so THERE ARE 8 MILLION Purple plasmids offspring carry exact copies of their parent\u2019s GENETIC COMBINATIONS control their genes. The only way they can change over POSSIBLE IN THE SPERM OR own transfer generations is when mutation produces variety. But most organisms, because of EGGS PRODUCED BY \u25b2 Bacterium their sexuality, can vary much more. Sex mixes up DNA, enriching a population EVERY HUMAN Bacteria have sex by transferring DNA to another with new combinations. A plant species individual. Some of the genes that control the might have genetically-determined white or purple flowers, as well as tall or dwarf exchange are actually on the DNA that is moved, statures. These variants are produced by mutations (see pp.108\u201309), but sex mixes separates the doses into sex cells (usually so the DNA strand controls its own transfer, a them up, so both tall or dwarf plants can sperm and eggs), and fertilization restores little like an independent life-form. produce flowers of either color. the double dose. This ensures that each gene The simplest kind of sex happens when bacteria exchange bits of DNA. When they gets inherited and no information is lost. Single-celled Parent\u2019s DNA is on separate, each partner is genetically changed, but no new cells are made. So bacteria have VARIETY IN SPERM AND EGGS parent has a chromosome pairs\u2014 evolved sex, but not sexual reproduction. Fertilization mixes genes from different protective a single pair of casing chromosomes HOW TO SHUFFLE A HUGE GENOME is shown here Complex, or eukaryotic, cells (see pp.118\u2013 19), including those of all plants, fungi, and individuals, but meiosis ensures that all the animals, cannot exchange their genes as bacteria do: their long, unwieldy chains of sex cells coming from a single parent are DNA prevent it. Instead, they rely on first making special sex cells containing only half different, too. As a prelude to meiosis, DNA their DNA, and then fuse, or fertilize, them with half the DNA of another individual. is shuffled around in the cells of the sex To achieve the halving and fertilization, organs, so that all the sperm or egg cells they need two \u201cdoses\u201d of each kind of gene. The halving process, called meiosis, made by one parent are genetically different. Plants, animals, and other complex organisms evolved sexual lifecycles that were \u25bc Spawning molded by their capabilities. Fungi\u2014which PARENT Chromosome has Production of sex grow as microscopic threads\u2014adopted a versions of all the same cells can be prolific. method reminiscent of bacteria: their genes as its partner, but Corals release millions some versions differ of sperm and eggs simultaneously\u2014 threads fuse in places without producing \u25b2 Complex microbe increasing the chance true sperm or eggs. Plants\u2014rooted to the Chlamydomonas is a single-celled microbe, but as of fertilization in the open ocean water. ground\u2014evolved cycles that used dispersive a complex cell (eukaryote), it has a double-dose of spores or pollen. But in all these organisms, DNA divided into pairs of equivalent chromosomes. sex served to multiply the raw material for Each member of a chromosome pair has equivalent natural selection\u2014variation. genes to its partner, but these genes may differ, due to mutations accumulated over millions of years. PARENT\u2014A One of millions of cells in the These cells are from NEMATODE, A parent animal, each containing the nematode\u2019s SIMPLE ANIMAL ovaries\u2014they are paired chromosomes\u2014only about to make one pair is shown here eggs (sex cells) Each member of every chromosome pair has versions of all the genes of its partner \u25b2 Animal PARENT IN CLOSE-UP Animals are also eukaryotes. They carry out the same halving and fertilization processes as all eukaryotes, but their sex cells are short-lived eggs and sperm, produced by the halving process (meiosis) in the animals\u2019 sex cells\u2014either ovaries or testes. 120 THRESHOLD 5","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES Plasmid DNA passes into another individual GENE After DNA transfer, TRANSFER both bacteria have a copy of the plasmid GENE TRANSFER ACHIEVED Before dividing, chromosome Parent cell splits One sex cell fuses pairs stick together and swap into four sex cells with the cell of another corresponding material, making individual (there are new combinations of genes no males or females) HALVING Offspring has THE DNA a new, unique combination MIXING THE DNA of DNA FERTILIZATION Sex cell of another individual, carrying Each sex cell has a mixed-up half of a mixed-up half the parent\u2019s DNA; each is different of its own from the parent and from the others parent\u2019s genes SEX CELLS GENETICALLY MIXED OFFSPRING The halving produces a The fertilization The cells multiply variety of eggs (or sperm), produces offspring with to grow into a each with a mixed-up half new, unique mix of DNA new adult of the parent's genes Embryo begins dividing into OFFSPRING many cells, each with the Before halving the DNA, same DNA, eventually to chromosome pairs meet build the animal\u2019s body and swap genes Sex cell (sperm) HALVING from another THE DNA individual MIXING THE DNA An egg fuses with a genetically different sex cell from another individual SEX CELLS FERTILIZATION GENETICALLY MIXED (EGGS) OFFSPRING IN CLOSE-UP SEX MIXES GENES 121","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS CELLS BEGIN Cleft suggests that this is an embryo TO BUILD BODIES that has just made its first cell division, from one to two cells The step from microscopic, single-celled microbes to organisms such as plants and animals, with up to trillions of cells, was another quantum leap in the complexity of life. Maintaining order in a multicellular organism demands that cells not only stick together in the right way, but also communicate so that the entire body develops properly. There are limits to the capabilities of a functions to concentrate on specific jobs\u2014 TWO-CELL STAGE single-celled microbe. Cells cannot grow and increasingly rely on other cells around \u25b2 Temporary body beyond a certain size without becoming them to supply their deficiencies. Slime molds are unmanageable\u2014using diffusion, materials on the cusp of for life pass in and out of their bodies only In the Precambrian oceans, filter- multicellularity. They over microscopic distances, and the oily feeding sponges were among the first are usually solitary, cell membrane breaks up if a cell gets too multicellular animals, although they are amoebalike single big. Cells divide when they reach a certain just a step away from being a loose colony. cells, but in times of stage, so microbes stay microscopic. A sponge passed through a sieve can sprout stress, they band new individuals from each separated cell, together and form Bigger organisms with cooperating working and the same is true of some simple algae. multicellular parts can evolve new ways to live, but they Later, more complex, animals and plants fruiting bodies must become multicellular to do so. had cells more committed to their specific such as these. Some microbes refuse to separate after roles. Their fate\u2014to become skin, muscle, division, so their cells remain attached in a or another tissue\u2014is set by their location colony. The simplicity of this arrangement\u2014 in the early embryo. Cooperating tissues division without segregation\u2014suggests that then become organs, such as solar-powered multicellularity in itself is not such a leaves or beating hearts, and their cells monumental achievement\u2014but getting no longer survive alone. body parts (and therefore cells) focused on different tasks is another matter. Multicellularity might make cells forever dependent, but it reaps enormous benefits DIVISION OF CELLULAR LABOR for the bigger body. It allowed life to evolve True multicellularity happens when a working parts, such as stinging tentacles and colony\u2019s cells work together and specialize, sex organs. The variety of body sizes now relying on chemical cues from their possible multiplied the complexity of natural neighbors to do so. All cells in a colony carry communities, leading to elaborate food webs copies of DNA made by replication during and habitats built from the bodies of larger each cellular division. Although they keep organisms, from corals to trees. identical genetic blueprints, cells switch off selected genes as they forego certain Epithelial cell forms sponge\u2019s body lining 16-CELL STAGE Collar cell creates Collar Flagellum beats a feeding current to create a current that carries food Pore cell lets to the cell in water and food Choanoflagellate colony Amoeboid \u25c0 Creature or colony? cell attacks The distinction between colonies of cells and true invaders multicellular life is not always clear. Single-celled microbes called choanoflagellates form stalked colonies. Many cells in Sponge a sponge look and behave in much the same way. What makes the animal more than a colony are its different, specialized cell types, which must cooperate in an integrated way to survive. 122 THRESHOLD 5","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES One of four cells in this fossil, Granular texture is due suggesting it is an embryo that to the mineralization has divided twice process of fossilization FOUR-CELL STAGE A membrane encloses EIGHT-CELL STAGE the cells, just as it would in an animal embryo Cells in this \u201cembryo\u201d are more rounded\u2014perhaps due to loss of its enveloping membrane 32-CELL STAGE This membrane encloses what BLASTULA STAGE looks like a ball of cells\u2014called a blastula in animal embryos THE ANCESTORS OF THE HIGHER ANIMALS MUST \u25b2 Arrested development BE... ONE-CELLED BEINGS, SIMILAR TO THE AMOEBAE Astonishing fossils from the WHICH... OCCUR IN OUR RIVERS, POOLS, AND LAKES. Doushantuo Formation of China appear to show embryos frozen in time at their Ernst Haeckel, evolutionary biologist, 1834\u20131919 very earliest stages of cell division, as The History of Creation they change from a single egg cell to form first two, then four, and eight cells, and so on. This act of cell division without separating is at the root of multicellularity; it may be that these fossils represent very early multicellular animals beginning life around 635 MYA. CELLS BEGIN TO BUILD BODIES 123","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS Showing off Many males use color to impress females in species that have good daytime vision, such as big-eyed jumping spiders. This male peacock jumping spider combines color with choreography in his courtship display. 124 THRESHOLD 5","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES MALES AND FEMALES DIVERGE As well as evolving complex, multicellular bodies, plants and animals also diverged into two sexes. In each species of animal, half became females and\u2014through yolky eggs or pregnancy\u2014focused on nourishing their offspring. The other half\u2014the males\u2014became fighters and show-offs. Contrast between the sexes can be very investment in the next generation makes pronounced indeed. A female elephant a female choosy when it comes to selecting seal can be five times smaller than her mates and passing on her genes. mate\u2014and an anglerfish female 40 times bigger. All sexual organisms have a shared The cost of sperm production is far lower. genetic investment in producing offspring, In the drive to pass on their genes, males but males and females have dissimilar\u2014 invest more in beating other males to fertilize although complementary\u2014interests in the eggs, either in competition, such as a race way they help create the next generation. or fight, or by wowing females with advertisement displays. This has resulted WE CAN HARDLY BELIEVE THAT ... THE FEMALE ... IS NOT INFLUENCED BY THE GORGEOUS COLORS OR OTHER ORNAMENTS WITH WHICH THE MALE ... IS DECORATED. Charles Darwin, biologist, 1809\u201382, The Descent of Man and Selection in Relation to Sex MATING TYPES AND SEXES in extravagant male features, from the giant The lowliest of organisms manages to be jaws of stag beetles to a bird-of-paradise\u2019s sexual without having males and females plumes. Fossil evidence\u2014such as the crests of at all. Many microbes and fungi have male pterosaurs\u2014suggests that this is nothing multiple, but identical-looking, \u201cmating new. But male displays relying on color, voice, types.\u201d Subtle chemical differences dictate or behavior leave no trace; today these whether they can fuse to mix their genes. attributes provide some of the most dazzling natural spectacles\u2014as males fight, dance, Mating types have equal reproductive or sing their way to mating success. responsibilities. But the evolution of different sexes changes this. Although each sex \u25c0 Size contrast contributes the same amount of genetic information, the female sex supplies hers as An egg\u2019s package of an egg provided with nourishing yolk, while males make lightweight sperm devoted to cytoplasm and yolk racing to fuse with that egg. The battle of the sexes began when sperm started makes it one of the swimming toward food-packed eggs. biggest kinds of cells. CHOOSY FEMALES, SHOWY MALES Some females\u2014such as many insects A sperm\u2014one of and fish\u2014deposit tiny amounts of yolk in each egg so can still afford to produce the smallest\u2014has hundreds. Others make fewer, yolkier eggs or give birth to young after a costly a whiplike flagellum pregnancy. Either way, high bodily that helps it swim, powered by a single mitochondrion. MALES AND FEMALES DIVERGE 125","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS ANIMALS Nerve net GET A BRAIN extends into each tentacle All animals have a nervous system that detects and responds Nerve fiber, the to change. But only some evolved more complex behavior. long, thin part of a The animals that did are those that started swimming nerve cell, carries electrical impulses or crawling forward. They developed a battery of sense Nodes are points where organs and a decision-making brain to lead the way. nerve fibers meet and communicate \u25bc Fossil brain Some of the first animals, such as jellyfish, cells that processed all the incoming data: \u25b2 Nerve net Soft tissues, such as moved with tentacles radiating from the they evolved the first heads with the first An anemone does not have any nerve cells the brain, rarely body in all directions. Their body had brains. A central conduit\u2014a nerve chord\u2014 concentrated in a brain. Instead, they are fossilize, but the fossil a top and bottom, but no front or back\u2014 carried impulses through the body, allowing arranged into a net, with sensory ones collecting head of a Cambrian so no head and tail. It was enough to communication between brain, muscles, information and deeper ones communicating shrimplike animal respond to food and danger, and they and sense organs. It meant a fundamental with muscles. Behavior is in its simplest stimulus- called Fuxianhuia had a nervous system for that, made up rearrangement. Two sides of the newly response form. shows a detailed brain of long, interconnecting nerve cells. A elongated body developed as mirror images impression. The large stimulus, which can be any prompt from of each other, giving the new kind of animal Nerve chord\u2014a thick optic lobes suggest the the environment, triggered their system a single line of symmetry down the midline bundle of nerve fibers\u2014is animal relied on vision. to fire electrical impulses along the nerve of its body. This body plan came to dominate one of a pair running down cells\u2019 fibers\u2014and when the signal reached animals from the simplest flatworms to the a muscle, the muscle contracted to pull on most complex vertebrates. the ventral (belly) side a part of the body. But complex behavior of the animal was impossible: they had no brain to Brain power allows complex behavior, analyze sensory input and make decisions. so spiders, for instance, can spin webs to catch prey. But behavior can still be A HEAD FOR THINKING \u201chardwired\u201d and fixed by genes. Genuine More than 600 MYA, forward-moving versatility would come where traces animals introduced a key innovation. If they of the brain\u2019s electrical activity left moved in one direction consistently, one memories that affected behavior. Big- part of the body\u2014the front end\u2014always brained animals, such as mammals and encountered new territory first. Animals birds, can learn from experience. And concentrated sense organs at this end and among them, a few gained foresight\u2014the developed a corresponding mass of nerve ultimate expression of brain power that foreshadowed human creativity. Auricle is a projection on the side of the head that is sensitive to chemicals and is used to find food Eyespot, or ocellus, responds to light but doesn\u2019t produce detailed images Brain is simply a concentration of the biggest ganglia at the head end of the body Snout is the first part of the body to encounter new things and is touch-sensitive 126 THRESHOLD 5","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES PARTS OF THE BRAINS OF DIFFERENT ANIMALS HAVE EXPANDED Compound eye is AND PROSPERED IN IMPORTANCE\u2026 ALL IN ACCORDANCE much more complex WITH THE DEMANDS OF THE LIFESTYLE OF THE SPECIES. than an eyespot, and delivers detailed Susan Greenfield, neuroscientist, 1950\u2013 images to the brain Peripheral nerves are made of bundles Optic lobe is of fibers (nerve cells) and extend into made of many every part of the body\u2019s surface parallel nerve cells, leading to and from the eye Ganglion is a dense concentration Nerve tissue extends, as of nerve cells. Ganglia are arranged a lobe of the brain, into the sensory antenna in pairs and form chains down the twin nerve chords Brain is much bigger, showing that the nervous system has become \u201ccentralized\u201d around the brain Lobe of brain leading to mouthparts or other head appendages Head is more distinct than the flatworm\u2019s head. \u201cEncephalization\u201d\u2014 the evolutionary process of developing a head\u2014is complete \u25b2 Simplest brain Flatworms are among the simplest of living animals with a head and a brain. The central nervous system consists of a cluster of ganglia (bulges of nerve cells) making up a primitive brain, while nerve chords communicate with the rest of the body. Further neurons carry impulses to and from sense organs and muscles. \u25b6 Bigger brain Nerve chord is one of a Ganglion partially This arthropod, based on fossil pair, as in the flatworm, governs a segment Fuxianhuia (see opposite), shows of the body, forming how complex nervous systems had running down the a mini-brain devoted become by the early Cambrian period animal\u2019s underside to that segment (515 MYA). Fuxianhuia had a segmented body, with a pair of ganglia in each ANIMALS GET A BRAIN 127 A HUMAN HAS 85 BILLION segment. The brain was much larger NERVE CELLS, BUT A and features fat superhighways of nerve cells extending into the head\u2019s NEMATODE WORM GETS appendages and sense organs. BY WITH 302","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS ANIMAL LIFE EXPLODES The first big explosion of animal life occurred just over 600 MYA\u2014in oceans already alive with algae and microbes. From modest beginnings as creepers and grazers on the seabed, animals quickly evolved into all the main groups alive today. \u25bc Colonizing The oldest full-body fossils seem to appear left little more than tracks and traces. FROM THE BEGINNING TO the ocean floor so suddenly in the geological record that the Those fossil traces can be a rich source of THE END OF THE CAMBRIAN The earliest animals first chapter in the evolution of animals has data themselves, however, telling us about PERIOD, ANIMAL BURROWING hugged the ocean floor, been called an \u201cexplosion.\u201d A fuller picture animal lifestyles and communities. but their diversity and actually reveals what might be a series of DEPTH INCREASED FROM ecology escalated as explosions. An early wave of evolution left EARLY RECYCLERS \u00bdIN (1CM) TO 39IN (1M) some of them dug behind fossils worldwide, but notably in Animals evolved from single-celled deeper into the mud Newfoundland, Canada, and in Australia\u2019s organisms. The pre-Cambrian track marks dissolved food, some of these pioneers of the and others grew upward Ediacara Hills, which gave their name to show that the lives of these first animals sediment evolved into burrowers and began into the water, this period, the Ediacaran (635\u2013541 MYA). were tied to sediments on the ocean bed. churning the sediment in ways that had discovering new The animals preserved are unrecognizable\u2014 Some crawled over the surface or grew into never happened before. This swirled survival strategies some are disk-shaped, others frondlike\u2014and spongelike mats. Animals had evolved materials between the ocean water and and building complex scientists cannot place them in any modern muscle systems, which distinguish them the bottom muds\u2014adding oxygen to the communities. groups. These were not the first animals. from other multicellular life. Their muscles sediment and exchanging organic matter DNA evidence points to an even earlier helped them play an active role in shaping and minerals between the two habitats. pre-Cambrian origin, but the earliest forms their environment. In their search for Charnia\u2014an organism completely unlike any alive today\u2014had a leaflike appearance, but it was a food-absorbing animal Sponges filter-fed Sponge Trilobite-like on suspended food arthropod Sponge Surface trail of an Branching unknown animal burrow Charnia Dickinsonia Arthropod track marks Kimberella Arthropod Complex branching track marks burrows Life inhabited a thin, Dickinsonia left The first burrows beneath surface layer of the impressions of the surface were made by seabed, hardly its body as it unknown wormlike animals penetrating the was grazing sediment at all Deeper layer of mixed In the Ediacaran period (about 560 MYA), the sediment was created seabed was colonized by surface mats of algae, microbes, and possibly sponges. Scratch marks Early in the Cambrian period (about 540 MYA), a deeper were made by early animals, possibly including layer of mixed, recycled sediment was created by animals Kimberella, as they grazed the algae. burrowing and digging. The earliest known arthropods, probably resembling trilobites, left tracks\u2014long before the first trilobite body was fossilized. 128 THRESHOLD 5","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES SEABED COMMUNITIES \u25c0 Experimental body Opabinia is an example By the early Cambrian, animal communities of an experimental body were flourishing on and around the seabed. plan from the Burgess The fossil record of this time is less Shale. This creature incomplete, as many animals had chalky is not related to any exoskeletons\u2014protection from others but animal alive today, and also able to support taller bodies and some experts regard it colonies. As plankton became richer with as a failed body-plan bigger organisms, their dead bodies and experiment that waste were more likely to sink. For the first soon died out. time, life-forms in the water column were strongly linked to those on the ocean floor SOME 15\u201320 BURGESS SPECIES CANNOT BE by a primitive food chain. Deposit-feeders ALLIED WITH ANY KNOWN GROUP. MAGNIFY came to depend on this rain of food. SOME OF THEM... AND YOU ARE ON THE SET OF A SCIENCE-FICTION FILM. Now was the time of the full Cambrian Explosion, documented most famously by Stephen Jay Gould, paleontologist and evolutionary biologist, 1941\u20132002 Canada\u2019s Burgess Shale fossil assemblage Wonderful Life: The Burgess Shale and the Nature of History (505 MYA). All the major kinds of living animals\u2014flatworms, mollusks, and arthropods included\u2014had evolved. But other, less familiar, types evolved alongside them. Some fossils suggest the existence of animals very unlike anything alive today, and many scientists have described this period as a time of experimentation in body shaping. Many of these ancient types disappeared without leaving lasting descendants, but others went on to fill the planet with animal life. Anomalocaris reached nearly Eocrinoid 31\u20444 ft (1m) long and was the community\u2019s top predator Anomalocaris Wiwaxia Sponges Arthopods Hallucigenia Burrowing Fanworm anemone Vertical U-shaped worm burrows A range of worms Sediment filled with traces left Fan-shaped burrow and anemones held by burrowing and foraging trace made out tendrils or felt by a worm with a proboscis movements of wormlike animals for food With the Cambrian Explosion in full flow (520\u2013505 MYA), Later in the Cambrian (529 MYA), deposit-feeders subsisted on new lifestyles and experimental body plans really the \u201crain\u201d of detritus from plankton above. They included animals took off. Unique animals such as Anomalocaris, with food-grabbing tentacles, including burrowers similar to the Wiwaxia, and Hallucigenia evolved, but left no fanworms of today, and a diversity of trilobite-like arthropods, successful descendants. which left different types of tracks as they patrolled the seabed. ANIMAL LIFE EXPLODES 129","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS ANIMALS GAIN Cranium, or braincase, surrounds the A BACKBONE brain. In early vertebrates, this formed an open-topped cage, but this later Backboned animals\u2014from fish to mammals\u2014have a history stretching closed over, giving better protection back to small larvalike filter-feeders emerging in the evolutionary explosion of the Cambrian period. The internal skeleton they evolved Mandible is actually a former went on to support animal bodies far larger than was possible before. set of gill arches, reshaped by Vertebrates (animals with a spine, or over by the mouth and throat, evolution into a jaw vertebral column) emerged from small leaving the gills free to become \u25bc Step by step muscular swimmers in the Cambrian seas, better at extracting oxygen. Fossils show that the before 500 MYA. They had a rubbery rod\u2014a This happened in bottom-living, evolution of a spine notochord\u2014running through the back of a armored, jawless fish called happened during the tapering body and blocks of flexing muscle ostracoderms, which used throat Cambrian period, that curved the rod from side to side. Fish muscles to suck food in from mud. But 541\u2013485 MYA. The use the same technique to swim today\u2014but ostracoderms were also pioneers for another story began with a in most, the rod grows only in the embryo reason: they had the first bone. back stiffened by a and is replaced with a harder backbone by notochord (a rubbery adulthood. The Cambrian rod-backs were BONY BODIES rod) and passed modest filter-feeders, but a backbone gave Bone has its collagen hardened with at least through stages where their descendants dramatically new 70 percent mineral. It may have evolved vertebrae were made ways to live their lives. as a reservoir for the extra calcium and first from cartilage phosphate needed to trigger fast-acting then mineralized as CARTILAGINOUS BEGINNINGS muscles and nerves. But it had obvious a true backbone. The earliest elements of a skeleton were mechanical benefits, too. Ostracoderms made from cartilage: tough, but flexible, (\u201costrakon,\u201d shell; \u201dderma,\u201d skin) used bone tissue packed with collagen. Cartilage as outer armor, packed with so much mineral grew in the head of the first fish, such as it excluded living cells. Later fish permeated Haikouichthys, and protected the brain and their bone with life-supporting microscopic supported arches between their gill slits. channels, meaning it could grow from within In later animals, cartilage grew over the to make an internal skeleton. Most notochord and protected the spinal cord too, vertebrates alive today have a bony skeleton, becoming the first true vertebral column. with cartilage largely around joints. A few\u2014 The column allowed stronger swimming, such as sharks and rays\u2014reverted to a more while fins\u2014with cartilaginous supports of lightweight cartilaginous skeleton, but bony their own\u2014improved control and stability. fish diversified more, counteracting their heavier bone with a buoyant gas-filled swim Bodies with supporting cartilage could bladder. And a bony skeleton was critical for get bigger and more agile\u2014but demanded the evolution of the land vertebrates that more food and oxygen, too. The earliest fish followed. Only giant bones could bear the got both by straining water through their weight of the biggest dinosaurs. gills\u2014but feeding functions were later taken Nerve chord Notochord\u2014a stiff Cranium Cartilage gill arch stiffens supportive rod (braincase) gill, helping to hold it open of cartilage for feeding Brain Gill slit\u2014used for filter feeding Chordate\u2014a protofish with only a notochord Craniate\u2014protofish with a braincase 130 THRESHOLD 5","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES Rods of cartilage support the \u25c0 Modern vertebrate dorsal fin. They grow under A body reinforced by an internal jointed skeleton has the skin as part of the dermal enormous evolutionary promise. The great white shark is (skin) skeleton, unconnected a highpoint of this potential: one of the most formidable to the internal skeleton top predators in the world today. Its skeletal elements are made from cartilage reinforced with mineral deposits. Sharks share the main skeletal elements of most other living vertebrates, except that they lack ribs. Vertebrae surround and protect the spinal cord. They make up the vertebral column, or spine Gill arches are elements of the The pectoral fins\u2019 skeleton is part of the skeleton between the gill slits that internal skeleton. Later, this allowed other support feathery gills, holding them vertebrates to adapt these and the rear open to absorb oxygen from the water paired fins into limbs and to invade land (see pp.140\u201341) GREAT WHITE SHARK Bony plates covering Vertebrae made of cartilage, Notochord still First gill arch Cranium Backbone made of Swim bladder\u2014added because head\u2014fish first evolved making a chain called a present in some transformed into made vertebrae turned to bone it gives fish the buoyancy it bone as external armor vertebal column, or spine early fish primitive hinged jaws of bone by adding calcium minerals needs in the presence of its heavy, mineralized skeleton Cartilage gill arch Vertebrate\u2014a jawless fish with cartilage Gill now used for Vertebrate\u2014fish with bony internal skeleton internal skeleton and bony armor extracting oxygen from the water ANIMALS GAIN A BACKBONE 131","Myllokunmingia and Bonelike tissue is fossilized Whole body of an ostracoderm, Astraspis scales Haikouichthys, larvalike, for the first time, in the form of a bony-armored, jawless fish that uses CaarnatdsDileraoNasgasytAirtinserm);aofbeacoutvoosseondsdfliyevilsbireszhmyce4(amd5sAl.h0eossadMtrroekYafrAssnt,phies jawless animals with notocord dentine in fish scales, 510 MYA. gills for breathing, is fossilized for the and cranium but no spine, First possible vertebrae are preserved in what is now 500 mya first time, 465 MYA. Until now, gills fossilized 438 MYA\u2014 China as the earliest known were used for filter feeding. cartilaginous ones in fossil fish, 530 MYA. the fish Jamoytius. spkillniainJvactnihwoneeddsgee\u201dab)ranmorfdilnsie(hem\u201csp,ato4lrakd3etne6epor-rMwneY-snsAet.ryvleed, FISH 450 mya Conodonts\u2014mysterious Metaspriggina, a larvalike jawless Lampreys and hagfishes, wormlike creatures thought fish with possible rudiments of the only jawless fish living to be primitive vertebrates\u2014 today, evolve 482 MYA, leave abundant toothlike vertebral column, is preserved in according to modern fossils, 530 MYA. Canada\u2019s Burgess Shale, 505 MYA. DNA evidence. Astraspis, a type of armored, jawless fish called a LtuhnefgoffsiirsTssilhetpetiiliss4tamhF2fcfooue0oaslsrd4Mlsesye1iiYlplr5ibAizmzro,eMeeind4sndYe1yAftrf6ohi.vonreMerttdfYehiArsienn.hfaaiGrlssutkiyteiumle. eto, n n ostracoderm, lived 450 MYA. 400 mya TIMELINES TETRAPODS RISE OF THE First trace on land is left by a vertebrate 395 MYA: track marks of an unknown tetrapod (limbed vertebrate). VERTEBR ATEStTdsiBpohnkefrhaeescoleccteklrhduktfeioibyerudbnseicte,oednb1tnnnsas0itotoes\u20140iivndtmramhgeiteaneiijoealn\u201cclnwoAviiolsmoufsgn,teldthaayuhaoineltntesfadimgFrosmcsi,npoasoohdomaaf.ef\u201dsrnvpytecvlDeroeorhxtrueneutirobemslirbrryntsgaragoeataoerrtnt,nkehyeshseiavosuoso-bfotelmfiuslmsiietluggaiieohfcohn,ecntft,tes,hhihsi.sbestefjssuauuerlst\u2014aitninfgiaia,mnbaalinynldsteetravnostalevl.edEaPralffioniessdshstieinrlkirezainlceaopdthTelwatidishckntPotyolaadisavntelhTeidrkeemeb,traairifrnaclaotocphhstbefotesoehlstide-srlty,faaosEis3pnivrC,llg8eonsaelisade0ovnlldd,oeiefeMdioabsvfrtevsiYpeiefseendAoteh-lcraef.satiabieosAcnnnpiuhn3lcntlinoeszre7ledaoedei5dsnaadsntsMv.h3itetYe7fisAoo5f.insrMsstitlYosAs3h.7ar0kMtoYA.layAh3am6r1ndi-asMontYhaleAe,lysls(iemvsdoesredtugeegrgbgsnr)easDettvesN.soltAvheat AMNIOTES Some vertebrates that followed used these adaptations to be successful 350 mya on land. Their strong internal skeleton supported their weight; strongly jointed, bony fins became walking limbs; buoyant gas bladders became lungs; and an outer skin, in some, evolved feathers and fur. 132 THRESHOLD 5","Mdielkmciptdahrmueooovsmduieduasoecl)tfnteliaceeoeamtnhv2oea1(ndlf0iokdeMersnsYiAl . Mammals exist as a M Pfotrertohesafuirrstletaimvees2fo2s0sMilsYA. variety of small shrewlike DNwitAhsauargspglueapsciteas.lntsaaenvdolmveod1e7r6nMYmAa,mmals animals, including Megazostrodon, 200 MYA. Megazostrodon preAsedFreivrloesdtbma2s2ail5meuMmsY.aAl: is 200 mya Fuorflmeaavmesmaaltsrafocretihnethfiersftotsismiler,e1c6o5rdMYA. DINOSAURS MAMMALS Pilsafcoesnstilailzmeda1m5m6 aMlYA. AND BIRDS Aoplrrdceehsseatrekvonepdotwe1r5ny0bxMi,rYdtAh, .eis 2l3ed1aiEvMneaYo2srAs3lfii.ao0esuFssfMsritoriYslsAsst.iltiuzretdle 150 mya Ichthyosaur r synapsids would eventually give rise to mammals. 250 mya is fossilized Archaeopteryx 248 MYA. Dunkleosteus, a Varanops was a synapsid reptile living 275 MYA. Othe letaoevdegFasgiyr-fs\u2019losaitssypMmsfilnioaloasgtsrnys1mspoi2luuiat0zprmseeMioadmmYrlA1aeme.l2c,(a5mhlimkiModemdnYAeaar).ln Ct\u201cylp2eypra5noev9oteoodMsf-oYfrmoneApast. ,smtaiillmsiaanl,\u201d giant predatory placoderm, lived 380 MYA. Early placoderms were the first vertebrates w 100 mya Ma(mofsrdaepoelavgharocDsminbclaNvaoineanrAdnd2dsi6eenvr4gsid)MteoYnAc, e. related to horses, but it walked on its knuckles. cotherium was a hooved mammal living 28 MYA. It was 300 MYA ith jaws, 436 MYA. mafostsrhetersexiAtvttsregi,andedpcirootnaifdowtashnsmse,tu3poCc0haah5irbcabMlsiooaYAsnAnei.msf,wepdrihouthiurbisanacgmoAwuashlmicphhibamus Chali First human (member of the Sylneaamdpaisnmidgm\u2014tRoatleithsshp\u2014efetotiieyslsvepspoeiHrlleiuozystfeelioordrennvp3eootd1imfl8e3u1M2sYMAY. A. RodtehneEtafiirsrlsifetosttsismkinlpeiozr6ewes0dneMrfpvoYrerAidm. 5a6teMiYsA . 50 mya genus Homo) leaves fossils 2.8 MYA. Earliest known ape, Rukwapithecus, is fossilized 25 MYA. 0 mya RISE OF THE VERTEBRATES 133","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS Neck joint between the shieldlike Jaw muscles pulled the head back \u25bc Terror of the Devonian seas bones of the thorax and skull was here, aided by the flexible joint at Dunkleosteus was among the first predators not a true neck, but, unusually in the back of the head, to help open to catch fast-moving prey with snapping jaws. fishes, it was flexible up the jaws for a wider bite. Studies of its fossilized skull suggest it could have had one of the strongest bites in the history of vertebrate life. Jagged edge of jaw bone could slice through prey\u2014100 million years before sharks evolved their bladed teeth Joint connecting upper and lower jaws was powered by strong, fast-acting muscles to pull the jaws closed and give a formidable bite Thoracic shield was a plate of bone that anchored muscles that pulled on the lower jaw to rapidly open the mouth 134 THRESHOLD 5","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES JAWS CREATE TOP PREDATORS Predators have been a part of the natural world ever since organisms evolved the ability to eat one another. However, backboned animals started as filter-feeders that sucked mud from the ocean floor. It was not until they evolved jaws that they could sit at the top of long food chains. Many invertebrates\u2014such as predatory throughout the world\u2014evidence of its vertebrates\u2014notably the sharks\u2014had worms, sea scorpions, and centipedes\u2014 success. Growing twice the length of a car, evolved in the meantime, and they survived. have evolved sharp-edged jaws that can Dunkleosteus was the biggest predator of its Although their jaws were built from flexible grab prey. But vertebrates, using cartilage time\u2014and its jaws could easily puncture the cartilage, they had blade-edged teeth that and bone, made their jaws bigger and more armor of its contemporaries. Its size and could be serially replaced\u2014something that muscular. The first jawed vertebrates did so through an evolutionary rearrangement THE VERTEBRATES THAT CAME STORMING THROUGH... of the arches that support the gills. Over SWEEPING MOST OF THE [JAWLESS FISH] ASIDE DURING generations, the front arches were shifted THE DEVONIAN, WERE THE ONES WITH JAWS. forward into the roof and floor of the mouth and met toward the back of the skull, Colin Tudge, biologist and writer, 1943\u2013 forming a hinged joint. strength meant that it could prey on placoderms probably could never do. But it SUPER-PREDATORS bigger animals, including other predators. was bony vertebrates that took jaws and their Reshaping the gill arches into moveable jaws Devonian oceans had an extra link to especially hard, enamel-coated teeth to a may have helped to fill the gills with more their food chains: a top predator. new level. Crocodiles, dinosaurs, and oxygen, but the development of stronger mammals developed deeply-rooted teeth that muscles allowed the jaws to bite, too. This DIETARY DIVERSITY could better resist struggling prey. Dentition helped fish both to catch prey and also Despite their apparent supremacy, the was also modified in animals lower in the to kill and dismember it. Natural selection placoderms did not last. They disappeared food chain. Grazing mammals developed would have favored the evolution of bigger in the Late Devonian mass extinction\u2014an grinding teeth, and their biting jaws became fish with more powerful jaws\u2014opening up event that was probably triggered by a drop chewing jaws\u2014extending the ecological more ambitious avenues of predation. in oxygen levels. But other jawed range of vertebrates more than ever. The earliest-known jawed vertebrates were placoderms: mostly armor-plated fish that flourished during the Devonian period (419\u2013359 MYA). One of the largest known was Dunkleosteus, whose fossil remains are found \u25bc Top of the food chain The evolution of bigger jawed vertebrates broadened the size range of their potential prey to include other smaller predators. As a result, food chains lengthened. In this food chain, arrows show the flow of energy from prey to predator. PREDATOR PREDATOR HERBIVORES PRODUCERS Cladoselache shark Mimipiscis fish Animal plankton Plantlike plankton TOP PREDATOR Dunkleosteus PREDATOR DEPOSIT FEEDER DETRITUS Ammonite Trilobite JAWS CREATE TOP PREDATORS 135","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS Plant pores A scanning electron micrograph (SEM) of a pine leaf clearly shows rows of stomata. These open and close, allowing the plant to control the passage of gases\u2014a useful adaptation to life on land. 136 THRESHOLD 5","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES PLANTS MOVE ONTO LAND The first sign that the land was turning green probably came when algae crept above the tidal zone along ocean shores. However, the move to permanently drier environments farther inland required plants with roots anchored in soil, and shoots that could grow upright in dry air. Vegetation grew in water long before it the evolution of a complex substance invaded the land. Algae had evolved broad called lignin. By coating their fronds that intercepted the sun\u2019s light microscopic transportation vessels, energy and a \u201choldfast\u201d that stuck the lignin helped to form watertight tubes body to rock. These seaweeds still live in that could deliver water and minerals the ocean today. Many survive periodic up the stem. Lignified vessels were also exposure at low tide, but they are too flimsy physically strong, so these new plants to last long on dry land. grew and branched vertically. Tough vessels also grew downward as stronger, WATERPROOFING THE LEAVES branching roots penetrated the soil to Water screens out some of the sun\u2019s energy. anchor the weight and absorb dissolved On land, although plants bask in stronger minerals. Many of these taller plants radiation, they risk drying out. Land plants were already better suited to life on ...THE FIRST ZOOLOGICAL LANDFALL WAS CONTINGENT ON THE GREENING OF THE TERRESTRIAL LANDSCAPE BY PLANT LIFE...WHICH WAS... MORE AN INVASION OF AIR. Karl Niklas, professor of plant science, 1945\u2013 evolved a waxy waterproof coating on land by producing seeds. But thickened their epidermis\u2014the surface \u201cskin\u201d of cells. lignified tissue, called wood, helped Pores in the epidermis called stomata trunks get thicker and trees became helped to keep gases moving for processes taller still. such as photosynthesis (see pp.114\u201315) and respiration. The earliest land plants, like the mosses and liverworts of today, could only hug the land with creeping stems. They clung there with rhizoids\u2014 microscopic hairs that scarcely penetrated the ground to function as primitive roots. STANDING TALL \u25c0 Rigid stem A cross-section Strength is required to stand upright. Plant of a fossilized plant cells are surrounded by a scaffolding of (Rhynia gwynne- tough fibrous cellulose, and the thickening vaughanii) from the of this wall in places helps stems bear some Devonian period weight. Although mosses can do this, they about 410 MYA reveals can rise no more than a few inches. Other watertight tubes plants managed to grow taller because of that conducted water and nutrients. PLANTS MOVE ONTO LAND 137","HARD EVIDENCE WENLOCK Rugose coral was a solitary LIMESTONE horn-shaped extinct relative of the corals that live today Few organisms leave any kind of fossilized trace, but in some locations Clam was a conditions have preserved extraordinary snapshots of entire communities. free-swimming Their wonderful fossils\u2014rich in species and finely detailed\u2014offer rare insights into the ways groups of animals and plants lived and died. filter-feeder like today\u2019s scallops Wenlock Edge\u2014an outcrop of limestone on from scattered localities. Other Lagerst\u00e4tten the Welsh-English border\u2014holds an example may preserve communities intact. The of such a fossil assemblage, or Lagerst\u00e4tte. Burgess Shale in the Canadian Rockies has It is packed with animals of a tropical reef soft-body impressions of animals that were from more than 420 MYA. At this time, the smothered in mudslides 508 MYA. Although site straddled the coastline of the ancient their orientation is chaotic, the postures Iapetus Ocean,\u00a0where many of Earth\u2019s suggest they were killed instantly. But not all animals had evolved. The fossils show that Lagerst\u00e4tten result from violent slaughter. corals, sponges, trilobites, and brachiopods North America\u2019s Green River formation flourished in the warm shallows. comprises 50 million-year-old sediments left in lake basins that contain fish, leaves, insects, Lagerst\u00e4tten form under certain and even small birds complete with feathers. conditions that favor preservation. The The oxygen-poor conditions in the lakebed Wenlock assemblage includes hard-shelled muds slowed bacterial decomposition and animals that have been broken or uprooted\u2014 allowed fragile parts to fossilize intact. The suggesting that crashing waves left debris in same process happened in Germany\u2019s mud at the bottom of a slope. It means a Messel lake at a similar time. single Wenlock slab could contain animals Identifying extinct animals An abundance of fossils from the same age not only helps to reconstruct the interacting lives of prehistoric animals, but helps to resolve their diversity too. Species are described on the basis of specimens\u2014but fossil specimens are frequently incomplete. When so many individuals are preserved together, biologists have a better, more representative, view of anatomy\u2014helping them to divide one species from another. Giant flying ant with iridescent colors intact Frog, including an outline of soft body parts \u25c0 Messel lake pit Bird complete with fossilized feathers The site of Messel, Germany, shows very fine preservation of a community living 47 MYA. Its special conditions included poisonous gas emanating from the lake, which not only killed animals instantly, but also ensured no living scavengers ate the fallen remains before they were mineralized. A piece of colonial coral has been ripped from its position on the reef, like many other attached Wenlock animals 138 THRESHOLD 5","Fenestella, of which this is just a fragment, How was the community suddenly buried? was a fan-shaped colony of tiny filter-feeding animals called bryozoans Scientists studying taphonomy\u2014the history of a fossil\u2014note that this slab has an abundance of Crinoids, or feather stars lightweight animals\u2014such as brachiopods, crinoids, (relatives of starfish), have and bryozoans, but most of their shells and cases left many broken fragments are broken. Taphonomists believe wave-smashed of their branched arms fragments of the living reef were washed away on currents and collected in calmer spots, where their remains were buried. Other Wenlock fossils show Trilobite in defensive rolled trilobites that are partially enrolled\u2014suggesting posture that they were buried alive. Top part of a cystoid\u2014an extinct relative of starfish Where did the animals live? This slab shows a death assemblage, meaning it includes animals that died together. Paleoecologists (scientists who study ancient ecology) need further fossils to build a picture of where the organisms lived. From remains of the animals fossilized as in life, they have found that on the ancient reef, harder-shelled brachiopods\u2014 more resistant to wave action\u2014lived higher on the shoreline, whereas free-swimming animals lived in deeper waters. Brachiopods had two shells connected by a hinge, just like a clam, but they are not related to clams Supporting stems of crinoids were easily broken by strong currents and are abundant in some limestone rocks Restoring the past To reconstruct prehistoric life-forms, paleontologists use all the fossil evidence to put forward a hypothesis about how they looked and behaved, although they can never be certain about their conclusions. The Wenlock fauna consisted of attached animals\u2014such as crinoids and honeycomb corals\u2014that formed a reef habitat, which also contained bottom-feeding trilobites and predatory Orthoceras\u2014a shelled relative of squid. Brachiopod with both Restoration of Orthoceras in Ordovician seas shells open in death Brachiopod WENLOCK LIMESTONE 139","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS ANIMALS INVADE LAND For billions of years, much of life was confined to oceans, lakes, \u25c0 First air-breather and rivers. Such an ancient aquatic heritage meant that the first This modern millipede has complex organisms also lived only in water. Dry land offered so armoured segments similar many new opportunities that terrestrial colonization happened to those of Pneumodesmus, not just once, but many times. a millipede that lived 428\u00a0MYA. Pneumodesmus It is likely that the first microbes were LIVING ON LAND is the earliest body fossil invading land within a billion years of life\u2019s of an animal known to origin. For these bacteria, the wet coastal Above ground, colonization was walk on land and breathe rocks and moist sediments where oceans less straightforward. All living cells, air. Fragments of its lapped the shore were a natural extension of whether of single- or multicellular exoskeleton show that their range. As erosion and detritus formed organisms, must be surrounded by the first soil over 3 BYA, bacteria began to moisture. Land plants survived by evolving it had spiracles, or live between its particles. The earliest a thick, waxy outer layer (cuticle) that breathing holes. burrowing organisms would have churned both retained water and let gases in coastal sediment and added more organic and out (see pp.136\u201337). animals had evolved into some gigantic material that served as food for fungi and forms, but size was a liability on land. A other decomposers. Soils were becoming The first land animals had a cuticle, body is buoyed in water, effectively weighing so enriched, that by 470 MYA, land was too, that served the same water-retaining less, but on land, the same animal may be becoming an inviting place for plants, too. function, but there were other challenges too heavy to move. Early land animals to overcome in just getting around. In the needed stronger muscles and supporting Cambrian period (541\u2013485 MYA), marine skeletons, and compensated for this extra baggage by getting smaller. At first, THE TETRAPODS, WITH THEIR LIMBS AND FINGERS AND TOES, wormlike land animals probably survived INCLUDE OURSELVES AS HUMANS, SO THAT THIS DISTANT underground or in rocky crevices, where, DEVONIAN EVENT IS PROFOUNDLY SIGNIFICANT FOR in moist microhabitats, these small animals HUMANS AS WELL AS FOR THE PLANET. might have used their skin to breathe air. Track between the Jennifer Clack, paleontologist, 1947\u2013 The early terrestrial colonists also footprints suggests Gaining Ground: the Origin and Evolution of Tetrapods included jointed-limbed arthropods. the creature dragged Prehistoric arthropods, relatives of today\u2019s Small, thin prints crabs and spiders, were already thriving in its abdomen suggest at least the oceans. Their jointed limbs and armor eight pairs of legs gave them the potential to succeed on land. Fossil and DNA evidence indicates that millipedes and centipedes were part of the first big wave of land colonists, possibly more than 500 MYA. Their articulated, armored bodies helped them crawl over land without dehydrating and they evolved breathing holes in this armor, called spiracles, and got oxygen straight from the air. Millipedes would have been among the first grazers of land plants and centipedes the first predators of the terrestrial ecosystem. \u25c0 Life\u2019s first steps FILLING THE FORESTS These fossilized marks made in sand dunes in the early Cambrian period, Fossil evidence shows that by 380 MYA, the 530 MYA, represent the oldest trace of land was already supporting its first trees. By animals on land that we have discovered. the beginning of the Carboniferous period They were made by an arthropod that (359 MYA), Earth was home to rich, swampy divided its time between land and sea. forests teeming with life. Plants could grow taller because of the evolution of tougher 140 THRESHOLD 5","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES supporting materials, such as woody lignin. Forests gave height to the land ecosystems\u2014 providing new niches for tree-climbing and flying animals. In particular, they encouraged the biggest radiation of land animals of them all: the insects. The evolution of life on land was producing entirely new kinds of animals with new ecological interactions: web-spinning predatory spiders, browsing insects, and grazing snails. In terms of diversity and abundance, the organisms roaming the land were rivaling anything swimming in the water of the oceans. OUR ANCESTORS REACH LAND lobe-fins, this new kind of breathing mammals, as well as modern amphibians. \u25b2 Transitional fossil When invertebrate life conquered land, mechanism, later powered by chest An astonishingly complete fossil record Tiktaalik rosae is an vertebrates were still confined to water muscles such as the diaphragm, evolved documents the transition from fish to evolutionary wonder. habitats. As with invertebrates, when to become the first air-breathing lungs. tetrapod, via intermediate forms Although it resembled vertebrates started to move onto land sometimes called \u201cfishapods.\u201d a fish, its neck was 395\u2013375 MYA during the Devonian The first vertebrates with lungs are often more flexible than that period, their bodies needed to change. called amphibians, but these long-extinct The four-legged, air-breathing plan of true fishes and its creatures are only distantly related to was a major evolutionary step. Although fins, although small, Fishes use their paired fins for stabilizing today\u2019s frogs and newts. They were the some legs have since been lost or turned had strong joints that their swimming and although a few use first four-legged vertebrates, or \u201ctetrapods,\u201d to arms or wings, it is the basis for most may have supported them secondarily to \u201cwalk\u201d on the sea bed, and ancestors of all reptiles, birds, and land-based vertebrates today. its weight on land. for most of them, fins are not strong enough to fashion into legs. One group, however\u2014 the \u201clobe-fins\u201d\u2014had an advantage. A few, TIKTAALIK ROSAE LIVED 375 mya; THE FIRST FOSSILS WERE DISCOVERED IN 2004 IN THE CANADIAN ARCTIC such as lungfishes and coelacanths, are still Lobe-fin fish Lobe-fin fish \u201cFishapod\u201d \u25c0 Fins to limbs alive today, but in the Devonian, there were Sauripterus Eusthenopteron Tiktaalik Dozens of fossilized many different forms. They differed from all Radial bones have now developed Ulna and radius, once Digit number has reduced species show the other fishes in having a stronger bony support into finger bones, but the number differing in shape, to five\u2014a plan inherited evolution of fishes to for each of their paired fins. Their flexible of digits ranged from five to eight now grow together by all later tetrapods four-legged amphibians. joints allowed these fins to be used to walk as arm bones Over time, the same under water and later helped them emerge Early tetrapod Later tetrapod bones mold into from the water and crawl across land. Acanthostega Later tetrapod Proterogyrinus different shapes, Lobe-finned fishes may have done this in Tulerpeton and a few are times of drought, just as lungfishes do today. lost altogether. As lobe-fins wandered farther ashore, their fins evolved into limbs with fingers and toes. KEY Fishes had other features that prepared Humerus them for a terrestrial life. Most species have Radius a gas-filled bag\u2014the swim bladder\u2014used Ulna for controlling buoyancy. Modifications of Ulnare this swim bladder in some modern fishes Intermedium mean that the sac can communicate directly Radials with air, helping the fish to breathe and Bones missing from supplement the supply of oxygen it extracts the fossil record from the water with its gills. In early ANIMALS INVADE LAND 141","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS REINVENTING THE WING Often, the similarities seen in life are the result of a common ancestor, but not always. For instance, flapping wings required for flight evolved independently in at least four groups of animals at separate points in time, allowing them to take to the air. Organisms evolve adaptations that make apart, both ichthyosaurs and dolphins range of species. Mammals his laaknensebryfasplcroenading their feathers. them better suited to their lifestyles. needed to be fast swimmers in order to later evolved one of the Sometimes, natural selection can produce escape predators and catch fast prey, and more specialized groups of the same innovation in separate, unrelated therefore evolved flippers. flying animals\u2014bats\u2014most groups. This is convergent evolution. of which take to the air at EVOLVING FLIGHT night and use sonar, or SHARING CHARACTERISTICS Insects were the first animals to fly, and they echolocation, to navigate All plants that produce seeds share a are the only fliers to evolve wings that were and hunt in darkness. common ancestor\u2014in the same way not commandeered from existing limbs. that the stingers of jellyfish and coral are Vertebrates became fliers by refashioning Bisrhdoswwsebreirtdhse\u2019 afbirislittwy taormtu-rbnlotohdeierdwainnigms ainlstotoaifrlbyr. T related too. But sometimes, natural their existing limbs. Their forelimbs and selection can produce a similar adaptation hands, over time, evolved into different in unrelated groups\u2014such as the flippers types of wings. Pterosaurs probably of swimming ichthyosaurs (reptiles) and achieved this first and became the most dolphins (mammals). well-known of the reptilian fliers, before becoming extinct along with the dinosaurs. When different forms of life, living in Birds evolved from bipedal dinosaurs and separate environments or even time periods, fared better. They survived the same share an anatomical or behavioral extinction event, possibly due to their similarity, it is often because they live in warm-bloodedness, to thrive alongside similar environments that demand certain mammals and diversify into a wide adaptations. Despite living millions of years Bats true afrliegthhte. \u201coFlyi ng\u201dnslyqmuiarrmemls amlsetroelhyagvleidme.astered Juraatsesisct(o1e6v6ol\u2013v1e45wiMnYg-A). Oldest-known flying insect, a tehxeamfirpsltevferrotembtrhe \u25bc Taking to the skies mayfly or stonefly, is fossilized flaPptpeirnogsfaluigrhst,.cRohuasminps hoof rthhyendcihnuossaisuarsn, were The history of flying animals spans hundreds of millions in North America, 314 MYA early of years of evolution. At four separate points in time, a different group of animals evolved powered flight. 300 MYA 400 MYA REPTILES ARTHROPODS 142 THRESHOLD 5","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES Stretched \u25c0 Anatomy of a wing wing membrane The wings of a attaches to body pterosaur, bird, and bat each use bones of the Long arm and hand, but fifth digit evolution has molded the bones differently Pterosaur wings were relatively rudimentary compared in each case. The shape to a bird\u2019s or a bat\u2019s. Wing membranes were supported of pterosaur and bat mainly by a single finger. Pterosaurs did not have the wings depends on how musculature for coordinated flight, and probably relied the bones hold out the on soaring on air drafts to get around. flight membrane, made of skin. A bird\u2019s flight Shortened, fused surface is made of finger bones feathers and its shape depends on the form of its feathers. Long feathers KEY Humerus Radius and ulna Wrist bones Finger bones Thumb bones Birds have shorter arm and finger bones that bring the wing under better control. More powerful muscles also mean that the wing, equipped with long flight feathers, can be flapped at wider angles, providing birds with stronger flight than pterosaurs. Thumb-bone Long fingers free of wing support wing membrane aInpeseacktinwaintmgso,slpikheetrhicoosexyogfeanmtroidggeernreddramgoorneflayc,tmivaeylihfeavsetyelevso.lved when \u25b2 How did wings evolve? Bats have more finger bones than other flying The earliest proto-wings were flaps on insects\u2019 bodies vertebrates. Their wings are supported by four that previously served as gills, or as oars allowing them fingers. This makes their shape more flexible than to skim over water. Climbing insects may have used the wings of birds, helping them move in ways that them to parachute through air before evolving flapping improve maneuverability and save energy. wings, which gave them better aerial control. Pterosaurs, birds, and finally bats became airborne not by growing Oldest modern bird, Lithornis, the oldest known ratite (a type of new appendages, but by adapting existing ones. Vegavis, is fossilized flightless bird including kiwis and ostriches) in Antarctica, 66 MYA is fossilized in North America, 57 MYA BIRDS Oldest known pterosaur, Oldest known bird, At the end of their reign, Oldest known bats are fossilized Faxinalipterus, is fossilized Archaeopteryx, 68\u201366 MYA, some pterosaurs 56 MYA: Archaeonycteris in Portugal in Brazil, 220 MYA is fossilized in reach enormous sizes, with some and Marnenycteris in France Germany, 150 MYA exceeding 33ft (10m) in wingspan 200 MYA 100 MYA 0 MAMMALS REINVENTING THE WING 143","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS THE COMPLETELY PROTECTED EMBRYO WITHIN THE SEED GIVES... A GREAT ADVANTAGE. Douglas Houghton Campbell, American botanist, 1859\u20131953 The presence of fossilized seeds indicates that the tree had been pollinated and so successfully fertilized Woody cone has turned to stone during fossilization","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES THE FIRST SEEDS \u25c0 Monkey puzzle About 370 million years ago, a new kind of plant evolved. It produced This fossilized cone is 160 million years old but seeds, which are the ultimate embryo survival kit\u2014packed with nutrients is remarkably similar to the cones produced by and enveloped in a protective casing. Seeds would shape the history of trees today. This species, Araucaria araucana, life and play a key part in our own prehistory. is known as the monkey puzzle and still thrives in Argentina and Chile. The first algaelike plants completed their cell nucleus straight to the egg, dispensing entire life cycle\u2014alternating between spores with swimming altogether. Pollen allowed Scales in the cone are and gametes (eggs or sperm)\u2014under water. plants to spread farther inland than their modified leaves that As their descendants, mosses and ferns, water-reliant relatives. What is more, these protect the seed crept farther inland, more resilient spores plants completed their break from water by could be dispersed into the air. However, keeping embryos of their next generation their sperm still needed water droplets to in drought-resistant cases\u2014seeds. swim to the egg: no matter how much their deep roots and tough leaves helped them HOW SEEDS WORK survive droughts, plants still needed periodic Eggs develop inside a thin-walled sac called rainfall in order to reproduce. an ovule. After pollen fertilizes an ovule, its walls thicken, and it becomes a seed. At first, A new kind of plant broke this restrictive ovules grew exposed on foliage or the scales link with water by relocating its fertilization of cones\u2014reproductive shoots composed of into reproductive shoots away from the hard scales connected at their base, just like ground. Female shoots retained their spores, the cones produced by cycads and conifers today. Eventually, most seed plants buried MEDULLOSA\u2014A PRIMITIVE their ovules deeper inside the shoot, beneath SEED PLANT THAT LIVED a flower (see pp.160\u201361). When these ovules 350\u2013250 MYA\u2014HAD SEEDS turn into seeds, the succulent tissue around them becomes fruit. Seed plants had now THE SIZE OF EGGS evolved a method of enticing animals, a different form of complex life, to become which grew into eggs. Spores from male part of their life strategy (see pp.164\u201365). shoots became pollen grains that were blown inland to land on female shoots. SEEDS, THEIR SUCCESS, AND US In the most primitive seed plants, sperm The pollen method of fertilization and the then burst from the pollen grains and swam seed method of dispersal have both been through the shoot to the egg\u2014something so successful that seed plants now form still seen in cycads of today. But in most seed the basis of all land-based ecosystems and plants, sperm became redundant. Instead, food webs worldwide, including those with each pollen grain sprouted a tiny thread\u2014 humans at the top. Non-seed plants\u2014mosses, a pollen tube\u2014that conveyed a naked male ferns, and liverworts\u2014although widespread, no longer dominate any land habitats. Ovule Primitive Cupule \u25c0 Primitive seed plants fruiting The first seed plants structure Ovule Cupule are called seed ferns, containing a because of the shape LIDGETTONIA Capitulum single ovule of their leaves, although holding four they are unrelated to ovules the ferns we know today. They grew their ovules PLUMSTEDIA DENKANIA in packages attached to the leaves. Cones and flowers eventually evolved in later types of plants. THE FIRST SEEDS 145","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS \u25bc Life in a shell The shell is composed of a chalky material White shell membranes conceal the Prehistoric reptiles\u2014including dinosaurs\u2014were based on calcium carbonate that is hard chorion\u2014a transparent embryonic pioneers of the shelled egg. The embryo could membrane that completely encloses the develop inside, safe from dehydration. Its parents enough to withstand damage, permeable to embryo, amnion, yolk sac, and allantois may have guarded the egg from predators, just allow the exchange of respiratory gases, like many reptiles and birds do today. and sufficiently brittle so the infant can break free upon hatching The expansive allantois absorbs oxygen that seeps through the shell from the egg\u2019s surroundings and releases carbon dioxide Some blood vessels in the allantois carry The embryo has already oxygen into the embryo; others take waste developed all the major body carbon dioxide away from it. Nitrogen- parts it will need upon hatching containing waste products also build up in the allantois as deposits of uric acid. 146 THRESHOLD 5","530 MYA FIRST LAND 380 MYA FIRST TREES 220 MYA FIRST MAMMALS 65 MYA ASTEROID KILLS THE ANIMALS AND FORESTS AND DINOSAURS RULING REPTILES SHELLED EGGS ARE BORN The yolk sac is filled with The first backboned animals to live on land could walk, since they had foods, such as protein and fat, legs, and could breathe air. These early amphibians were still tied to that nourish the developing water, however, because they needed a wet place to breed. Reptiles broke embryo; it shrinks as the this link by producing hard-shelled eggs that could develop on dry land. embryo grows bigger and uses up its contents Backboned animals originated in water, time. But, in both cases, hatchlings are where fish and amphibians laid their soft ready to eat and breathe as soon as they The amnion is a thin transparent eggs encased in nothing but a protective emerge from the egg. membrane that encloses the jelly coat. Reptiles not only evolved hard, amniotic fluid, which surrounds scaly, waterproof skin as protection from PREPARED FOR LAND the embryo and cushions it from dehydration, but transformed their breeding The shelled egg and its life-supporting physical harm habits too. They covered their eggs in a shell membranes enabled the amniotes to hard enough to protect and contain the complete their life cycle on land. They embryo on land, yet permeable enough mated on land and laid eggs in a dry nest. for it to breathe. A few living reptiles have abandoned their egg-laying ways and give birth to live young. EMBRYO SURVIVAL KIT The shelled eggs produced by most reptiles THE EARLIEST ANIMAL and all birds are amazing structures that THOUGHT TO LAY SHELLED contain all their embryos need to develop. Until the invention of these eggs, all living EGGS IS PALEOTHYRIS, A embryos developed surrounded by fluid. To REPTILE LIVING 330 MYA reproduce those fluid conditions on land, it was a small and manageable evolutionary But one group of amniotes, the mammals, step to enclose the fluid within a membrane. turned live birth into a major asset. They The membrane is called the amnion, giving commandeered two membranes\u2014the the first animals to possess it the name allantois and chorion\u2014into a placenta, which \u201camniotes\u201d as well as the more familiar draws oxygen and nourishment straight from \u201creptiles.\u201d Within the egg, the embryo the mother\u2019s blood. By nurturing the embryo also has its own larder, the yolk sac, just as in the mother\u2019s body, mammals improved fish and amphibians do. But it also has an their offspring\u2019s chances of survival beyond allantois\u2014a waste-disposal pouch absent in those of their larvae-producing ancestors. its ancestors. The yolk sac grows smaller and the allantois enlarges as it absorbs oxygen \u25c0 Land colonizer and accumulates waste products while the Dimetrodon, a reptile embryo grows. A final membrane\u2014the that lived 290\u2013270 MYA, chorion\u2014serves to contain the entire is an example of an early embryo \u201csurvival kit.\u201d amniote. Its ability to lay eggs with a shell and By the time they hatch, reptiles are ready amnion allowed it to to lead independent lives; on the other hand, colonize arid habitats most bird chicks need parental care for a where water was not readily available. SHELLED EGGS ARE BORN 147","4.1 BYA FIRST TRACE 2.4 BYA OXYGEN 936 MYA ESTIMATED ORIGIN OF OF POSSIBLE LIFE FILLS THE AIR ALGAE AND PLANTS HOW COAL FORMED The trees that formed Earth\u2019s first forests were giant fernlike plants that resisted decay. Their dead bodies built up, trapping carbon and energy underground. These were the coal forests\u2014and 300 million years later, their compacted remains would fuel an industrial revolution. The Carboniferous period (359\u2013299 MYA) rich store of energy that formed coal. The passive breathing through their skin or body was a time when life on land prospered more trees concentrated lignin in their tissues to surface became enormous. The biggest than ever before. Trees grew from mosslike more than 10 times the quantity found in insects that ever lived evolved during the ancestors, insects took to the air in a world today\u2019s trees. This not only helped to deter Carboniferous, and amphibians grew to already crawling with invertebrates, and herbivores, but it also resisted decay, because the size of crocodiles. giant amphibians were evolving into reptiles. few microbes could digest it. As trees died, This time in Earth\u2019s history would have their fallen trunks lingered. The lignin, huge implications for our own history. along with the carbon it contained, would be converted to carbon dioxide (CO2) if it THE FIRST FORESTS decayed, but it sank into the swampy Earth, For the first time, terrestrial life could live locking away its chemical energy. As CO2 in the trees, imbuing habitats with an extra in the atmosphere diminished, oxygen richness. The first big invasions of land increased, since it would normally be animals, involving millipedes, insects, and consumed by the same processes of arachnids, had already taken place, but now decomposition, which were now suppressed. these groups exploded into a multitude of Oxygen built up in the air to become more species, including predators such as spiders, than one-third of it by volume. Today, scorpions, and centipedes. Carboniferous oxygen accounts for only one-fifth of the gas trees could grow tall because they had in the atmosphere. The effects of such high evolved a tough supporting material called oxygen levels would have been bizarre. lignin that formed a protective layer. It Ignition would have happened more readily, would also eventually become the carbon- sparking wildfires. Animals that relied on \u25b6 Coal formation Prehistoric trees Coal began as and other plants undecomposed matter topple and die from dead trees. The dead matter was Dead matter Pressure of Pressure THE ORIGINS OF COAL buried as new dead in swampy soil sediment layers of sediment material accumulated partially decays squeezes out layers builds Much of the bulk of the Carboniferous trees on top, and it became to form peat water and air sank intact beneath the swampy waters, compacted under forming layer upon layer of a deposit called high pressure. Over millions of years of LEPIDODENDRON increasing pressure TREES GREW UP TO 130FT and temperature, the material turned first (40M) TALL IN THE to the rock lignite, then CARBONIFEROUS PERIOD eventually to coal. Pressure solidifies and compacts As pressures and temperatures Rock is now coal, a more peat. In the peat, oxygen was low and layers of peat into a rock called rise, deposits lose water and gas, combustible rock with acidity high, and instead of decomposing, lignite, which is combustible concentrating their carbon levels maximum carbon the carbon-rich remains built up. The peat became compacted under its own weight, squeezing out water and gases, turning first into a form of rock called lignite, and finally 148 THRESHOLD 5"]
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