compelling. Researchershave found in 2.6-billion-yearo1d rocks,for example,that a striking resemblanceto cyanobac- teria,the lineageof bacteriathat carriesout photosynthesis.That's exactlywhen the first evidenceof atmosphericoxygen appearsin the record; between 2.45 and 2.32 billion ago,oxygen increaseddramatically. The rise in oxygen was likely the result of the emergenceof ,which releaseoxygen during photosynthesis.While oxygen levels increaseddramatically dur- ing this time, they were still very low compared to today. As a result,purple sulfur bacteriawere still abundant1.6 billion years ago,as reflectedby the presenceof okenane. Archaeaalso make an early but ambiguous-appearance in the fossil record. ln 2006, Yuichiro Ueno and his colleaguesat Tokyo Institute of Technologywere able to extract methane from 3.S-billionyear-old rocks from Australia.The methane had a low fraction of carbon-13,indicating that it had been producedbio- logically(Ueno et al. 2006).Only one group of organismsalive todayreleases methane: a lineageofarchaea called . Among the placesthey live today is the digestivetract of cows; they'rethe reasonsthat cow belchescontain methane. Eukaryaemerge in the fossil record only about 1.8 billion yearsago. Their first fossilsare single celled organisms measuring about 100micrometers across. While they would havebeen invis- ible to the nakedeye, they marked a giant leap in size,measur- ing about 100times biggerthan a typical bacterium.These early eukaryoteshad ridges,plates, and otherstructures that aresimilar to thoseof living singlecelled . Over the next billion years,the diversityof thesesingle-celled eukaryotes increased, as some lineagesevolved to carry out photosynthesiswhile others preyed on bacteriaor grazedon their photosyntheticrelatives (Knollet al.2OO6\. If you could travel back in time to 1.5billion yearsago, the world would look like a desolateplace. On land there were no trees,no flowers,not evenmoss. In somespots, a thin varnishof single-celledorganisms grew. In the ocean,there were no or lobstersor reefs.Yet the oceanteemed with microbiallife, from the organisms that lived around hydrothermal vents on the seafloorto free-float- Figure3.13 A: Bacteriacan grow and ing and photosynthetic eukaryotes at the ocean's surface. Along the coasts, divideindividually, but they can also microbial mats stretched for miles in the shallow waters. form multicellularstructures, sucn as Today our attention may be distracted by and , but the world gelatinoussheets called biofilms. remains dominated by microbes. By weight, microbes make up the bulk of Earth's B'.Dictyosteliumdiscoides,asoil biomass. They live in a tremendous range of habitats that would kill the typical - ,istypicallyunicellular. nral or -from Antarctic deserts to the bottom of acid-drenched mine shafts. The BuID. discoides individuals can come genetic variation among single-celledlife also far exceedsthat of animals or plants. togethertoformaslug-likemassthat Most genes on the planet belong to microbes or their viruses. It's a microbial world, cancrawl away and form a stalkof in other words, and we just happen to live in it. spores.Along with bacterialbiofilms, theyoffer clues to howmulticellularity first evolved. 3.5 LifeGets Big O* The body is radically different from a single-celledbacterium. It's made of a trillion cells glued together with adhesivemolecules and differentiated into organs and tissues that work together. Only a minuscule fraction of cells in the human body-the sperm or -have the potential to pass on their genetic material to future generations.

3.5 LrFEGErS BrG 67 Figure3.14 In 2010,scientists publisheddetails of enigmaticfossils measuringup to 12centimeters across. Datingback 2.1 billion years, they are the oldestfossils of multicellularlife. (Albaniet a1.2010.)

The tree of shows that multicellularity did not evolve just once. Instead, multicellularity evolved on dozens of different occasions. Animals, for example, are closely related to fungi, which can develop mushrooms and other multicellular structures.But animals have lots of even closerrelatives that are single celled,and it is likely that multi- cellularity evolvedin animals and fungi independently. Studieson living organismsoffer some clues to what the precursors of multicellular lineages may have looked like. Although bacteriaare consideredsingle celled,for example, they often live together in films that line surfaces ranging from our intestinesto rocks on the seafloor.In thesebiofilms, they sendsignals to one anotherthat regulatetheir growth and activity. Among eukaryotesa model organism used to study multicellularlty is Dictyosteliumdiscoides, which most of its life as a single-celledpredator on bacteria.When its prey runs out, it joins with thousandsof other D, discoidesto form a slug-like "body" that crawls through the soil. Eventually it stops,and some of the cells produce celluloseto build a stalk, on top of which a ball of sporesforms (we discussDictyoste- lium socialbehavior in Chapter 16). In 2010,an internationalteam of scientistsdescribed the oldestknown fossilsof a (Albani et al. 2010).The fossils,dating back 2.1 billion yearsrwere found in Gabonin WestAfrica. They arescalloped disks, measuring up to 12 centimeters across.It's not clear whether the fossils were formed by early eukaryotes,some type of bacteria,or (Figure3.1a). The oldest recognizable multicellular eukary- otes-filaments of some type of -are 1.6 billion years old. The oldestknown fossilsof red algaedate back 1.2billion years (Figure3.15), while green algaefirst appear750 million 25 pm r years ago. KeyConcept Thetransition to multicellularlife began at least 2.1 billion years ago, but multicellularitylikely evolved ina numberof lineages.

Figure3.15 A fossilof redalgae, known as Bangiomorpha, dating back 1.2billion years.

58 CHAPTERTHREE WHAT THE ROCKSSAY: HOW GEOLOGYAND PALEONTOLOGYREVEAL THE HISTORYOF LIFE

i', ..:ri.irisrrn|Ndl*l Figure3.15 ln 2010,scientists described650-million--old fossils from Australia,which they interpreted assponges. A: A iossj)ishighighted in red.B: By slicingthrougha rock, scientistsare able to getclues to the three-dimensionalstructure of another \ssir\.\\.{ss\e\*.a\\\

3.6 TheDawn of theAnimal Different lineagesof multicellularorganisms evolved different ways of capturing energ'yto grow. Greenplants, green algae,brown algae,and red algaeall use photo' synthesis.Fungi releaseenzymes to break down food,which they can then absorb. Only one lineageof multicellularorganisms evolved a body that allowedthem to swallowother organisms:animals. Figure3.17 Thesemicroscopic fossils Today,animals include familiar groups such as and , but less of clustersof cellsdate back about 580 familiar onesas well, like .Sponges may not seemmuch like otheranimals- millionyears. Based on studiesof living they lack a brain,, or evena mouth,getting their food by trapping particlesthat cells,paleontologists initially conclud- drift through the poresof their bodies.Yet they sharethousands of geneticmarkers edthey are probably . with other animalsthat are not found in nonanimal species.And it's spongesthat ln 20.1.1,other researchers argued that appearto mark the earliestappearance of animalsin the fossilrecord. In 2010,Adam theyare instead single-celled relatives Maloof of PrincetonUniversity and his colleaguesreported 65O-million-year-old fos- of animalsthat reproduceby forming silsthat appearstrikingly like living sponges-down to -likepores. a cyst.The cell divides into daughter Meanwhile,Gordon Love, a geochemistat the Universityof Californiaat River- cells,which then escape the cyst. side,and his colleagueshave been finding biomarkersof animalsof aboutthat same age.In 2009they reportedthe discoveryof a cholesterol- like molecule,in 635-million-year-oldrock in Oman,that is made today by only one group of sponges(Love et al. 2009).In 2011,they reportedfinding the samemolecules in rocksof a similarage in Siberia(Kelly et al.,in press). Thesebiomarkers and the Australianfossils of possible sponges suggest that animals had already evolved at least100 million yearsbefore the start of the period. The search for early fossils of animals inspires strong debatesbecause it can be difficult to distinguish the ancient remains of a primitive animal from a colony of single-celledeukaryotes. A ChineseLagerstiitte called the DoushantuoFormation has yielded what were ini- tially proposedto be 58O-million-yearold animal embryos (Figure3.17;Raffet aI.2008; Chen et aI.2009). But in 2011, StephenBengtson of the SwedishMuseum of Natural History and his colleaguesmade a compelling casethat thev were actuallv fossils of close relatives of animals. Eachof the unicellularorganisms reproduced by dividing

3.6 THEDAWN oF THEANTMAL KtNGDoM 69 Figure3"18 Between about 575 and 535million years ago, a bizarrecommu nityof multicellularorganisms l

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Figure3.19 TheEdiacaran fauna was a groupof organismswith diverse and uniquemorphologies and body plans. Thesefossils show the range of varia- tion in form andsymmetry.

70 cHAprERTHREE wHAT THERocKs say: How GEoLoGyaND paLEoNToLoGyREVEAL THE HrsroRy oF LrFE cambrianl

mva i Rangeomorphs s4s-l Schwarzrand E : EE E A i ! XF F rn HE 6 5sol Kuibis os 0 Bilatera DiscoidalffirriradialP qJ I l"l i EdiacaraMbr c t;] ffi gE L:I H H [q [qPt ffi i_el Etltt t|l l+iHne* 5ss-lI o ffi ZimmieGory N H b.l lal H tal tol ffiffiEE F a tr<'o!o O F e e €*s :Eqr\+ I P OOi:cih E -^E E € *- -3 P ! s#x -co:!g __l Charnwood ih*a,S H Hb d o )ou"l ACY oi FCI II ICU>C fi I E, I - qJl rLi Z cec-l MistakenPoint D E, I UI $ s UIo 570-l ffi I Tribrachidium 57s"1 I 5801I I (laSKIerSLrlaclallon {)dl IVla) ,I

Doushantuocysts Doushantuo-Pertatakaacfl tarchs Marinoan Glaciation(635 Ma)

Figure3.20 Theevolution of the Edia- caranfauna. Some fossils share traits withliving organisms, but others are difficultto placesystematically. rnto many daughter cells inside a cyst. The cyst then ruptured, allowing the daughter cellsto escape(Huldtgren et al. 2011). More widely acceptedevidence for animal remains are tracks that appear to have been made by tiny like creatures as well as shells measuring 1 to 2 millimeters wide (Budd 2008). And starting at 575 million years ago, a host of nruch bigger fossils appear in the fossil record. Measuring over a meter in length in some cases,they were bizarre in appearance.Some looked iike fronds, others like geo- nretrical disks, and still others like blobs covered with tire tracks. Collectively,these enigmatic speciesare known as the fauna (named for a region in Australia Ediacaranfauna: A groupof animal rvhere paleontologists first recognized that these kinds of fossils dated back to before speciesthat existedbetween 575 and tl-reCambrian period). 535million years ago. Paleontologistshave compared Ediacaran fossils to living speciesto determine their place in the tree of life. Some fossils share many traits with living groups of ar-rimals.Kimberella, for instance, has a rasp-shaped feeding structure found today in nrollusks, a group that includes clams and snails. But many Ediacarans have proven f ar more difficult to decipher. Some fossils may be animals, but they are only distantly related to living lineages. Others may not be animals at ali, having independently evolved multicellularity at about the same time (Xiao and Laflamme 2008). Many of these enigmatic Ediacaran forms had disappeared by the beginning of the Cambrian period, 542 million years ago, and virtually all were gone by about 535 nrillion years ago. In the meantime, some of the earliest recognizable members of liv-

3.6 THEDAWN oF THEANTMAL t(|NGDoM Figure3.2lTheEdiacaranfaunawasreplacedbynewgroupsofanimalsduringtheCambrian period.This painting is a reconstructionof the 505-million-year-oldecosystem recorded in the BurgessShale.

Chordates:Members of a diverse ing animal lineages had emerged (Marshall 2006). The early Cambrian, from 542 mil- phylumof animalsthat includes the to 511 million years ago, is divided into four stages,and each stage saw more first ,, and . appearances of living groups than the previous one did. We belong to the chordates, Chordatesall havea ,a for example, a group that makes its first appearance in fossil-rich rocks in China hollownerve cord, pharyngeal slits, called the Chenjiang Formation, dating back 515 million years ago (Shu et al. 1999). anda post-analtail asembryos. Not all of the major groups of animals that emerged during the Cambrian period can be found on Earth today. After trilobites emerged during the Cambrian period, Trilobites:Extinct marine for example, they endured until 251 million years ago. The last trilobite species disap- thatdiversified during the Cambrian peared at around the same time that about 90 percent of all other species vanished. periodand gradually died out during (For more on the causes and effects of mass extinctions, see page 458.) the Devonianperiod. Key Concepts Althoughearly Ediacaran fossils were highly diverse and had unique body plans, only a fractionshare traitswith livingspecies. Nearly all Ediacaran species disappear from the fossil record within 40 mil- lionyears.

Nearlyall living animal lineages, including chordates, evolved during the Cambrianperiod.

72 CHAPTERTHREE WHAT THE ROCKSSAY: HOW GEOLOGYAND PALEONTOLOGYREVEAL THE HISTORYOF LIFE Figure3.22 Theearliest members of manyliving groups of animals suchas a brain,a stiffeningrod (called a notochord)running next to firstappeared during the Cambrian period. We belong to its spinalcord, and arches that mayhave supported . B: By 380 thechordate lineage that first appears inthe fossil record during the millionyears ago, large predators had evolved, such as Cambrianperiod and that gave rise to vertebrates.A:Hoikouichthys ,which grew up to 5 meters(18 feet) long. wasa small,fish-like animal with some traits found only in chordates, Notochord:A flexible,rod-shaped structurefound in the embryosof all 3.7 ClimbingAshore chordates. served as the first"backbones" in earlychordates, Along with the rise of multicellular life, another major transition documentedin the andin extantvertebrates the embry- fossil record is the transition of life from the oceanto land (Labandeira2005). As life onicnotochord becomes part of the evolvedin the sea,dry land remained bare.The earliesthints of terrestrial life come vertebralcolumn. from prokaryotes. In South African rocks dating to 2.6 billion years ago, scientists have found remains of microbial mats that grew on land. Fungi, plants,and animals Prokaryotes:Microorganisms lacking did not arrive on land until much later. In Oman, scientistshave found 475-million- a cellnucleus or anyother membrane- year-oldfossils of sporesthat appearto have embeddedoriginally in plant tissues- boundorganelles. Prokaryotes the oldestplant fossilsfound so far. The earliestland plants resemblemosses and liv- comprisetwo evolutionarilydistinct erworts (Wellman, Osterloff, groups,the Bacteriaand the Archaea. and Mohiuddiu 2003). Over the next 100 million years, fossils show that plants beganto establishlarger and larger ecosystems on land, until full-blown forests were growing. Today, land plants live in intimate associationwith fungi. Some fungi feed on dead plants, helping to con- vert them into soil. Others causediseases in plants,such as chestnut blight, which wiped out almost all Ameri- can chestnut trees in the twentieth century. Still oth- ers help plants, supplying Figure3.23 Theoldest tree-like plant, nutrients to their roots in knownas Wattieza,was a 385-million- exchange for organic carbon year-oldplant that 8 meters that the plants createin pho- stood (26feet) tall. toslmthesis.The oldest

3.7 cLTMBTNGAsHoRE 73 Figure 3.24 A: Theoldest fossil of a landanimal belonged to a 428-mil- lion-year-oldmillipede known as Pneumodesmusnewmoni. B:A reconstruction. ll lr l I I :

fossils,which date back to 400 million years ago,belong to this last category.Their fossilsare mingled with the fossilsof plants. It appearsthat fungi and plants helped eachother move from water onto land (Berbeeand Taylor 2007).(See Chapter 15 for more on how different speciesform intimate partnerships.) Animals left only tentative marks on the land at first. In rocks dating back to about 480 million yearsago, there are tracksthat appearto havebeen made by inver- tebrate animals-probably ancient relativesof insectsand spiders.The tracks were made on a beachdune; whether the animal that made them could actuallyhave lived full-time on land is a mystery.The oldestknown fossil of a fully is more than 50 million years younger than the first trackways:a 428-million-year-old :Vertebrates with four limbs relativeof today'smillipedes, found in Scotlandin2004 by a bus driver who hunts for (or descendedfrom vertebrateswith fossilsin his free time (Wilson and Anderson 2004).The oldestknown trackwaysleft four limbs).Living tetrapods include by a vertebratedate backto 390 million yearsago (Niedzradedzkiet al. 2010),while the mammals,birds, , and oldest known fossils of vertebrateswith legs-known as tetrapods-are about 370 amohibians. million years old. (In Chapter4, we'll look at the origin of tetrapodsin more detail.)

B

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Figure3.25 A: ln 2010,paleontolo- gistsreported a 390-million-year-old trackway.B: The spacing of the tracks I suggeststhey were made by an animal with analternating gait. C: lt's possible thatthis animalwas an earlv tetraood.

74 cHAprERTHREE wHAT THE RocKs sAV: How GEoLoGyAND paLEoNToLoGy REvEALTHE HrsroRy oF LrFE Figure 3.26 Silvonerpeton,shown here,was one ofthe oldestterrestrial vertebrates(known as tetrapods) that left fossils.

3.8 RecentArrivals One of the most important lessonsfrom the fossil record is that some of the most familiar kinds of life today did not emerge until relatively recently. Most species of fish on Earth today,for example,belong to a group known as the .They Teleosts:A lineageof bonyfish that include many of the most familiar fish, such as , ,and . But 350 comprisesmost living species of million years ago there were no teleostsat all. Likewise,350 million years ago there vertebrates.Teleosts include goldfi sh, were no mammals,which today are the dominantvertebrates on land. Some15,000 salmon,and tuna. Teleosts can be dis- speciesof birds fly overheadtoday, but not a singlebird existed350 million yearsago. tinguishedfrom other by unique Before today's most common groups of speciesemerged, the planet was domi- traits,such as the mobilityof anupper nated by other groups. Before the rise of fishes,for example, some of the jawbonecalled the premaxillary. ocean'stop predatorswere giant sea scorpions,which measuredup to 6 feet long. On land, 280 million years ago, the dominant vertebrateswere relatives of today's mammals: ungainly, sprawling creaturescalled .The first synapsidsthat Synapsids:A lineageof tetrapodsthat evolved into something that looked even remotely like today's mammals emerged emerged3oo millionyears ago and about 200 million yearsago. It was not until about 150million yearsago that the first gaverise to mammals.Synapsids can members of the living groups of mammals evolved (Luo 2007). bedistinguished from other tetrapods Meanwhile, new lineagesof reptiles were also evolving. One of the most suc- by the presenceof a pairof openingsin cessful was the branch. emerged about 230 million years ago the skullbehind the eyes,known as the and steadily grew more diverse.Their ranks included giant long-neckedsauropods temporalfenestrae. that were the largest animals ever to walk the Earth as well as fearsomepredators. Dinosaursdominated ecosystemson land until they disappearedin a pulse of mass extinctions65 million yearsago. The only survivorsof this lineagetoday arethe birds, which branchedoff from other dinosaursabout 150million yearsago (Chiappe2007). (ln Chapter4, we will examine the origin of birds more closely.) Most of the plants we seearound us today are also relatively new in the . As we saw earlier,the earliest fossils of plants resembledliving liverworts and .They likely formed low, ground-huggingcarpets. The of lignin and other plant compoundsallowed somelineages to grow stems,stalks, and trunks. Starting in the Carboniferousperiod, a number of large-sizedplant lineagesbegan to

3.8 necrlrr ARRTvALs 75 appear.Manv lincageslater became extinct, but sorne- such as ferns and gingko trees have survived until today.They are no longer the dominant plant lineages, however.Today, rnost ecosystemsare insteaddominated by flowerir.rgplants. The oldestfossils of flowering plants date back to only 132 rnillion years ago-some 300 miiliorr years younger than the oldest known plant fossils.During the Jurassicand Cretaceousperiods, flowering plar-rts becamemore abundantand diverse.In Chapter15, we'11 considerone explanationfor their rise:their coevolution witlrpollinating irrset l:. Today,one of the most widespreadforms of flower- ing plantsis thegrasses. Grasses cover the greatexpanses of savannasand prairies of the world; they thrive ir-r suburbanlawns and city parks.Farmers plant don'resti ^^'^-r--^^^^^ ---^L^- ldttrtt qtclJDtrD )ttLll di wheatand corn over much of the planet'sarable surface. Yet grassfossils are alsolate arrivers.The first evidenceof grassesin the fossilrecorcl is tiny bitsof tissuein the 70-millionyear olcl droppings of clinosaLrrs.Grasses remained rare for tl-renext 50 rnil lion years. Only abor-rt20 million yearsago did grassesexpancl into widespreadterritories. The fossilrecord is fillecl Figure3.27 Mammalsare descended with n.rarryexarnples ol such lons-term shifts in biologicalrlivcrsity, anrl, as we'll see fromsprawling, like vertebrates in Chaptcr 14,oner of the rnajor soals of evolrrtionarybiology is to test explanations calledsynapsids that first emerged 32O for thr:set:hiirrges. ln tlie caseol grasses,n)arry resear(:hcrs have argut:rl that a gradual millionyears ago. shift in the clierrristryof the atrnospherehas been responsiblc.Ovcr thc past 50 mil lion years,ciirbon rlioxirlc lias lrt:c'ngrarhrally rler:lining in the atrnosphere.Crasses .lrc lrorc efficierrt at extrar:tinrrcarborr rlioxitle flont tlic atnrosphcrc than C-3plants (l'iperno and Srrcs2005; Soltiset al. 2(X)6). At about the sanrctirnc that flowt:ringplants were ernergirtg,the nroclernlineages of nriirnrnalswere also bec

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Figure3.?8 Theoldest insect fossils are400 millionyears old. But many of theIargest groups of livinginsect speciesevolved much later. The first flies,for example,evolved about 250 millionyears ago. This fly (a gall midge) wastrapped in amberabout 30 million yearsago.

CHAPTER THREE WHAT THE ROCKS SAY: HOW GEOLOGY AND PALEONTOLOGY REVEAL THE HISTORY OF LIFE Figure3.29 A: A reconstructionofthe known hominid,Sahelan- primateswere small, like creatures,but they sharedmany traitsfound in all liv oldest bipedal thropus.B: This fossil, discovered in the ing primates(including ourselves), such as forward facing eyes and dexteroushands. Saharadesert in 200],is estimated to The oldesthominin fossilsinclude , which was discoveredin 2001in havelived millionyears ago. Chadand datesback about 7 million years(Brunet et al. 2002).Early hominins were 7 sirnilarin someways to ,both in termsof their body and brain size.The oldestknown fossilsmatching our own statureemerged only about 2 million years ago;and the oldestfossils that clearlybelong to our own species,found in Ethiopia, are estimatedto be nearly 200,000years old (page63; fohansonand Edgar2006, Hominin:Humans and all species McDougallet al.2005). In Chapter4, we'Il explorehominin phylogenyin moredetail, moreclosely related to humansthan to and in Chapter17, we'll investigatesome of the key geneticand behavioralchanges chimpanzees. overthe courseof hominin evolution. Two hundred thousandyears-the age of our species-is such a vast span of time that it's hard for the human mind to fathom. And yet, as we've seenin this chapter,it is just a tiny fractionof the 3.5billion or moreyears that life hasexisted on Earth.If you wereto shrink that time down to a day,our specieswould haveemerged 5 secondsbefore midnieht.

Figure3.30 Theoldest known fossil of our ownspecies, discovered in Ethiopia,isestimated to beless than 200,000years old.

3.8 REcENTARRTvALs 77 ThePresent and the Pastin Science

Predrctionsare essentialto science. When scientists devise a hypoth- constructphylogenetic hypotheses based on the morphologyof esisexplaining a nal-ralphenomenon, they can useit to generate fossrlsand ,ivrngspecies. Each of thesehypotheses makes a predic nrpdrefinns:nd in snmoeases lhev c2n set rrnexnertnentStOtest tion:namely, that other linesof evidencewill support the proposed thosepred ctions. lf onescientist publrshes an experiment that sup- phylogeny,As we'llsee in Chapter9, scientistscan nowexamine the portsa prediction,other scientists try to come up with experiments evidenceencoded in DNA.In a numberof cases,the resultsof these of their own to see if theyget the same result.Even a scrupulously moLecularstudies match their predictions, honestscrentist can get a misleadingresult from an experiment.lf Anotherexample of the predictionsthat evolutionarybiologists an experi.nentis not set up particularlywell. it may producea "false make comesfrom one of the greatestcatastrophes in the history positive"-inother words, a seeminglypositive result that meetsthe of life,About 252 millionyears ago, over 900/o of all speciesbecame scientist'spredictions, but not for the reasonsthe scientistthinks. extinct(page 459). Basedon the chemistryof rocksthat formed Experimentscan also producefalse negatives.A predictionmay at that time,some researchersproposed that much of the ocean's seemto fail,but onlybecause the experimentwasn't set up carefully oxygendisappeared, starting in deepwater and spreading out to the enoughto testit. lt mayalso fail because a scientisthappened to ana- coasts.That hypothesis leads to a prediction:if paleontologistsfound lyzean unrepresentativesample. a goodfossil record, they'd f ind that deepwater species would become Evolutionarybiologists have run many experimentsto test pre- extinctfirst, and then shallow-water ones. dictions.They have constructed experiments in whichnatural selec- CatherinePowers and DavidBottjer at the Universityof California tion hasreshaped organisms rn their laboratories, for example(page foundjust such a record,in the fossils of bryozoans,coral-like animals 170).But evolutionarybiologists also study eventsthat happened that anchorthemselves to the seafloor.Many deepwater bryozoan millionsor evenbillions of yearsago. In these cases, they must make specieswent extinct f irstduring mass die-offs, and then shallow ones predictionsabout the past.In Chapter4, we'lllook at how scientists disappearedlater on, just as predicted(Powers and Bottler2007).

The geologicalrecord of fossils,biomarkers, isotopes, and other tracesof past life is clear evidencethat life on Earth is immensely old. It also documents a profound transformation. For more than 1.5 billion years,the planet was inhabited solely by single-celledorganisms. Over the next 2 billion years, multicellular life-forms also emerged.To understandhow thesesorts of changesoccurred, scientists do not simply cataloglists of bonesand stromatolites.They also figure out how different species- either alive today or long extinct-are relatedto one another.By determining these relationships,scientists can form hypothesesabout the processesand patternsof evo- lution. How they discoverlife's kinship is the subjectof the next chapter.

KeyConcept Manyof themost diverse animal and plant species alive today belong to relativelyrecent radiations.

Geologistsuse the breakdown ofradioactive isotopes to estimatethe age of rocks. Organismsonly rarely become fossils. Biomarkers,such as molecules from cell walls, can be preserved for hundreds of millionsof years. Theratio of isotopes in fossils can give hints about the diets and ecology of extinct species. Theoldest proposed evidence of life is about 3.7 billion years old. Stromatolitesand other fossils of microbes date back about 3.5 billion years. Thethree main branches of Iife are Bacterla, Archaea, and Eukaryotes. r4rtBtccUtttnanelraatgtioJrosiJndetrt€'oaL,?e,J?[,eEJ6J'iJt,;no",'gec,tsug-. Biomarkersofanimals date back as far as 650 million years ago. TheEdiacaran fauna is a puzzlingcollection ofanimals that existed between about 575and 535 million years ago.

78 CHAPTERTHREE WHAT THE ROCKSSAY: HOW GEOLOGYAND PAI.EONTOLOGYREVEAL THE HISTORYOF LIFE