compelling. Researchershave found fossils in 2.6-billion-yearo1d rocks,for example,that bear a striking resemblanceto cyanobac- teria,the lineageof bacteriathat carriesout photosynthesis.That's exactlywhen the first evidenceof atmosphericoxygen appearsin the fossil record; between 2.45 and 2.32 billion years ago,oxygen increaseddramatically. The rise in oxygen was likely the result of the emergenceof cyanobacteria,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 Euryarchaeota. 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 eukaryotes. 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 fish or lobstersor coral 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 bacteria 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 animals and plants, 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 ani- eukaryote,istypicallyunicellular. nral or plant-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 human 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 eggs-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 life 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 lives 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 multicellular organism (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 archaea (Figure3.1a). The oldest recognizable multicellular eukary- otes-filaments of some type of algae-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-year-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 Kingdom 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 mammals and birds, but less of clustersof cellsdate back about 580 familiar onesas well, like sponges.Sponges may not seemmuch like otheranimals- millionyears. Based on studiesof living they lack a brain,eyes, 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 animal embryos. 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 sponge-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 Cambrian 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 THE DAWNoF THEANTMAL KtNGDoM 69 Figure3"18 Between about 575 and 535million years ago, a bizarrecommu nityof multicellularorganisms l<nown asthe Ediacaran fauna dominated the world'soceans. Some of themgave riseto livinglineages of animals,while manyothers had become entirely extinctby 535 million years ago. .e. "s, rll) , ,'.;i: *d;i i{-!#: # :rw, :;lr 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