uring the revisionof Invertebrates my brother Gary passed away. For a while the project was stalled. But, buoyed by the support of family, friends, and colleagues, I eventually returned to the task, which, at times, seemed overwhelming. The field of invertebrate biology is so vast, and cuts across so n1any disciplinary lines, that even in a book of this size it is necessary to generalize about some topics and to slight others. As university instructors, my brother and Irealized early onthat the teaching of invertebrate zoology should not be compartmentalized. Thus, in planning this book we were concerned about two potentialdangers. First, the text might become an encyclopediclist of "facts" about one group after another, the sort of "flash-card" approach thatwe wanted to avoid. Second, the book might be a rambling series ofstoriesor vignettes about randomly selected animals (or "model organisms'') and their ways of life. The first book would be dull, would encourage rote memorization instead of understanding, and might give the misconception that there is little left todiscover. Thesecond bookmight befull of interesting "gee whiz" stuff but would seem disorganized and ,,vithout continuity or purpose to the serious student. Either approach could fail to present the most important aspectsofinvertebrates their phenomenal diversity, their natural history, and their evolutionary relationships. We also held to the belief that v.-hat we kno,v about these animals is not as important as how we think about them. You should be prepared to assimilate much new material, but you should also be prepared for a great deal of uncertainty and mystery, as much remainstobe discovered.
To avoid the pitfalls noted above, and to establish threads of continuity in our discussions about invertebrates, we developed our book around two fundamental themes: unity and diversity. The first theme we approach by way of functional body architecture,
or whatwe callthebauplanconcept.The second theme \Ve approach through the principles of phylogenetic biology. Our hope was thatweaving the book tightly to these themes would provide a 1neaningful flow as readers move fromone phylum to the next. The first fivechapters provide backgroundfor thesethemes and thus provide an importantfoundation upon which the restof the book rests. Please readthese chapters carefullyand refer back tothemthroughoutyourstudy.
The bulk of this bookis devoted to a phylum-byphylum discussion of invertebrates. Fairly detailed classificationsor taxonomic synopses for eachphylUJn are included in separate sections of each chapter to serveasreferences.A consistent organization ismaintainedthroughout each chapter, although �ve did yield tothe important and sometimesdifferent lessons to be learned by investigating the special attributes of each group of animals. Inaddition,becauseof their size and diversity, some taxa receive more attention than others although thisdoes not n,ean that such groups are more "i1nportant" biologically than smaller or more ho1nogeneous ones. (Five chapters are devoted to the arthropods and their kin.) In certain chapters more than one phylum is covered. In some cases the phyla coveredarethought to be closelyrelated toone another;in other cases the phyla merely represent a particular gradeof complexity and their inclusion in a single chapter facilitatesour comparative approach.
Certain aspects of this book have, of course, been influenced byourown biases; thisis especially true of the discussions onphylogeny. We use acombination of phylogenetic trees (cladograms) and narrative discussions to talk about animal evolution. Cladograms are used when appropriate, because theyprovidethe least ambiguous state1nents that can be made about animal relationships. We always knew that some of you, professors and students both, would disagree with our methods and ideas to various degrees at least we hopedthat youwould.Never placidly accept what you see in a textbook, or anyplace else for that matter, but try tobe critical in your reading.
Thebook's final chapter isa phylogenetic summary of the animal kingdon1.It reinforces the point that
GuestContributors
Jesus Benito, He1njchordata (with Fernando Pardos), Universidad Complutense, Madrid, Spain
C. Sarah Cohen, Urochordata, CaliforniaState University at San Francisco,California,USA
Gonzalo Giribet,Onychophora, Nen,ertea, Chelicerata (with Gustavo Hormiga), Annelida: Sipuncula, Metazoan Phylogeny (withRichard C. Brusca), Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts, USA
Rick Hochberg, Gastrotricha, University of Massachusetts Lowell, Lowell, Massachusetts, USA
Gustavo Hormiga, Chelicerata(with Gonzalo Giribet), The GeorgeWashingtonUniversity, Washington, DC, USA
Reinhardt M0bjerg Kristensen, Tardigrada (with Rjchard C. Brusca), Loricifera, Micrognathozoa (with Katrine Worsaae),NaturalHistory Museum of Denmark, University of Copenhagen, Copenhagen, Denmark
David Lindberg, Mollusca (with Winston Ponder andRichard C. Brusca), University of California, Berkeley,California, USA
Carsten Li.iter, Brachiopoda, Museum fi.ir Naturkunde, Berlin, Germany
Joel W. Martin, Crustacea (withRichard C. Brusca), Natural History Museum of Los Angeles County, Los Angeles,California, USA
Alessandro Minelli, Myriapoda, University of Padova, Padova, Italy
Rjch Mooi,Echinodern1ata, California Academy of Sciences,San Francisco, California,USA
Ricardo Cardoso Neves,Cycliophora, Biozentrurn, University of Basel, Basel, Switzerland
Claus Nielsen, Entoprocta, Bryozoa, Natural History Museun1 of Denmark, University of Copenhagen, Copenhagen, Denmark
Fernando Pardos, Hemichordata (vvithJesus Benito), Universidad Complutense, Madrid, Spain
Winston Ponder, Mollusca (with David Lindberg and Richard C. Brusca), Australian Museum, Sydney, Australia
GregRouse, Annelida: non-Sipuncula, Scripps Institution of Oceanography, University of California, SanDiego, California,USA
Scott Santagata, Phoronida, Long Island University, Greenvale, New York, USA
Andreas Schmidt-Rheasa, Nematomorpha, ZoologicalMuseum, University of Hamburg, Germany
George Shinn, Chaetognatha, Truman State University, Kirksville, Missouri, USA
Martin Vinther S0rensen, Kinorhyncha, Priapula, Gnathostomulida,Rotifera, Natural History Museum of Denmark, University of Copenhagen, Copenl1agen, Denmark
S. Patricia Stock, Ne1natoda, University of Arizona, Tucson, Arizona, USA
Katrine Worsaae, Micrognathozoa (vvithReinhardt M0bjerg Kristensen), University of Copenhagen, Denmark
Chapter reviewers for the Third Edition include the following:
Nicole Boury-Esnault,VlaamsInstituut voor de Zee, Oostende, Belgium
Jose Luis Carballo, Universidad Nacional Aut6noma de Mexico, Estacion Mazatlan, Mexico
Allen Collins, SmithsonianInstitution, Washington, D.C.
Alexander V. Ereskovsky, French National Center for Scientific Research, Institut Mediterraneen de Biodiversite et d'Ecologie Marine et Continentale (IMBE), Marseille, France.
Daphne G. Fautin, Professor Emerita, University of Kansas, Lawrence, USA
Gonzalo Giribet, Harvard University, Ca,nbridge, Massachusetts, USA
Gordon Hendler, Natural History Museumof Los Angeles County, Los Angeles,USA
Jens H0eg, University of Copenhagen, Denmark
Matthew Hooge, University of Maine, Orono, USA
Michael N. Horst, Mercer University, Georgia, USA
Michelle Kelly, NIWA (National Institute of Water and Atmospheric Research), New Zealand
KevinKocot, University of Alabama, Tuscaloosa, USA
Reinhardt Kristensen, University of Copenhagen, Denmark
Christopher Laumer,Harvard University, Cambridge, Massachusetts, USA
Brian Leander, University ofBritish Columbia, Vancouver, Canada
Sally Leys, University of Alberta, Edmonton, Canada
Renata Manconi,Universita degli Studi di Sassari (UNlSS), Italy
Mark Q. Martindale, Director, Whitney Laboratory and Seahorse Key Marine Laboratory, and Professor ofBiology, University of Florida, Gainesville, USA
Rick McCourt, Academy of Natural Sciences, Philadelphia,USA
Catherine S. McFadden, Harvey Mudd College, Clare,nont, California, USA
Claus Nielsen, University of Copenhagen, Denmark
David Pawson, National Museum of Natural History, Smithsonian Institution, Washington, DC, USA
Hilke Ruhberg, University ofHamburg, Germany
Rebecca Rundell, State University of New York-ESF, Syracuse, New York,USA
Jean Vacelet, Universite de la Mediterranee AixMarseille, France
R. W. M. van Soest, University of Amsterdam, The Netherlands
Media and Supplements to accompany Invertebrates, Third Edition
eBook
Invertebrates,ThjrdEditionis availablefor purchase asaneBook,inseveraldifferentfonnats,includingVitalSource,Yuzu,BryteWave,andRedShelf.TheeBook can be purchased aseither a180-day rental or a permanent(non-expiring)subscription. Allmajormobile devices are supported.For details on the eBook platformsoffered, please visitwv.,w.sinauer.com/ebooks.
FortheInstructor
(Availableto qualified adopters)
Instructor's Resource Library
TheInvertebrates,ThirdEditionInstructor'sResource Libraryincludes an extensive collection of visual resourcesforuseinpreparinglecturesandothercourse materials.The IRLincludesthefollowing:
TextbookFiguresandTables:Allofthetextbook'sfigures andtablesare includedasbothhjgh-andlow-resolutionJPEGs,foreasy useinpresentationsoftware, learning management systen,s, and assessments. New for theThudEdition, trus nowincludesall of thetextbook'sphotographs.
Supplen1entalPhotoCollection: Expandedfor theThird Edition,thjs collection of over 1,000 photographs depicts organisms that span the entire range of phylacoveredin the textbook.
PowerPointPresentations:Twoready-to-usePowerPoint presentationsareprovidedfor eachchapter of the textbook: one that containsall ofthetextbook figures andtables,andonethatcontainsalloftherelevantphotosfromthesupplementalphotocollection.
TABLE1.1 Numbers ofDescribed Living Species inthe 32 Animal syn1biotic a-proteobacterium, Phyla (-1,382,402total; -1,324,402, or96% are and then subsequently diversiinvertebrates)8 fiedbroadly. Modernfree-Jiving relativesof thisbacterium harbor
Number of Percent ofTota about 2,000 genesacross several Described DescribedAnimal Taxon Species Species millionbases,buttheir1nitochond.rialdescendantshavefarfewer, PhylumPorifera
9000 0.65 sometin1esasfewasthree genes.
Phy umPlacozoa 1(or2) 0.0001 Andhumanmitochondrial DNA
PhylumCnidaria
13,200 0.95 harborsonlyabout 16,000bases. On the other hand, so1ne plants Phy umCtenophora
PhylumXenacoelomorpha
Phy umPlatyhelminthes
100 0.007 have greatly expanded theirmi-
400 0.03 tochondrial genome, the largest
26,500 1.92 so far discovered in the genus Silene, with around 11 million PhylumChaetognatha 130 0.009 bases. Another prokaryotic intraPhyumGastrotricha
800 0.06 cellularguest, acyanobacterium,
PhylumRhombozoa (=Dicye,nida) 70 0.005 became the ancestor of chloroplaststhroughthesan1esymbioPhy umOrthonectida 21 0.002 genie process; chloroplasts, of
PhylumNemertea 1300 0.09 course, are the photosynthesiz-
PhylumMollusca
5.79 ing organelles that made plants and algaepossible.Insomepla11t
PhylumAnnelida 20,000 1.45 and algae Jilles,the origillal chloPhy umEntoprocta 200 0.014 roplastwaslost,and a new one
Phy umCycliophora 2 0.0001 waspicked up vvhen a host cell took in an alga and co-opted its
PhylumGnathostomulida 100 0.007 chloroplast in another kind of
PhylumMicrognathozoa 1 0.0001 symbiogenicevent(seeChapter3 for a detaileddiscussion of these PhylumRotifera
bio1narkerssuggest that thefirst eukaryoticcellsmight have ap- Phy umBrachiopoda
0.03 peared as early as 2.7 billion
Phy umNematoda(=Nemata)
years ago (late Archean), alPhy umNematomorpha
0.03 though the earliest fossils that have been proposed to be euPhylumKinorhyncha
PhylumPriapula
PhylumLoricifera
PhylumTardigrada
PhylumOnychophora
0.014 karyotes-based on cell surface
0.001 features and theirlargesize-are
0.003 about 1.8 billion years old (early Proterozoic). Multicelled algae
0.087 (protists) date as far back as 1.2
0.014 billion years. Eukaryote-likemicrofossils have been described
PhylumArthropoda from 1-billion-year-oldfreshwa-
SubphylumCrustacea
ter deposits, suggesting thatthe eukaryotes might have left the PhylumArthropoda sea and invaded the terrestrial SubphylumHexapoda
realm longago. Even though the PhylumArthropoda eukaryotic condition appeared earlyin Earth'shistory, it took a SubphylumChelicerata
fewhundredmillion more years
PhylumArthropoda for multicellular organisms to
Subphy umMyriapoda
PhylumArthropoda
firstevolve.
Disputed trace fossilshaveled
some to suggest that the earliest
TABLE 1.1 (Continued)
PhylumEchinodennata
PhylumHemichordata
PhyumChordata
SubphylumCephalochordata
Phylw11Chordata
SubphylumUrochordata
PhylumChordata
SubphylumVertebrata
PhylumChordataTOTAL
There is no argun1ent that
Described Animal Metazoa are n1onophyletic (i.e., a clade),andtheanimal kingdom is defined by numerous synapomorphies,including: gastrulation and en1bryonicgern1layer formation; unique modes of oogenesis and spermatogenesis; a unique sperm structure; mitochondrial gene reduction; epidermal epithelia withseptatejw1ctions, tight junctions, or zonula adherens; striate myofibrils; actin-myosin
315,000;andFungi=100000. lagen; andthepresenceof a basal lanli.na/basement membrane beneathepidermallayers(of course, some of these features have been secondarily lost in son1e groups).
animals (Metazoa)mighthaveoriginated a billion years ago, but recent n1oleculai· clock estimatesput the originofMetazoa at875 to650 million yearsago.Theoldest generally acceptedmetazoan fossils are from the Ediacaran period, found in the Fermeuse Formation of Nev-•foundland (-560 Ma) and the Doushantuo Formation of southernChina(600-580 Ma). A560-million-year-old likely cnidarian (namedHaootinq11adrifonnis) has been described from Newfoundland, with quadraradialsymmetry and clearly preservedbundled n1uscular fibers.Haootin appears to be a nearly 6-cmlong polyp, or perhaps an attached medusa-it re sembles modern speciesofStaurozoa.Cnidariansand otherapparent diploblastic animals have beenreported from theDoushantuodeposits, although these have been met with skepticistn in some quarters. However, in2015,aseemingly reliable600-million-year-oldfossil sponge(Eoci;nthispongiaqiania) was describedfrom the Doushantuo Formation.In 2009, Jun-Yuan Chen and colleagues reported on embryos ofreputed bilaterians (triploblasts) in the Doushantuodeposits (dated600580 Ma)-32-cellstage embryoswith micromeresand macromeres, apparent anterior-posterior and dorsoventra1patterning, and ectoderm-likecellsaroundpart of their periphery. This finding was challenged, and the fossils v-•ere variously declared as prokaryotesor protists byother workers. However, furtherdiscoveriesof additional embryos seemed to support the vie,-v of these being bilaterian embryos and, in some cases, perhaps diapause embryos ("restingeggs") of bilaterians. Good trace fossils (tracks) of aminute wormlike bilaterian animal, possibly with legs, have also been describedfrom585-million-year-old rocksinUruguay. These fossil records put the appearance of "higher metazoans" (i.e.,bilaterians) millions of years before the beginningoftheCambrian period.
Evidenceis strong that Metazoa arose out of the protist group Choanoflagellata, or a common ancestor, and the two comprisesister groups in aln1ost all recent analyses. They are,in turn, part of alarger clade known as Opisthokonta that also includes the fungi and several smallprotist groups(seeChapter3).
The three great lineages of life on Earth-Bacteria, Archaea, Eukaryota are very different from one another.Bacteria and Archaea havetheirDNAdispersed throughout thecell, whereas in Eukaryotathe DNA is enclosed within a membrane-bound nucleus. The cell lineages that gave rise to the Eukaryota are still wlknown.Themany millennia betweenthe origin of Eukaryota and theexplosiveradiation that apparently began in theEdiacaranissometimescalled the "boring billion years," but the fossil record is fairly sparse for that time period,so we'renot surehow "boring" it actuallywas. A popular hypothesissuggests that oxygen levelswere too low duringthat time for larger organismstoevolve (seebelow).
Today, thereare an estimated 2,007,702 described and named living species. About 58,000 of these are vertebratesand 1,324,402 are invertebrates(Table 1.1). Inaddition, about 200,000protists have beendescribed, 315,000 plants (290,000 seedplants),and 100,000 fungi. And 15,000 to 20,000 new species are described every year.It seemslikely thata significantportion of Earth's biodiversity, at the levelof both genes and species, resides inthe "invisible"prokaryotic world, and we have co1ne to realize howlittle weknow about thishidden world.About10,300species of prokaryotes have been described,but there areanestimated10 million (or perhaps more) undescribed prokaryote specieson Earth. An esti.n1ated 135,000 more plantspecies remain to be described. Overall, estimatesof undescribed eukaryotes range fromlows of 3-8million tohighsof 100 millionor more.
(or "sea slugs") also refers to agroup ofanimalsthat do not share a single ancestry (the slug forn1 has evolved many times among gastropod1nolluscs), so slugs are polyphyletic (see chapter opener photo of aSpanish shawl slug, Flnbellinniodinen). We explore these concepts more fully inChapter2.
We know approximately how many genes are in organisms from yeast (about 6,000 genes) to humans (about 25,000 genes), but we don't knov.• how many living species inhabit ourplanet and the range in estimates is surprisingly broad. How many undescribed species are lingering out there, waiting for names?However derived, predictions of global species diversity rely on extrapolations from existing realdata. Methods of estimation have included rates ofpast species descriptions, expert opinion, the fraction of undescribed species in samples collected, and ratios between taxa in the taxonomic hierarchy. Each method has its limitations. Two recent estimates of undescribed marine animals (Mora et al. 2011 and Appeltans et al. 2012) reached the conclusion that 91% vs. 33-67% (respectively) of the world's eukaryotic marine fauna is stilJ undescribed. More recently, a large research program sampling the Western Australian upper continentalslope for Crustacea and Polychaeta found 95%of the species to beundescribed (with the rate of new species obtained by the sampling program not even leveling off). Given the vast extent of the poorly-sampled world's continental slopes, not to mention the deep sea, rainforests, and other littlesampled habitats, these data suggest that estimates of over 90% undescribed eukaryotes on Earth are not unreasonable.
Our great uncertainty about how many species of living organismsexist onEarthis unsettling andspeaks to the issue of priorities and funding in biology. At our current rate of species descriptions, it might take us 10,000 years ormore to describejustthe rest of Earth's eukaryotic life forms. Notall of the species remaining to be described are invertebrates between 1990 and 2002 alone, 38 new primate species were discovered and named. And if prokaryotes are thrown into this mix, thenumbers become even larger. Recent gene-sequencesurveys of the world'soceans (based largelyon DNA "barcodes"-16S ribosomal gene sequences for Bacteria and Archaea, 18S ribosomal gene sequences foreukaryotes) have revealed a massive undescribed biota of microbes in the sea. Similar discoveries have been made withgenetic searchesfor soil microbes. For example, there areabout30,000 forn1ally namedbacterial varietiesthatare in pure culture, but estimates of undescribed species range from 10 million to a billion ormore! Andv.•enow know thatthousands of bacterial species inhabit the human body, almost all of which arenot yet even namedanddescribed.Viruses still lack auniversal molecularidentifier, and theworldscopeof viral biodiversity is essentiallyunknown.
There are currently several atte1npts to compile a List of all known species on Earth. The United States Geological Service (USGS) hosts ITIS-the Integrated Taxonomic Information System. The goal of!TIS is to create an easily accessible database ,.vith reliable inforn1ation on species nan1es and their classification. Recently, ITIS and several other initiativesturnedtheir data over to the Catalogue of Life(Col) project, which is building the species list (up to 1.5 million species as this book went to press) and maintaining a "consensus classification" of all life (see www.catalogueof life. org, andRuggiero et al.2015).TheEncyclopediaof Life (EOL) project is building a website that offers not just species names, but also ecological information about each species; it currently contains more than 175,000 vetted species pages. WoRMS(The World Register of Marine Species), an open-access online database \Vith the goal of listing all described eukaryotic marine species, predicts the con1pleted inventory will catalog 222,000 to230,000 species.
However, at our current rate of anthropogenically driven extinction a majority of Earth's species will go extinct before they are ever described. In the Unjted States alone, at least 5,000 nan1ed species are threatened with extinction, and an estimated 500 known species have already gone extinct since people first arrived in North America. Globally, the United Nations Environn1ent Programme estimates that by 2030 nearly25% of the world's mammalscould go extinct, and recentcountsindicate 322 vertebrate species have already become extinct since 1500. Some v.,orkers no,,v refer tothe time since the start of the Industrial Revolution as theAnthropocene--a period n1arked by humanity'sprofoundglobal transformation of the environment. Morethan halfof Earth'sterrestrial surface is now plowed, pastured, fertilized, irrigated, drained, bulldozed, compacted, eroded, reconstructed, mined, logged, or otherwise converted to new uses. Humandriven deforestation removes15 billiontrees per year. E. 0. Wilson once estimated that about 25,000 species are goiJ1g extinctannually on Earth (v.re just don't know what theyare!).
Pimm et al. (2014) calculated extinction rates as fractions of species going extinct over time-extinctions pernilllion species-years(E/MSY). For exan1ple, 1,230 birdspecies have beendescribed since1900, and 13 of these are nowextinct. This cohort accun1ulated 98,334species-years, meaning an average species has been known for 80 years. The extinction rate is thus 13/98,334 x 106 = 132 E/MSY. They calculated that, beforeHo1nosapiens arrived on the scene, the overall background animal extinction rate was 0.1 E/MSY; today itis aboutJOOE/MSY(a thousand times higher). ProportionalJy speaking, larger animals (e.g., vertebrates), higher in the foodchain andfev.•er in number, are more likely to go extinct than invertebrates. This means invertebrates(and protists),which already run
Even thoughinvertebratesmake up96% of the described animal kingdom(Table1.1),they account for amere 38%ofthe500orsospecies now under protection bytheU.S.EndangeredSpeciesAct.NatureServe has argued that more than 1,800 invertebrate species need protection, ,,vhile the IUCN Red List of Threatened Species documents the extinction risk of nearly50,000species of animals and plants. In2002, theU.N.Conventionon BiologicalDiversitycommittednationstosignificantlyreduceratesofbiodiversity loss by 2010, and in2010this call wasrenewed•,,vith aset of specific targetsfor2020.However,several recentstudieshaveshown thattheconventionhassofar failed and rates ofbiodiversity loss donot appear to beslowingatall.Further,the Conventionhascrippled scientific fieldwork around the1vorldby instilJingin many nations a fear ofoutside researchers"stealing their geneticbiodiversity."
Thesinglegreatestthreattospecies survivalforthe past200yearshas been habitatloss.Although wehear mostly about deforestation,30 to50% of theEarth's coastalenvirorunentshave beendegradedduringpast decades,atratesexceedingthoseoftropicalforestloss. Looking forward,thedamaging effectsof habitatloss will likely be matched (and escalated) byanthropogenically-drivenglobal clin1atechange.Theconcentrationofcarbondioxide(CO2) inEarth'satmospherehas risenbyabout 38%sincethe startoftheindustrialera as a resultof fossil fuel burningandlandusechange, and aquarterto athirdofallthe CO2e1nittedthrough human activitieshas been absorbed by theocean, resulting in acidification ofsurface waters. In fact, the oceans overall are novv about30%more acidic than theywere100yearsago. ThedropinoceanpHiscreatinghardshipson animalswithcalciumcarbonateskeletons,anddamagehasbeendocumentedineverything from coralsto sea butterflies (pteropodmolluscs). In May2013,theconcentration of CO2intheatn1osphere reached400ppm,the highesti.thas beenover thepast 2tnillionyears.Neverbefore hassucharapidglobalscaleincreaseinCO2(andtemperature)occurredduring thehistory ofhumancivilization. Rising concentrations of greenhouse gassesin the atmosphere are leadingtoincreasingglobaltemperaturesandchanges inprecipitationregimes,andthesechangesarein1pactingthedistribution ofbiota acrosstheplanet.Globally, averageairtemperatureshaverisen about0.8°Csince 1880, mean land surface temperature has warmed 0.27°Cper decade since 1979,and projections from globalclin1atemodelspredict globalatmospherictemperaturesto increase by about4°C by theend of this century.Meltingpolariceand glaciers,combined1-vith expansion of •,,vanning ocean vvaters, are driving up sea level,whichisexpectedtobeasmuchasfourfeet higherbytheendof thiscentury.
Prokaryotes and Eukaryotes
The discoverythatorganisms witha cellnucleusconstituteanatural(monophyletic)groupdividedtheliving world neatly into twocategories,the prokaryotes (Archaea and Bacteria:thoseorganismslackingmembrane-enclosed organelles and anucleus,andwithout linearchromosomes),andthe eukaryotes(thoseorganisn1s that dopossess men1brane-bound organelles, a nucleus,and linearchromosomes). Investigations by CarlWoeseand others,beginningin the 1970s,ledto thediscovery thattheprokaryotesthemselvescomprise two distinctgroups,calledBacteria(=Eubacteria)and Archaea (= Archaebacteria), both quite distinctfrom eukaryotes(BoxlA).Bacteria correspondmoreorless toour traditional understanding of bacteria. Archaea strongly resemble Bacteria,butthey have genetic and n1etaboliccharacteristics thatmakethem quiteunique. For exan1ple, Archaea differfrom bothBacteria and Eukaryotain thecomposition of their ribosomes, in theconstructionof their cell walls, and in the kindsof lipids in their cellmembranes.Some Bacteriaconduct chlorophylJ-based photosynthesis,atrait that isnever present in Archaea (photosynthesis is the harvesting of lightto produce energy/sugars and oxygen). Current thinkingfavors the viev., thatprokaryotesruled Earth forabout a billion years before the eukaryotic cell appeared.
As the prokaryotes evolved,they adapted tocolonize everyconceivableenvironmentonEarth.During the early evolution of prokaryotes, Earth's airhad almostnooxygen,consistingprimarilyofCO2,1nethane, and nitrogen. The n1etabolismof the earliest prokaryotesreliedon hydrogen,methane,andsulfur,anddid notproduceoxygen asabyproduct.ltwastheappearance ofthe first oceanicphotosynthesizingprokaryotes thatled to increased atmosphericoxygenconcentrations,settingthe stagefor theevolutionofcomplex multicellular life.And aquatic species v.,ereprobably ableto colonizelandonly because the oxygen helped create theozonelayerthat shieldsagainstthesun's ultravioletradiation.Just whenoxygen-producingphotosynthesis beganisstillbeingdebated,but whenit happened,mostoftheearlychemoautotrophicprokaryotes werelikelypoisonedbythe "newgas"intheenvironment. Alargebody ofevidencepointstoasharprisein theconcentration of atmosphericoxygenbetween2.45 and2.32billionyearsago(thisissometimescalled the "greatoxidation event"),aroundthesametimetheeukaryoticcellfirstappeared.Thisevidenceincludesred bedsor layerstingedbyoxidizediron(i.e.,rust)andoil biomarkersthat may be there1nainsofCyanobacteria (true Bacteria). However, in western Australia, thick shaledepositsthat are3.2billionyearsoldhavebacterial remains thathintatoxygen-producingphotosynthesis.Theseancientoxygenlevelsmighthavereamed around40% of present atmospheric levels. There is
