-12
-10
-8
-6
_A
-2
Even the individual atoms in rocks can _ 1 4
rffer scientists clues about ancient life. As
3 mya
'.\'esaw in Chapter 1, the oxygen atoms in the
',\'aterconsumedby whales end up in their
Browsers
tceth.The ratio of different oxygen isotopesin
A. africanus
Grazers
-{-ivhale fossil teeth indicateswhether they con.umed freshwater,seawater,or water with an
2.F2.6 mya
intermediatelevel of salinitv (seeBox 3.1 for
nloreon isotopes).
Browsers
--*A. africanus
Isotopes not only offer clues to the habi
Grazers
--*-tats in which organismslived, they also provide clues to their metabolism. Plants, for
1.7mya
example,obtain their carbon from the atmo
sphere,incorporating a mixture of carbon-l2
Browsers
A. robustus
and carbon13 isotopes into their biomass.
{-*-Grazers
Becausecarbon-13is heavierthan carbon-l2,
it's more difficult for the plants to absorb it.
fi, cryorrc,
{As a result,the ratio of carbon-l3to carbon-12 r t n l t t l t
n
rllttlll
-12
-10
-8
-4-202
is lower in plantsthan it is in the atmosphere. -14
b'3c^^Different plants have slightly different
ratios of carbon isotopes,dependingon how
Figure3.1O Usingcarbonisotopesexthey carry out photosynthesis.Most plant speciescarry out C3 photosynthesis,so
tractedfrom the enameloffossilteeth,
named becauseit incorporates carbon dioxide into a molecule with three carbon
andMatt Sponheimer
JuliaLee-Thorp
atoms. Grassesand certain other plants carry out Ca photosynthesis,in which the
of the University
of CapeTownhave
carbon dioxide is incorporatedinto four-carbonmolecules.Chemistshave found that
comparedthe dietsof earlyhominin
Caplants have lower levels of carbon 13 than C3plants. Analysis of plant fossils has
fossilsto thoseof browsingmammals
revealedthis samedifferencebetween extinct C: and C+plants.
t h a tf e do n s h r u b sa n dg r a z i n g
mamThe ratio of carbon isotopesin animals reflectsthe kinds of animals and plants
malsthat ate grasssampledfrom the
that they eat. Cows and horsesthat graze on Cagrasseshave a lower carbon 73fcar
samegeological
time periods.
Across
bon-i2 ratio than giraffesor elephants,which browse on the leavesof C3plants.This
periods(3 million
threesuccessive
difference is preservedin their fossils. |ulia LeeThorp and Matt Sponheimer have
yearsago,2.4-2.6millionyearsago,
taken advantageof this finding to use carbon isotopesto get clues to the diet of our
a n d1 . 7m i l l i o ny e a r sa g o ) h, o m i n i n
hominin forerunners.(Homininsare a group that includeshumansand speciesmore
e n a m esl h o w e dc h e m i c asli g n a t u r eosf
closely related to humans than to other apes.For more details, see Chapter 4 and
a m i x e dd i e tt h a t i n c l u d e a
dsurprising
Chapter15.)
(i.e.,isotopevalues
amountof grasses
Lee-Thorpand Sponheimermeasuredthe carbon isotopesin tooth enamel from
offsetfrom that of purebrowsers
and
hominin fossilsdating back to three periods: 3 million years ago,2.4-2.6 million
morein the directionof grazers).
Our
years ago,and 1.7million years ago.For comparison,they measuredcarbon isotopes
homininancestors
appearto havebeen
from the teeth of C3browsersand C+grazersfrom the same sitesand from the same
associated
with grasslands
wherethey
time periods. They found that hominins fell between the two. They ate a diet that
ate grasses
or, morelikely,ate animals
included substantialamounts of carbon from C+plants,which they could have gotten
that fed on grasses.
Suchstudies
from eating grassesor from eating animals that grazedon grasses(Lee-Thorp,Spon
provide
v a l u a b lgel i m p s eisn t ot h e
heimer, and Van Der Merwe 2003).
+-
KeyConcepts
(Modibehaviorof our earlyancestors.
fied from Lee-Thorp
et al. 2003.)
Technology
allows
scientists
to gainnewinsights
intothebehavior
andappearance
of extinctspecies
fromtheirfossils.
lsotopes
and biomarker
molecules
aboutthe historyof life.
carryinformation
3.3 Life'sEarliestMarks
Scientistsuse all the methods we've just explored to extract information from the
fossil record.We'll now use the evidencethey'vegatheredto take a tour of the history
of life on Earth.
After the Earth formed from the primordial solar disk 4.568 billion years ago,
it cooledover millions of years.Its rnolten crust hardened,and lighter formations
s A R L T EM
sT
ARKs
3 , 3 L r F E 'E
61
R a d i o a cti ve
C l o cks
When physicists discoveredthe structure of atoms in the early
1900s,theyalsomadeit possiblefor geologists
to preciselymeasure
the agesof rocks.Allatomsaremadeof threefundamentalparticles:
protons,neutrons,and electrons.
The numberof protonsin an atom
determineswhichelementit belongsto, but the numberof neutrons
can vary. For example,all carbon atoms have six protons.While
of all carbonatomson Earthhavesix neutrons,I.07o/o
98.930/o
have
seven,and one in a trillioncarbonatoms haveeight.Theseso-called
isotopesof carbon are named,respectively,
carbon-l2,carbon-13,
and carbon-14.(lsotopesareoften representedwith the total number
of neutronsand protonsas a superscriptprecedingiheir elemental
symbol,such as raC.)In some arrangements,
the protonsand neutronsin an atom are perfectlystable,but in otherarrangements
they
wrllsooneror laterfallapart.
lf the decayof an atom leadsto the lossor gainof protons,it becomesa drfferentelement.Uranium-238,
for example,breaksdown
by releasing
a pairof neutronsand a pairof protons,therebyturning
jntothorium-234.fhorium-234is alsounstable,
and in time it decays
into protactinium-234,
which in turn decaysagain.lt takesa chain
of 13 intermediatesfor uranium-238to settle into a stableform:
lead-206.
Eachradioactiveisotopehas a distinctivedecayrate,knownas
its half-life.
The half-lifeof uranium-238is 4.47 billionyears,which
meansthat in this periodof trme,half of a givenquantityof uranium-238will decayinto lead-206.Eachyear,the probability
that any
givenatom of uranium-238wil\decayis 1.55125x 10 10,(The number of atoms,N, left remainingfrom the originalsupplyNo,can be
of
calculatedfrom the equationN: Noe-rt,where\ is the probability
an atom decayingin a giventime interval.also knownas the decay
constantILanphere2001]).
When the Earthformed 4.568 billionyearsago,it incorporated
atoms presentin the solarsystem'sprimordialdust cloud.A small
fractionof those atoms were radioactive.
The originalradioactive
atoms havebeengraduallydecayinginto stableisotopeseversince.
Whenfreshlavacoolsand forms new igneousrocks,traceamounts
of radioactive
atomsget lockedintotheir minerals.lmprisonedin the
rocks,the atoms continueto decay.The olderthe rock gets,the tewer
originalradioactive
atomsremain(Dalrymple1991).
Radiometricdatingis a powerfulmethod,but it is not simple.To
seewhy,imaginetryingto date a rock by measuringthe proportions
of rubidium-87and its decay product,strontium-87(lts half-lifeis
4B.Bbillionyears.)Let'ssaythat the rockcontainsequalamountsof
Youmightconcludethat halfof the rubidium-87presboth isotopes.
ent whenthe rockformedhasdecayedintostrontium-87Thatwould
meanthe rockis 48 billionyearsold-older thanthe universeitself.
Thisabsurdresultis dueto a mistakenassumptionthat the strontium-87in the rock was all derivedfrom rubidium-87In fact.a rock
may incorporatestrontium-87when it forms.Afterward,the subsequentdecayof rubidium-87in the rockmay add onlya tiny additional
amountof strontium-BZ
We cannotknow how much strontium-87was orrginallypresent
in a rock,becausewe weren'ttherewhen it formed.Fortunately,
the
rock itselfprovidesa way to get aroundthis problem-by measuring
the ratiosof differentisotopesin differentsamplesin a rock.
A
Theslopeof the isochron
increases
with time(t).
Rock
+"4"
with
Thoseminerals
moreinitialRbwould
havemoredecays
and
the ratioof
change
87Sr
to 865rmore.
e272 keY
Half-life
yrs
4.88x 1010
All minerals
wouldhavethe
to 865r
at
sameratioof 87Sr
sincetheyare
crystallization
\dentica\.
chemica\\y
eachhaving
Box Figure3.1.1A: A rockcontainsvariousminerals,
regardless
ofthe proportionof rubidiumto strontium.Thevalues
traceamountsof rubidium(Rb)andstrontium(Sr).B: Rubidium-87
Aftera rockcrystallizes,
this radioactive
decaysto strontium-87.
decayincreasesits strontium-87.C: Whena rock first forms,the ratio
is the samethroughoutthe rock,
of strontium-86
to strontium-87
As the stronform a straighthorizontalline,calledan isochron.
the slopeofthe
tium-87decays,
the ratioschange,increasing
isochron.(Adaptedfrom Lanphere2001.)
62
w H A T T H ER o c K s s A y : H o w c E o L o G y a N D p a L E o N T o L o G yR E v E A LT H E H t s r o R Y o F L I F E
c H A p r E RT H R E E
There are two factors that make this possible.One factor is that
the strontium in a rock may be either strontium-87producedfrom
rubidium-82or strontium-86.The two strontiumisotopeswill be
mixedtogetheruniformlythroughthe rock,becausetheir chemistry is the same.Oncethe rock has formed,the amountof the stable
strontium-86will not change.But the strontium-87will increaseas
the rubidium-87breaksdown.
Whilethe ratioof strontium-86and strontium-87will be uniform
throughthe rock,the ratio of strontiumto rubidiumwill not. That's
becausedifferent mineralsin a rock will have differentprooortions
of the elements.Somemineralgrainswill havea lot of rubidiumand
a littlestrontium,and some will havea lot of strontiumand a little
rubidium.
Assumingthat no additionalstrontiumentersthe rock,the only
newsourcefor the elementis the decayof the rock'ssupplyof rubidium-87As the supplyof strontium-87goesup in eachmineralgrain,
so does the relativeproportionof strontium-87to strontium-86,
sincethere'sno sourceof additionalstrontium-86.Meanwhile,
the
proportionof rubidium-87to strontium-87goesdown.In a mineral
that startedout with only a Iittlerubidium,the proportionsof these
two changeswillbe relatively
small.In a mrneralthatstartedout with
a lol o{ rubidium.the changeswillbe bigger.
The prooortionof strontium-87to strontium-86after a time intervalt can be exoressed
as
87Srt/86Srt
: 87510/86510
+ 87Rbt/86srr
[erf-l
To date the age of a rock,geochronologists
sampleseveraldifferent mineralsfrom it and then plot the proportionsof the isotopesin
each mineral.As you can see in Box Figure3.1.1,
these proportions
form a straightline,calledan isochron.Whena rock first forms,the
proportionof the two isotopesof strontiumare identicalin all the
minerals.As a result,the line is horizontal-thatis, it has a slopeof
zero.Overtime, as the strontium-87increasesin each mineral,the
slopeof the Iineincreases.
The slopeof the isochronat time t is ert-1.
Thus,by knowingthe decayconstantand the valueof the slope,scientistscan calculatethe ageof the rock.
This is just one of many differentkinds of radiometricdating
geochronologistscan carry out. They can measuretime at different
scalesby measuringthe decayof differentelements.Rubidium-87's
long half-lifemakes it good for measuringvery old rocks. Potassium-40,by contrast,takesonly1.25billionyearsto breakdowninto
argon-3S.As a result,it providesa more accurateclock for daiingthe
age of youngerrocks.In some cases,scientistscan measuretwo different isotopesin the same rock. The fact that they can derivethe
sameagewith independent
methodsconfirmsthat radiometricdating is a validwayto measurethe ageof rocks.
The radiometricdatingof rocksallowsscientiststo estimatethe
agesof fossils.lf a fossiljs sandwrched
betweenlayersof volcanicash
rich in potassium-4O,
for example,the layerscan createupperand
lowerboundsfor the age of the fossilitself.In 1962 scientistsdiscoveredfossilsof humansat a site nearthe villageof Omo,Ethiopia.
The
scientistsknewthe fossilswereold,but it was hardto determinejust
how old they were.Almostthree decadeslater,a team of scientists
went backto the site to take a closerlook.They discoveredtwo layers
of volcanicash,oneabovethe rockswherethe fossilshad beenfound
and anotherrisht belowthem.
(continued)
Volcanic
eruptionlayer;
ashdatedto 104,000
yearsago
Fossil
discovered
close
to the lowereruption
rayer
Volcanic
eruptionlayer;
ashdatedto 196,000
yearsago
Box Figure 3.1.2 Paleontologists
useas manylinesof evidenceas
possibleto estimatethe ageof fossils.By calculatingthe agesof layersof volcanicashaboveand belowa fossil,they can establishupper
and lowerboundsfor whenit formed.In Ethiopia,
the oldestfossils
of Homosapiens
aresandwichedbetweentwo layersof volcanicash.
Usingargon-potassium
datingto estimatethe ageof the layers,the
scientistsdeterminedthat the fossilswereabout195.000vearsold.
s TA R K s
3 . 3 L r F E ' sE A R L T E M
63
Radioactive
Clocks{continued)
The argon in the upper layeryieldedan age of 104,000years,
with a marginof error of 70OOyears.The lowerlayerwas 196,000
yearsold, with a marginof error of 2OOOyears.A carefulstudy of
the sedimentsbetweenthose two layersindicatedthat the fossils
werecloserto the olderboundarythanto the youngerone-perhaps
as oid as 195,000years.Thanksto this research,the Omo fossils
becamethe oldest known fossilsof membersof our own species
( M c D o u g ael lt a l .2 O O 5 ) .
E l e m e n t s u c h a s u r a n i u ma n d a r g o nc a n h e l pg e o l o g i s tess t i mate the agesof rocksonly.But carbon-l4,which has a half-lifeof
5730 years, can allow scientiststo determinethe ages of fossils
themselves.Unlike other radioactiveisotopes,new carbon-l4 is
continuallygeneratedon Earth.Chargedparticlesfrom spacetravel
throughthe atmosphere,
collidingwith nitrogen-14
atoms and turning them into carbon-l4.As plantstake up carbondioxidefrom the
atmosphere.thev accumulatecarbon-14in their tissues.Animals
Figure3.11Tinyspecksof carboncan
be preserved
for billionsofyearsin
mineralsknownaszircons.
Thebalance
of carbonisotopescanprovideclues
to what lifewaslikewhenthey were
trappedin the mineral.
64
also build up a supplyof carbon-I4when they eat plants,or when
theyeatotheranimalsthat haveeatenplants.At everymeal,you pick
too.
up a littleextracarbon-14
Once an organrsmdres,the carbon-l4 in its remainssteadily
breaksdownfor thousandsof years.Scientistscan use radiocarbon
datingto estimatethe age of biologicalmaterialthat's up to about
geologrsts
40 000 yearsold. By measuringcarbon-14,
can date not
just bones,but any materialwith some organiccarbon in it. They
can date the age of woodentools,or the age of ash from a fire. In
1994,cave explorersin Francediscoveredhiddenchambersfilled
with beautifulpaintingsof horses,lions,and otheranimals.Scientists
came to this cave,knownas Chauvet,and scrapedtiny samplesof
charcoalfrom the walls.Backat their lab,they isolatedcarbon isotopesand usedthem to estimatethai the paintingswere made between26,000and 32,000yearsago,makingthem the oldestknown
i nt h e w o r l d .
e x a m p l e os f p a i n t i n g
of rock rose to form continents. Gasesescapedfrom the rocks to form the Earth's
atmosphere.Water arrived on the surface of the planet, possibly escaping from
Earth'srocks as vapor or deliveredby comets and asteroids.The basins between the
continents filled with the water,forming oceans.
For hundreds of millions of years,the Earth collided with debris remaining from
the original solar disk. One such collision was so big that the rocky rubble thrown up
from the impact beganto orbit the Earth and eventuallycoalescedto form the Moon.
The giant impacts began to taper away about 3.8 billion years ago.Over the next bil
lion yearsor so,the crust of the planet broke into plates.Hot rock rose up in some of
the cracksbetween the plates and added to their margins. Meanwhile, the opposite
margins of the plateswere driven down under the crust. As this rock sank,it became
hotter, until it melted away.
The burial of Earth'scrust and the hearrybombardmentsearly in its history have
together destroyedalmost all of the planet's original surface (Sleep2010).The only
tracesof the first few hundred million years of the crust's
history are preserved in microscopic crystals known as
zircons (Figure3.11).As a result, scientistscannot hope to
find conventional fossils from the early Earth. So theyve
shifted their attention to the isotopesin ancient rocks,in
the hopes of finding a chemicalsignatureof early life.
Before life began, the only source of carbon on the
surface of Earth would have come from lifeless sources,
like volcanoes.But once life emerged on Earth, it would
have produced abundant amounts of organic carbon,
which gradually would have become incorporated into
sedimentaryrocks.Carbon from organismsis a lighter isotope than carbon from volcanoes.So rocks formed after
the origin of life should record this shift (Gaines2008).In
2004, Minik Rosing and Robert Frei of the University of
Copenhagenannouncedthey had found this shift (Rosing
and Frei 2004).They extractedbits of 3.7-billion-year-old
carbon from rocks in Greenlandand discovereda biologi
cal ratio of carbon in them. Rosing and Frei concludedthat
this was the earliest sign of life, produced most likely by
oholosvntheticbacteria.
C H A P T E RT H R E EW H A T T H E R O C K S s A Y : H O W G E O L O G YA N D P A L E O N T O L O G YR E V E A L T H E H I S T O R Y O F L I F E
Not surprisingly, such striking conclusionsusually neet with a great deal of
healthy skepticism. After Rosing and Frei published their study, other researchers
challengedthe results.They argued that geologicalprocessescould have createdthe
ratio of carbon isotopesin the rocks-without any need for life (Westall2008).Such
uncertainty hovers over much of the earliestevidencefor life on Earth. In the 1980s
in Australia,|. William Schopfof UCLA discoveredwhat he proposedwere 3.5-billionyear-oldfossilsof bacteria.Martin Brasierof the University of Oxford has challenged
's
Schopf results,arguing that the fossilswere actually formed by tiny blobs of mineral-richfluids (Brasieret al. 2006).
To better understand the early history of life, scientistsare continuing to scour
ancient rocks. Abigail Allwood and hei colleaguesdiscovered their strange, eggcarton-likerocks in some of the oldest geologicalformations on Earth. The researchers then found striking microscopicsimilarities betweenthe rocks and large mounds
built today by coloniesof bacteria.Thesemounds, known as stromatolites, grow on
the floors of lakesand shallow seas.Stromatolitesform when biofilms of microorganisms, especiallycyanobacteria,trap and bind sedimentsto form layeredaccretionary
structures.Sedimentsand minerals accumulateon the bacteria in thin lavers.and
more bacteriagrow on top of the sedimentsand minerals.Thesestructuresgradually
enlargeinto cabbage-likeor even meter-highdomed structures.Modern stromatolites
occur in only a very few extreme environments,such as salinelakesand hot, shallow,
lagoons where the high salinity keeps grazers away. However, despite their rarity
today, stromatolite fossils are abundant in Precambrianrocks worldwide, and these
bacterial mats are thought to have blanketed vast stretchesof warm, shallow seas
around 1.25billion years ago.
The rocks that Allwood studies come from much earlier than this-3.45 billion
years ago.In2006, Allwood and her colleaguespublished their findings, arguing that
they had discoveredstromatolitefossils.If they're right (and many of their colleagues
think they are),they may have found evidenceof some of the earliest life on Earth
( A l l w o o de t a l . 2 0 0 6 ; A l l w o o de t a I . 2 0 0 9 ) .
Stromatolites:Layeredstructures
formedby the mineralization
of
bacteria.
KeyConcept
Potential
signsof lifedatebackasfaras3.7billionyearsago.Theoldestknownfossils
thatare
generally
accepted
are3.45billionyearsold.Theearliest
signsof lifearemicrobial,
andmicrobes
still
constitute
mostof theworld'sbiomass
andgenetic
diversity.
3.4 TheRiseof Life
In The Origin of Species,CharlesDarwin noted that the oldestknown fossilsdated as
far back as the Early Cambrian period, which is now known to have stretchedfrom
542 to 510 million years ago.Those fossilsbelongedto a wide diversity of animals.If
Darwin's theory was right, then life must have been evolving long beforehand."During thesevast periodsthe world swarmedwith living creatures,"he wrote. Yet Darwin
recognizedthat no fossilsof thosecreatureshad yet been found. "To the questionwhy
we do not find rich fossiliferousdepositsbelonging to theseassumedearliestperiods
prior to the Cambrian system,I can give no satisfactoryanswer,"he wrote.
Today we know the answer: the fossilshad yet to be discovered.Scientistshave
assembleda record of life that stretchesback about 3 billion years before the Cambrian period-a record that continuesto improve eachyear.
One of the great challengesof Precambrian paleontology is determining how
early fossilsare relatedto the diversity of life on Earth today.Figure3.12illustratesthe
large-scalephylogeny of life, basedon the analysisof DNA from living species.(See
Chapters4 and 9 for more details about how scientistsinvestigatephylogeny with
DNA.) The figure showshow the history of life was dominated by three great branchings. As a result, living things can be divided into three domains: Bacteria,Archaea,
and Eukarva.
Bacteria:One of two prokaryote
domainsof life.BacteriaincludesorganismssuchasE.coli andotherfamiliar
microbes.
Archaea:One of the two prokaryote
domainsof life.Archaearesemble
bacteria,
but theyaredistinguished
b y a n u m b e or f u n i q u eb i o c h e m i c a l
features.
Eukarya:A domainof lifecharacterized
b y u n i q u et r a i t si n c l u d i n m
g embranee n c l o s ecde l ln u c l eai n dm i t o c h o n d r i a .
I n c l u d easn i m a l sp,l a n t sf,u n g i a
, nd
protists(single-cel
ledeukaryotes).
3 . 4 r H E R r s Eo F L r F E
65
Eukaryotesincludemulticellular
lineages
such
plants,andfungi,buttheyalso
asanimals,
include
a widerangeof single-celled
lineages
knownasprotists.
Eukaryotic
cellsareeasily
distinguished
frombothbacteria
andarchaea.
Theyareroughly100timesbigger,
for example.
All eukaryotic
cellshavea nucleus-a
membrane
thatenvelops
tightlypackedDNA.
All eukaryotes
alsocontainmitochondria,
or
descend
from ancestors
that possessed
them.
EUKARYA
u4
o)
-o
rS
qJ
d
.E
z8
^","J:"qr\
g
a--
"6r
ta
bAV
c
c.'
0^9
v
,s"/
\
V,C^
"%
'q^.
%^o^-\
o^
/z
-.€r..
"d.,
'q
".Tjr"":,:;
TtvtT
Actinobacteria
-
BACTERTA
^*"o;_";'
*-*tt\;
*".:"."""-t1,
a
"'",tCtt
Depending
on the
Bacteriaaresingle-celled
organisms.
or spheres.
species,
theymaybe shapedlikerods,filaments,
niches.
as
Bacteria
existin a widerangeof ecological
predators
as
on otherbacteria,
asphoiosynthesizers,
Bacteria
sharecertain
heterotrophs,
andaschemoautotrophs.
a
of life,including
traitsnotfoundin the othertwo domains
peptidoglycan,
anda uniquesetof
membrane
thatcontains
fiveproteins
thatcarryout RNApolymerization.
ARCHAEA
'liilfrfifi
branching
order
Unresolved
organisms
that
archaeaarealsosingle-celled
Likebacteria,
Theyalsolive
or spheres.
canbe shapedlikerods,filaments,
cannotcarryout
Whilearchaea
in a widerangeof habitats.
photosynthesis,
of manyotherformsof
theyarecapable
Archaea
havea
including
methanogenesis.
metabolism,
or
features
notfoundin bacteria
numberof biochemical
of archaea
containglycerolThemembranes
eukaryotes.
haveproposed
for example.
Someresearchers
etherlipids,
ancestor.
evolvedfrom an archaean
that eukaryotes
groupof species.
This
of a representative
tree showsthe relationships
Figure3.12 An evolutionary
The
Archaea,
and Eukaryotes.
of all livingthings:Bacteria,
treeincludesthe threemajorlineages
formsof lifewith whichwe aremostfamiliar,suchasanimalsandplants,makeup only
macroscopic
a tiny portion of the full diversityof the tree of life.(Adaptedfrom Pace2009.)
as the 3.5-billion-year-old stromatolites founcl
The earliest signs of life-such
by Allwood and her colleagues-strongly resemble living bacteria. Yet it's also pos
sible that they're the vestiges of an extinct branch of life and that bacteria evolvecl
Iater. Later in the fossil record, however, the evidence for bacteria becomes more
66
C H A P T E RT H R E EW H A T T H E R O G K SS A Y : H O W G E O L O G YA N D P A L E O N T O L O G YR E V E A LT H E H I S T O R Y O F L I F E
rr{. Researchers
have found fossilsin 2.6-bi11ion-year-old
: crample, that bear a striking resemblanceto cyanobac,i neageof bacteriathat carriesout photosynthesis.That's
'.hen the first evidenceof atmosphericoxygen appearsin
- record; between 2.45 andZ.32bllhon years ago,oxygen
I dramatically.The rise in oxygen was likely the result
:rrergence
which releaseoxygenduring
of cyanobacteria,
- . nthesis.While oxygen levels increaseddramatically dur- tirne, they were still very low compared to today. As a
:,,.rrplesulfur bacteriawere still abundant1.6billion years
- :eflectedby the presenceof okenane.
'
:, haeaalso make an early-but ambiguous-appearance in
-.il record. ln 2006, Yuichiro Ueno and his colleaguesat
'
lnstitute of Technologywere able to extract methane from
-ion year-oldrocks from Australia.The methane had a low
,n of carbon-13,indicating that it had been producedbio- ,11r'(Ueno et al. 2006).Only one group of organismsalive
. r eleasesmethane:a lineageof archaeacalledEuryarchaeota.
'rg the placesthey live today is the digestivetract of cows;
. : t the reasonsthat cow belchescontain methane.
: rrkaryaemerge in the fossil record only about 1.8 billion
- agoT
. h e i r f i r s t f o s s i l sa r es i n g l ec e l l e do r g a n i s m sm e a s u r i n g
;t 100micrometersacross.While they would have been invisr r r t h e n a k e de y e ,t h e y m a r k e da g i a n t l e a p i n s i z e ,m e a s u r
- .ibout 100 times bigger than a typical bacterium. Theseearly
.:..rr\rotes
had ridges,plates,and other structuresthat are similar
:hose of living single-celledeukaryotes.Over the next billion
,rs,the diversity of these single-celledeukaryotesincreased,as
:re lineagesevolvedto carry out photosynthesiswhile others
' :,'\'€d on bacteria or grazed on their photosynthetic relatives
r . n o l le t a I . 2 0 0 6 ) .
If you could travel back in time to 1.5 billion years ago,the
',,rrldwouid look like a desolateplace.On land there were no
': f es,no flowers,not even moss.In some spots,a thin varnish of
-.ngle-celied
organismsgrew In the ocean,there were no fish or
rbstersor coral reefs.Yet the oceanteemedwith microbial life,
:rorn the organismsthat lived around hydrothermal vents on the seafloorto free-float.ng bacteria and photosynthetic eukaryotesat the ocean'ssurface.Along the coasts,
rnicrobialmats stretchedfor miles in the shallow waters.
Today our attention may be distracted by animals and plants, but the world
remains dominated by microbes.By weight, microbes make up the bulk of Earth's
biomass.They live in a tremendousrange of habitats that would kill the typical ani
nral or plant-from Antarctic desertsto the bottom of acid-drenchedmine shafts.The
senetic variation among single-cellediife also far exceedsthat of animals or plants.
\'lost geneson the planet belong to microbes or their viruses.It's a microbial world,
in other words,and we just happento live in it.
cangrowand
Figure3.13 A: Bacteria
d i v i d ei n d i v i d u a l lbyu, t t h e yc a na l s o
f o r m m u l t i c e l l u l satrr u c t u r e s u
, c ha s
gelatinous
sheetscalledbiofilms.
B: Dictyosteliumdiscoides,
a soil
eukaryote,
is typicallyunicellular.
But D. discoides
individualscan come
massthat
togetherto form a slug-like
cancrawlawayandform a stalkof
spores.Alongwith bacterialbiofilms,
they offercluesto how multicellularity
first evolved.
3.5 LifeGetsBig
One of the most dramatic transitions in evolution was the origin of multicellular life.
The human body is radically different from a single-celled bacterium. It's made of a
trillion cells glued together with adhesive molecules and differentiated into organs
and tissues that work together. Only a minuscule fraction of cells in the human
sperm or eggs-have the potential to pass on their genetic material to
body-the
future generations.
1 . 5 L I F EG E r sB r G
67