CHAPTER26
EARLY EARTH AND
THE ORIGIN OF LIFE
OUTLII\E
lntroduction to the History of Life
A. Life on Earthoriginatedbetween3.5 and4.0 billion yearsago
B. Major episodesin the historyof life: a preview
II. PrebioticChemicalEvolutionandthe Origin of Life
an
A. The first cellsmay have originatedby chemicalevolution on a young Earth:
overview
as a
B. Abiotic synthesisof organic monomersis a testablehypothesis:science
process
polymers
C. Laboratorysimulationsof early Earth conditionshave producedorganic
D. Protobiontscan form by self-assembly
E. RNA was probablythe first geneticmaterial
F. The origin of hereditaryinformationmadeDarwinianevolutionpossible
G. Debateaboutthe origin of life abounds
III. The Major Lineagesof Life
A. Anangingthe diversityof life into the highesttaxais a work in progress
L
OBJECTTVES
to:
After reading this chapter and attending lecture, the student should be able
Provide at least two lines of evidence for the antiquity of life.
I.
and Harold
Describethe contributions that A.I. Oparin, J.B.S.Haldane, Stanley Miller
2.
molecules.
organic
of
synthesis
for
abiotic
model
a
Urey made towards developing
provide plausible evidenceto support the hypothesis that chemical evolution resulting
3.
in life's origin occurred in four stages:
a. Abiotic synthesisof organic monomers
4.
5.
b. Abiotic sYnthesisof PolYmers
c. Formation of Protobionts
d. Origin of genetic information
Describe the basis for Whittaker's five-kingdom system'
rationale for
Describe three alternatives to the five-kingdom system and explain the
each.
KEYTERMS
stromatolites
protobionts
ribozyme
396 Unt V The EvolutionaryHistory of BiologicalDiversity
LECTT]RENOTES
The historyof living organismsand the historyof Earthare inextricablylinked.
Examples:
' Formationand subsequent
breakupof Pangaeaaffectedbiotic diversity.
' The first photosynthetic organismsreleasedoxygen into the air and
altered Earth's
atmosphere.
' Homo sapienshaschangedthe land,waterand air on a scaleand at a rate
unprecedented
for a singlespecies.
In orderto reconstructlife's history,scientistsuseevidencefrom:
'
'
The fossil record,which is lesscompletethe older the strata studied.In fact. there is no
fossil recordfor the seminalepisodeof the origin of Earth'slife.
Contemporaryorganismswhich, in their moleculesand anatomy, carry tracesof their
evolutionaryhistories.
Introduction to the llistory of Life
A. Life on Earth originated between3.5 and 4.0 billion years ago
Life probablyappearedrelatively early in the Earth'shistory. Scientistshave found
isotopesof carbonin 3.8 billion yearold rocksin Greenland.
Becauseof the relatively simple structureof prokaryotes,it is reasonableto assume
that the earliestorganismswere prokaryotes.
The fossil recordsupportsthis notion.
' Fossilssimilarto sphericaland filamentousprokaryoteshave beenrecovered
from stromatolites
3.5 billion yearsold in westernAustraliaand southernAfrica
(seeCampbell,Figire 26.2).
Stromatolites= Bandeddomesof sedimentsimilar to the layeredmatsconstructedby
coloniesof bacteriaand cyanobacteria
currentlyliving in salty marshes(seeCampbjll,
Figure26.1)
' The westernAustralianfossils appearto be of photosyntheticorganisms,which
indicateslife evolvedbefore these organismslived-perhaps some 4 billion
yearsago.
' Otherfossilssimilarto the prokaryotes
havebeenrecoveredfrom the Fig Tree
Chertrock formationin southernAfrica which dateto 3.4 billion vears.
B. Major episodesin the history of life: a preview
Campbell,Figure26.3,is a diagramof somemajorepisodes
in the historyof life.
Fossil evidencesuggeststhat prokaryotesappearedat least2 billion yearsbeforethe
oldesteukarvotes
. Two distinct groups of prokaryotes, Bacteria and Archaea, diverged early,
between2 to 3 billion ye{lrsago.
Photosynthetic bacteria started the production of oxygen about 2.5 billion
years ago, setting the stage for aerobic life.
Eukaryotes emerged some 2 billion years ago
.
'
Strongevidencesupportsthe hypothesisthat eukaryoticcells evolved from a
symbioticcommunityof prokaryotes
!lan]s, fungi, and animalsarosefrom distinctgroupsof unicellulareukaryotesduring
the Precambrian.
. Plantsevolvedfrom greenalgae.
Chaper26 Ealy Eartr andtheOriginofl-i€
397
Fungi and animals arose from different groups of heterotrophic unicells. Based
on riolecular evidence, fungi are more closely related to animals than they are
to Plants.
The oldest fossils of animals are those of soft-bodied invertebrates from about 700
million years ago. The basic body plans of most of the modern animal phyla probably
arose in the late Precambrian.
The transition from the aquatic environment to land was a pivotal point in the history
.
of life.
.
The first terrestrial colonization was by plants and fungi some 475 million
years ago; the move may have dependedupon a beneficial association between
the two groups.
.
The transformation of the landscapeby plants created new opportunities for all
forms of life.
tr.
Prebiotic Chemical Evolution and the Origins of Life
A. The first cells may have originated by chemical evolution on a young Earth:
an ovemiew
Life originated between 3.5 and 4.0 billion years ago. During this timespan the Earth's
crust bJgan to solidiff (4.1 billion) and bacteria advanced enough to build stromatolites
(3.5 billion).
The origin of life was possible in Earth's ancient environment, which was different
from today:
.
There was little atmospheric oxygen.
. Lightning, volcanic activity, meteorite bombardment, and ultraviolet radiation
were more intense.
One hypothesis about the first living organisms is that they were the products of a
chemical evolution that occurred in four stages:
l. Abiotic synthesisand accumulationof monomers, or small organic molecules,
that are the building blocks for more complex molecules
2. Joining of monomers into polymers (e.g., proteins and nucleic acids)
3. Formation of protobionfs, dropletswhich formed from aggregatesof_abiotically
produced mollcules and which differed chemically from their surroundings
4. Origin of heredity during or before protobiont appearance'
B. Abiotic synthesis of organic monomers is a testable hypothesisz science as a
process
In the lgZO',A.I. Oparin and J.B.S. Haldane independentlypostulated.thatthe reducing
atmosphere and greater UV radiation on primitive Earth.favored reactions that built
organic'moleculesfrom simple monomersas building blocks' This is not
;;pft*
possibletoday because:
. Oxygen in Earth's oxidizing environment attacks chemical bonds, removing
electrons. An important charicteristic of the early atmosphere must have been
the rarity of oxYgen.
.
The modern atmospherehas a layer of ozone that screensW radiation, so the
energy required to abiotically synthesize organic molecules is not available. On
prim'iiive Earth, energy was available from frequent lightning and intense W.
iadiation that penetratedthe atmosphere'
Stanley Miller and Harold Urey tested the Oparin/Haldane hypothesis (see Campbell,
Figure 26.4). They simulated conditions on early Earth by constructing an apparatus
containing H2O, H2, CI{+ and NH:.
398 Unit V The EvolutionaryHistory of BiologicalDiversity
'
'
'
Their simulated environment produced some amino acids and other organic
molecules.
Now we know the atmosphere of early Earth probably included CO, CO2, and
N2, and was less reducing than the Miller-Urey model, and thus, less favorable to
formation of organic compounds.
Additional experiments have produced all20 amino acids, ATP, some sugars,
lipids and purine and pyrimidine basesof RNA and DNA.
C. Laboratory simulations of early Earth conditions have produced organic
polymers
The forming of complex organic molecules,or polymers, from simpler building-block
molecules may have been inevitable on the primitive Earth.
Polymers : Chains of similar building blocks or monomers
Polymers are synthesizedby dehydration (condensation)reactions.For example:
o
.
n-H+oH- tr -)
tr-tr +H2o
fl and OH groupsare removedfrom the monomers.
HzO is producedas a by-product.
Abiotic polymerizationreactionsin early-Earthconditionsmust have occurred:
. Without the help of enzymes
' With dilute solutionsof monomers(spontaneousdehydration reactions that
producewaterwould be unlikely in alreadydilute solutions)
Abiotic polymerizationdoesoccur with dilute solutionsof monomersunder certain
laboratoryconditions:
Dilute solutionsof organicmonomersaredrippedonto hot sand,clay, or rock.
Water vaporizesand concentrates
the monomerson the substrate.
SidneyFox (University of Miami) usedthis method to abiotically produce
polypeptidescalledproteinoids.
Clay may have beenan importantsubstratefor abiotic synthesisof polymerssince:
' Monomers bind to chargedsites in clay, concentratingamino acidsand other
monomers.
' Metal ions (e.g., iron and zinc) could catalyzedehydrationreactions.
' The bindingsiteson clay couldhave broughtmany monomersclosetogether
and assistedin forming polymers.
' Pyrite (iron and sulfur) may also have been an important substrate.It has a
chargedsurfaceand electronsfreed during its formation could support bonding
betweenmolecules.
D. Protobionts can form by self-assembly
Living cells may havebeenprecededby protobionts.
Protobionts: Aggregatesof abioticallyproducedmoleculesableto maintainan internal
environmentdifferent from their surroundingsand exhibiting somelife propertiessuch
as metabolismand excitability
There is experimentalevidencefor the spontaneous
formation of protobionts:
' When mixed with cool water, proteinoidsself-assemble
into microspheres(see
Campbell, Flgure 26.5a) surroundedby a selectively permeable protein
membrane.Thesemicrospheres:
+ Undergoosmoticswellingand shrinking
=r Have potentialenergyin the form of a membranepotential
' Liposomes can form spontaneouslywhen phospholipids form a bilayered
membranesimilarto thoseof living cells.
Chag,u26 Ealy EarffrandtheOriginofl-i&
399
nucleicacids,andpolysaccharides)
(colloidaldropsof polypeptides,
Coacervates
self-assemble.
E. RNA was probably the first genetic material
This
Today'scells transcribeDNA into RNA, which is then translatedinto proteins.
control'
chainof commandmusthaveevolvedfrom a simplermechanismof heritable
. One hypothesisis that beforeDNA, there existeda primitive mechanismfor
whichwerethe first genes.Evidence
aligningaminoacidsalongRNA molecules,
to supportthis hypothesisincludes:
= RNA moleculesmay have beenableto self-replicate.Short polymers of
ribonucleotidesthat can basepair (5 - 10 baseswithout errzyme,up to 40
baseswith zinc addedas cataiyst) have been producedabiotically in test
Figure26'6)'
tubes(seeCamPbell,
- RNA is autocatalytic,as indicatedby ribozymes(RNA that actsas a catalyst
to removeintroni, or catalyzesynthesisof mRNA' tRNA or rRNA).
(unlike DNA), therebyprovidingraw
. RNA folds uniquelydependingon sequence
materials for'natural' seleciion-different molecular shapes (phenotypes)
varying in stability and catalytic properties..Replicationerrors (mutations)
pro-UuUtycreated additional variation within families of closely related
sequences.
. In additionto molecularcompetition,molecularcooperationprobablyevolved
as RNA-directedProtein syntiesis producedshort polypeptides that catalyzed
.
RNA rePlication.
genetic
Once this simple machinery for replication _and translation of
molecular
information became sequesteredinto membrane-bound protobionts'^
entire
cooperationcould be refined as natural selectionacted on the level ofthe
Protobiont.
possible
F. The origin of hereditary information made Darwinian evolution
Perhapsthis hypothetical membrane-boundprotobiont:
.
'
.
Incorporated genetic information
Selectively accumulated monomers from its surroundings
. Used enzymes programmed by genes to make polymers and carry out other
chemical reactions
.
Grew and split, distributing copies of its genesto offspring
the
If these cell precursorscould also grow, divide, and distribute genesto^offspring'
RNA
of
descendantprotobionts would uury b""aute of errors in the copying
(mutations).
selection'
.
The variation among related protobionts would be subject to natural
. Evolution in the Darwinian sense--differential reproductive. .successpr.ro-ubly accumulated refinements to primitive metabolism and inheritance,
including ih" upp"u.unce of DNA as the hereditary material.
+ Initially, RNA could have provided the template to produce DNA.
store of
+ Becauseit is more stable, DNA would have replaced RNA as the
genetic information.
= RNA,s role would change as it became an intermediate in translation.
G. Debate about the origin of life abounds
described and
No one knows how life actually began on Earth. The chemical evolution
supporting lab simulations indicate key steps that could have occurred'
Several alternatives have been proposed'
400 Unit V The EvolutionaryHistory of BiologicalDiversity
'
'
'
III.
Panspermia - Some organic compounds may have reached Earth by way of
meteorites and comets. Organic compounds (e.g., amino acids) have 6een
recovered from modern meteorites. These extraterrestrial organic compounds
may have contributed to the pool of molecules which formed eirly life.
Most researchersbelieve life first appearedin shallow water or moist sediments.
Some now feel the first organisms developed on the sea floor due to the harsh
conditions on the surface-duringthat time. This position was strengthened in
the 1970s by discovery of the deep sea vents. Hot water and minerals emitted
from such vents may have provided the energy and chemicals necessaryfor
early protobionts.
Simpler hereditary systemsmay have precedednucleic acid genes.JuliusRebek
synthesized a simple organic molecule in 1991. The importance of this
molecule was that it served as a template for self-replication (see Campbell,
Figure 26.8). This discovery supported the idea held by some biologistj that
RNA strandsare too complicatedto be the first self-replicatingmolecules.
The Major Lineages of Life
A. Arranging the diversity of life into the highest taxa is a work in progress
Systematistshave traditionally consideredthe kingdom to be the highest, most
inclusive taxonomic category.
'
The two kingdom system (animals and plants) long prevailed, but was not
suitable as biologists learned more about the structures and life histories of
different organisms.
'
The_.fiveftingdom system was proposed by Robert H. Whittaker (1969) and
modified by Lynn Margulis (seealso Campbell,Figure 26.9).
Prokaryotic
MONERA
Simplemulticells
or unicells
Multicellular
I
PROTISTA
Autotrophic
Heterotrophic
PI,AI{TAE
Absorptive
nutrition
I
FUNGI
Ingestive
nutrition
I
AI\IMALIA
Classifring living systemsis a work in progressthat reflects our increasedunderstanding
of the phylogeny of living organisms.
'
Using the tools of molecular systematics,biologists have gatherednew data that
leads them to challengethe traditional five-kingdom system.
Chgu_26
.
EarlyEarrhandtheOriginofl-iF
401
This new information has reopened issuesof biological diversity at the highest
taxonomic levels. Three alternative classification systems are outlined below
(see also Campbell,Figure 26.10):
Six-kingdom system. The prokaryotes are split into two kingdoms based on molecular
evidenJe for an early evolutionary divergence between eubacteria and archaebacteria.
M@
Protista
Plantae
Fungi
Aninalia
to the ancient
system.This schemeassignsmore significance
Three-domain
a superkingdomtaxon
using
by
and
archaebacteria
eubacteria
evolutionarysplit between
of
eukaryotic
four
kingdoms
includes
Eukarya
called theiomain The domain
organisms.
trtr
Eukarya(Eukaryotes)
Eight-kingdomsystem.In additionto two separateprokaryotickingdoms,this system
also splitsthe protistsinto threekingdoms.
@@
rE@
REFERENCES
1998.
Campbell,N., et al.Biology.5th ed. Menlo Park,California:Benjamin/Cummings,
of Life on
the
Phyla
to
Guide
Margulis,L., and K.V. Schwartz.Five Kingdoms: An Illustrated
1987'
Earth.2nded.New York: Freeman,
"The
Origin of Life on Earth." ScientificAmerican,October1994.
Orgel,L.E.
1969.
163l.150-160,
^gcience
of Kingdomsof Organisms."
Whittaker,R.H. "New Concepts