This document discusses evidence for evolution from comparative embryology. It notes that more complex organisms share many initial developmental steps with more primitive forms, as Darwinian evolution suggests they elaborated on existing developmental patterns. Comparative embryology studies reveal that all multicellular animals start as a single zygote and share fundamental developmental processes like gastrulation and the formation of germ layers, despite some differences. Recapitulation, where embryos of higher animals resemble embryos of lower forms, provides further evidence. Genetic factors explain recapitulation, as organisms retain genes regulating early development from ancestral forms.

1. EVIDENCESFROMCOMPARATIVEEMBRYOLOGY
One of the mostfascinatingsourcesof evidence forthe validityof Darwin’stheoryof evolutionhas
come from studiesof comparative embryology.The suspicionsof acommonancestryforverydiverse
setsof organismsfromcomparative anatomical studieshave beensupportedbyembryological data.As
Darwinianevolutionsuggestedthatmore complex organismswouldhave achievedtheirstate by
elaboratingonthe existingdevelopmental patternsof more primitive forms,soone wouldexpecttofind
that certainrelativelysimple organismsandmore complex oneshave manyinitialdevelopmental steps
incommon.In fact, the more developmental stepstwospecieshave incommon,the more closely
relatedtheyare to a commonancestral form.Comparative embryological studieshave revealedthat
there wasone developmentalpatternthatcouldbe viewedashavingundergone aseriesof branchings.
All multicellularanimalsstarttheirdevelopmentasa single zygote,andthroughaseriesof mitotic
divisions,increaseincell numberuntilablastulaisformed.The developingembryoelaboratesuponthe
blastulastage byformingtwofundamental germlayers,ectodermandendoderm, duringthe course of
gastrulation.Afterthe differentiationof the ectodermandendoderminthe gastrula,the thirdgerm
layer,mesoderm, isformed.There are three distinctpatternsbywhichdevelopingembryoof different
speciesproduce amesodermal layer.Forinstance,inannelids,molluscsandcertainotherinvertebrates,
the mesodermdevelopsfromspecial cellswhichare differentiatedearlyincleavage.Thesecellsmigrate
to the interiorandcome to lie betweenthe ectodermandendoderm.Theythenmultiplytoformtwo
longitudinalcordsof cellswhichdevelopintosheetsof mesodermbetweenthe ectodermand
endoderm.Inprimitive chordates(i.e.,Amphioxus,etc.) the mesodermarisesasa seriesof bilateral
pouchesfromthe endoderm.These losetheirconnectionswithgutandfuse one withanothertoforma
connectedlayer.The mesoderminamphibia,reptilia,birdsandmammalsisformedfromthe cellswhich
remainassociatedwitheitherectodermorendodermorbothand whichgetdisassociatedfromthemto
migrate andcome to lie inbetweenbothgerminal layersinthe formof mesoderm.Despite some
differencesindevelopmental patterns,comparative embryological studiesoverwhelminglysupportthe
conceptof Darwinianevolution.The speciesthatfollow the same pathwayforseveralstepsafterthe
gastrulaproduce adultformsthat are more similarthantwospecieswhose
Fig.4.8. Vestigial hindlimbsof Python(snake).
ribs
ilium
femur
claw
back bone
Darwinpointedoutthatboth frigate birdsanduplandgeese have webbedfeet,yetneithergoesinto
water.He explainedtheirfeetasaleftoverfromtheirpast, bothbeingdescendedfromwaterbirds.
Contents
EVOLUTION (EvolutionaryBiology)42

2. pathwaysdiverge afterthatstage.The differentdevelopmentalpatternsare thoughttobe indicationsof
differentlinesof evolution.Anotherfeature of embryological developmentthatservestolinkall
multicellularanimalsisthe embryological source of organsystems.Regardlessof the wayinwhichthe
developinganimalsare programmedforfurtherdifferentiationof mesodermtheyexhibitsimilaradult
structuresderivedfromthe twoprimarygermlayers.The outercoveringof all multicellularanimals,be
it skin,scales,orgelatinousmaterial,isderivedfromectoderm.The universal featuresof ectodermally
and endodermallyderivedtissuesalsoindicate the presence of acommonancestral type earlyin
evolutionaryhistory.Besidesthese basicembryologicalsimilaritiesinall multicellularanimals,there
existcertainembryologicalaspectswhichmaycompel one tobelieve inDarwinianevolution.For
instance,itismostcommonlyobservedthatthe embryosof higheranimalsrepeatmanyof the stages
throughwhichembryosof higheranimalshave passed.Thishasbeenreferredtoasrecapitulation.This
concept,as originallyusedbyvonBaer(1792–1876) indicatedthatsome of the developmental stagesof
an organismare similartosome of the developmental stagesof itsancestors.Unfortunately,however,
E. Haeckel in1866, modifiedthe conceptintoa“biogeneticlaw”whichstatedthat“ontogeny
recapitulatesphylogeny”,i.e.,initsdevelopment,the individual passesthroughstageslikethe adult
stagesof its ancestors.The Haeckelianideaiswronginthe eyesof modernembryologistsyet,ithas
stimulatedresearchinembryologyandfocussedattentiononthe general resemblance between
I
II
III
FishSalamanderTortoise Chick Pig Cow RabbitMan
Fig.4.9. Comparisonof three embryonicstagesfromfishtoman.
embryonicdevelopmentandthe evolutionaryprocesses.Accordingtobiogeneticlaw of Haeckel,the
fertilizedeggcanbe comparedto the hypothetical single-celledflagellate ancestorof all animals,and
the blastulacan be comparedto a colonial protozoanorto some hypotheticblastula-like animalwhich
has beenpostulatedtobe the ancestorof all Metazoa.Haeckel believedthatthe ancestorof
coelenteratesandall the higheranimalswere agastrula-likeorganismwithtwolayersof cellsanda
Contents
INDIRECTEVIDENCESOF EVOLUTION 43
central cavityconnectedbya blastopore tothe outside.ModernscientistsprefervonBaer’s
recapitulationconceptthanbiogeneticlaw of Haeckel.There are numerousexamplesof recapitulation
phenomenonduringembryonicdevelopment,butone of the bestisaffordedbya comparisonof
differentvertebrateembryosatcomparable stagesindevelopment(seeFig.4.9).Duringthe early
embryonicstage inthe series,all the embryoslookverymuchalike.All have similarpharyngealarches
and pharyngeal clefts.Atsomewhatlaterstage inthe series,the limb-budprimordiaof fore-and
hindlimbsare forminginall embryosinasimilarwayandall of themhave embryonictails.The embryos
of the lizard,chick,opossum,monkey,andmanhave strongresemblances,yetthose of the fishand
salamanderare beginningtoassume recognizableforms.At thisstage,gillshave formedfromthe
tissuesliningthe gill-cleftsof boththe fishandsalamander.Later,eachembryohasdevelopedfeatures

3. that indicate fairlyclearlyitsdefinitivenature.Further,inthe developmentof anymammalianembryo,
the heart isa four-chamberedinseriesstructure,asitisin fishembryo;thenitdevelopspartitionsof
the auricle (atria) similartothose of amphibianembryos,followedbyventriculardivisionthatis
incomplete foraperiod,asitis inthe embryosof reptiles.A similarexample of recapitulationisfoundin
mammalianembryosinthe formationof pronephric,mesonephricandmetanephrickidneys.The
existence of acommontrochophore larval stage inthe developmentof manymolluscsandmany
annelidsisalsotypical of thisprocess.Plantsprovide afew examplesof recapitulation.One good
example isfoundinthe Acaciatree.Thistree haswell-developedcompoundleaves,yetthe seedlinghas
simple leaveslikethose foundinall stagesof developmentof itsancestors.Further,itisgenerally
believedthatmossesandfernsare more highlyevolvedthanalgae.Protonemaof mosseswhichisa
green,filamentousstructure thatoriginatesfromanasexual spore,resemblescertaingreenalgae.This
providesaclue to theirevolutionaryrelationship.Bothbryophytesandpteridophyteshave ciliated
spermsandrequire waterforfertilization.Gymnospermsdonotneedwaterforfertilization,butCycads
and Gingko,the primitivegymnosperms,have ciliatedspermslike the pteridophytes.Thissuggeststhat
gymnospermshave descendedfrompteridophyte-likeancestors.
GeneticBasisof Recapitulation
Our increasingunderstandingof physiological geneticsprovidesuswithanexplanationof the
phenomenonof recapitulation.Allchordateshave incommonacertainnumberof geneswhichregulate
the processesof earlydevelopment.Asmammalianancestorsevolvedfromfishthroughamphibianand
reptilianstages,theyaccumulatedmutationsfornew characteristicsbutkeptsome of the original “fish”
genes,whichstill control earlydevelopment.Laterindevelopmentthe geneswhichthe humanorany
othermammal shareswithamphibiansinfluencesthe course of developmentsothatthe embryo
resemblesafrogembryo.Subsequently,some of the geneswhichhave incommonwithreptilescome
intocontrol.Onlyafterthisstage,mostof the peculiarlymammaliangenesexerttheirinfluence,and
these are followedbythe actionof geneshumanhasincommonwithotherprimates.The anthropoid
apes,which have the mostimmediate ancestorsincommonwithman,have the mostgenesincommon
withhimand theirdevelopmentisidentical withhim(human) exceptforsome fine details.Ingeneral,
duringdevelopmentthe general characteristicsthatdistinguishphyla andclassesappearbefore the
special characteristicsthatdistinguishgeneraand,finally,species.Withineachphylum, the higherforms
pass througha sequence of developmental stageswhichare similartothose of lowerformsbutachieve
a differentfinalformbyaddingchangesatthe endof the original sequence andbyalteringcertainof
the earlierembryonicstagestheyshare withthe lowerforms(Villeeetal,1973).
EVIDENCESFROMCOMPARATIVEPHYSIOLOGYANDBIOCHEMISTRY
Basically,evolutionis biochemical phenomenonanditis,therefore,natural thatphysiologyand
biochemistryhave givensome of the followingmostimportantanddependable evidencestosupport
the ideaof evolution:
Contents
EVOLUTION (EvolutionaryBiology)44
Human serum
Human serum

4. Chimpanzee serum
Baboonserum
Dog serum
Rabbitserum
antiserumcontainingantibodiesagainsthumanserumplacedineachtube
1. ProtoplasmChemistryBiochemical analysisof the living matterinthe protoplasmwhichisconsidered
as “the physical basisof life”,suggeststhatprotoplasmfromavarietyof sources(i.e.,bacteria,blue
greenalgae,plantsandanimals) hasthe same biochemical constitution.Itmainlyconsistsof substances
like proteins,lipids,carbohydrates,water,etc.Thiswouldsuggestthatduringevolutionthe most
fundamental propertyof livingthingshasremainedintact,whilevariationsincertainessential respects
producedthe variabilityaccordingtothe needsof differential forms.2.Chromosome ChemistryLike
protoplasm,anotherremarkablesimilarityatthe biochemical level isfoundinthe chemistryof
chromosomes.The chromosomesof all livingorganismsbasicallyconsistof nucleicacids(DNA andRNA)
and proteins(histonesandprotamines).The moleculesof these chemical substancesremainarrangedin
all chromosomesinanalmostidentical fashion.Suchauniformityinthe compositionof chromosome
againsuggestsa commonoriginof most livingbeings.3.Enzyme SimilaritiesA large numberof animals
and plantscontainidentical enzymes.Several enzymesfoundinthe digestivetractare commonina
varietyof animals.Forexample,trypsinandamylase are foundfromspongestomammals.A numberof
enzymesusedinphotosynthesisare commoninavarietyof greenplants.Suchcommonenzymesand
consequentlyacommonmechanismof processof photosynthesissuggestacommonancestryof green
plants.4. Hormonal SimilaritiesLike enzymes,hormonal similaritiesare alsofoundinall vertebrates.For
example,thyroidhormone iscommonlyfoundinall vertebratesandthishormone fromone classof
organismcan be substitutedforthatinanotherclassof organisms.For example,infrogsdeficiencyof
thyroidhormone canbe correctedby feedingthemonmammalianthyroidtissues.Likewise,another
commonlyoccurringhormone of vertebratesismelanophore expandinghormone.Itisconcernedwith
the pigmentationof the skintoexpand,thus,renderingthe skincolourdark.Thishormone isfoundin
amphibiansandmammals.Inthe latteritisa vestigial hormone,butif itisgraftedintothe amphibian
skin,the skinpigmentationexpands.The presence of these hormonesinvertebratesisunderstandable
onlyon the basisof descentfroman ancestorto whomthese hormoneswere useful.5.Comparative
SerologyWhenaforeignproteinisinoculatedintothe bloodof ananimal,the latterproducesacomplex
proteincompoundagainstthatforeignproteininoculated.Thesecompoundsare familiarlyknownas
antibodiesandthe foreigninoculatedproteinisknownasantigen.Whenareactionoccurs between
antibodyandantigen,asoftwhite precipitate will be formed.The strengthof precipitate dependsupon
the concentrationof antigen.The precipitateisthe precipitinandthe testisprecipitationtest.One of
the remarkable featuresof this testisthatthe antibodiesformedagainstone antigen,canalsoreact
withantigensof othersource,providedthe Fig.4.10. Principle of precipitintestappliedtoinvestigation
of animal relationship.
latterischemicallysimilartothe firstantigen.Antibodiescontainingserumisknownasantiserum.
Antiserumof antigenof one animal canbe testedwithantigensof otheranimalsinordertoshow their
relationships.The testcanbe interpretedthatif precipitateresultswithmore dilutedantigenof one
animal againstthe testanimal,thenthe formerismore closelyrelatedtothe latter; if precipitate results

5. withlessdilutedantigenof thatanimal,thenitisdistantlyrelatedtothe testanimal.Suchprecipitin
testshave beenconductedtoresolve the disputedrelationshipsof organismsinrecentyears.Of scores
of examples,here we givetwoillustrations.Till recently,itisbelievedthatwhaleshave relationship
withfishes.Itisbecause,almostall of theiranatomyare sostronglymodifiedtoaquaticfish-like life.
Onlyfewanatomical cluestoshowtheirrelationshipstoothermammals,remained.However,
comparative serologyof whaleswithothermammaliangroupsindicatesthattheirserumproteinsare
mostlike those of the even-toedhoofed(OrderArtiodactyla)mammals.Thismightsuggestthatwhales
sprang fromprimitive artiodactyl stock.The same serological testswhenperformedinslightlymodified
formamong the membersof primates –man, an anthropoidape,anoldworld-monkey,anew world-
monkeyanda lemur– the amountof precipitate of serumproteinswoulddecreaseindescendingorder,
that is,the anthropoidape ismore closelyrelatedtomanthanother organisms.Evenamonganthropoid
primates,testsdone accordingtoOuchterlonytechnique (Goodman,1962, 1967) reveal thatthe serum
proteinsof chimpanzee are more alike toman’sserum proteinsthanthe serumproteinsof asiaticapes,
gorillaandbaboon.Similarcomparative serological testsreveal the factthatcats, dogs,and bearsare
closelyrelated.Cows,sheep,goats,deerandantelopesconstitute anothercloselyrelatedgroupsin
termsof “bloodrelation.”Serological testsalsosuggestthatthere isacloserrelationshipamongthe
modernbirdsthanamong the mammals,forall of the several hundredspeciesof birdstestedgive
strongand immediate reactionswithserumcontainingantibodiesforchickenserum.Fromothertestsit
was concludedthatbirdsare more closelyrelatedtothe crocodile lineof reptilesthantothe snake-
lizardline,whichcorroboratesthe pal-aeontological evidence.Similartestsof the seraof crustaceans,
insectsandmolluscshave shownthatformsregardedasbeingcloselyrelatedfrommorphologicand
palaeontologicevidence alsoshowsimilaritiesintheirserumproteins.6.Amino-AcidSequence Analyses
Molecularbiological investigationsof the sequence of aminoacidsinthe α andβ chainsof
haemoglobinsfromdifferentspecieshave revealedgreatsimilarities,of course,andspecificantiserum
containingantibodiesagainstsheepserumplacedineachtube SheepserumGoatserumserial dilutions
Sheepserum serial dilutionsBeef serumserial dilutionsDogserumserial dilutions12 3 4 5 6 7 8 9 10
Fig.4.11. Principle of precipitintestemployingserial dilutionsof antigenandthe interfacial reaction.
Rabbitserum
Contents
EVOLUTION (EvolutionaryBiology)46
differences, the patternof whichdemonstratesthe orderinwhichthe underlyingmutations,the
changesinnucleotide base pairs,musthave occurredinevolution.The evolutionaryrelationships
inferredfromthese studiesagree completelywiththose basedonmorphologicstudies.Analysesof the
aminoacid sequence inthe proteinportionof the cytochrome enzyme provide furtherconcurring
evolutionaryrelationships.Furtherthe patternandratesof reactionsof lactate dehydrogenase and
certainotherenzymeswiththe normal pyridine nucleotide coenzyme(NAD)andwithanaloguesof NAD
can be usedtodemonstrate evolutionaryrelationships.7.ExcretoryProductAnalysesAnanalysisof the
urinarywastesof differentspeciesprovidesthe evidence of evolutionaryrelationship.The kindof
nitrogenousexcretorywaste dependsuponthe particularkindsof enzymespresent,andthe enzymes
are determinedbygenes,whichhave beenselectedinthe course of evolution.The waste productsof
the metabolismof purines,adenine andguanine are excretedbymanandotherprimatesasuric acid,by
othermammalsas allantoin,byamphibiansandmostfishesasurea,andby mostinvertebratesas
ammonia.Vertebrate evolutionhasbeenmarkedbythe successive lossof enzymesforthe stepwise

6. degradationof uricacid.J. Needhammade the interestingobservationthatthe chickembryointhe
earlystagesof developmentexcretesammonia,lateritexcretesureaandfinallyitexcretesuricacid.
The enzyme uricase,whichcatalyzesthe firststepinthe degradationof uricacid,ispresentinthe early
chickembryobut disappearsinthe laterstagesof development.The adultfrogexcretesurea,butits
tadpole larvaexcretesammonia.These biochemical examplesare the repetitionof the principle of
recapitulation.8.PhosphagensThe phosphagensplayakeyrole inmuscle contractionandare the
sourcesof energyforthe resynthesisof ATP,once theyare brokendown.Inthe musclesof most
vertebrates,phosphagenisalwaysaspecificcompoundcalledcreatine phosphate,while inmost
invertebratesitisarginine phosphate.Hemichordates,the mostprimitivechordates,have boththe
phosphagens,the creatinephosphateaswell asargine phosphate.Suchasituationisalsofoundin
echinodermsandonmorphologicalgroundsechinodermshave beenconsideredclose tothe ancestorof
chordates.