Quirks of human anatomy: an evo-devo look at the human body

274 Pages·2009·11.905 MB·English

by  Held, Lewis I

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QUIRKS OF HUMAN ANATOMY
AnEvo-DevoLookattheHumanBody
Withtheemergenceofthenewfieldofevolutionary–developmentalbiology,weare
witnessingarenaissanceofDarwin’sinsights150yearsafterhisOriginofSpecies.
Thusfar,theexcitingfindingsfrom“evo-devo”haveonlybeentricklingintocollege
coursesandintothedomainofnonspecialists.Withitsfocusonthehumanorgan-
ism,QuirksofHumanAnatomyopensthefloodgatesbystatingtheargumentsof
evo-devoinplainEnglishandbyofferingacornucopiaofinterestingcasestudies
andexamples.Itsdidacticvalueisenhancedby24schematicdiagramsthatinte-
grateahostofdisparateobservations,byitsSocraticquestion-and-answerformat,
andbyitsunprecedentedcompilationoftheliterature.Byframingthe“hows”of
developmentintermsofthe“whys”ofevolution,itletsreadersprobethedeepest
questionsofbiology.Readerswillfindthebooknotonlyeducationalbutalsoenjoy-
able,asitrevelsinthefunofscientificexploration.
LewisI.Held,Jr.,earnedhisB.S.inlifesciencesfromtheMassachusettsInstitute
ofTechnologyin1973.OncompletionofhisPh.D.inmolecularbiologyfromthe
UniversityofCalifornia,Berkeley,in1977,hebecameaPostdoctoralFellowatthe
DevelopmentalBiologyCenterattheUniversityofCalifornia,Irvine,from1977to
1980.Followinga6-yearperiodasanAssistantProfessionalResearchBiologistat
theUniversityofCalifornia,Irvine,hejoinedTexasTechUniversityasanAssociate
ProfessorofBiologyin1987.HewasawardedthePresident’sExcellenceinTeaching
MedalfromTexasTechUniversityin1995andistheauthorofModelsforEmbryonic
PeriodicityandImaginalDiscs:TheGeneticandCellularLogicofPatternFormation.

QUIRKS OF HUMAN ANATOMY
An Evo-Devo Look at the Human Body
LEWIS I. HELD, Jr.
TexasTechUniversity
CAMBRIDGEUNIVERSITYPRESS
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Published in the United States of America by Cambridge University Press, New York
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Information on this title: www.cambridge.org/9780521518482
© Lewis I. Held, Jr., 2009
This publication is in copyright. Subject to statutory exception and to the
provision of relevant collective licensing agreements, no reproduction of any part
may take place without the written permission of Cambridge University Press.
First published in print format 2009
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ISBN-13 978-0-521-51848-2 Hardback
ISBN-13 978-0-521-73233-8 Paperback
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Contents
Preface pagevii
CHAPTER1. BACKGROUND 1
Whatroledidchanceplayinourorigin? 1
Howmuchdidinternalfactorsinfluenceourevolution? 2
Arefruitfliesreallyourkissingcousins? 11
CHAPTER2. SYMMETRYANDASYMMETRY 17
Whyarewesymmetricoutsidebutasymmetricinside? 17
Isvisceralasymmetryduetomolecularchirality? 20
Whydofingerprintsviolatemirrorsymmetry? 25
Whydobloodvesselsviolatemirrorsymmetry? 25
Howhaveanimalsevolvedexternalasymmetries? 27
CHAPTER3. MYSTERIESOFTHEMIDLINE 33
Whydoopticnervescrossthemidline? 33
Whydopre-motornervescrossthemidline? 35
Whydosymmetricorgansfuseatthemidline? 37
Wheredidtheneuralcrestcomefrom? 38
Howdidourthirdeyebecomeourpinealgland? 39
Arehumancyclopesevolutionarythrowbacks? 40
Whydoconjoinedtwinstendtobesymmetric? 41
Whatcausesidenticaltwinning? 44
Istwinningeveradaptive? 46
CHAPTER4. MERISMANDMODULARITY 49
Whatmakesthethumbouronlyopposabledigit? 49
Couldweevolveagiantdigitlikepterosaurs? 53
Wheredidourtailgo? 57
Whydoourteethhavedifferentshapes? 60
v
vi Contents
Whyaresomefeatureseasiertoevolvethanothers? 60
Howdoupperandlowerteethachievequasi-symmetry? 62
Whydoourearsstartoutonourneck? 65
Whyisourarmbuiltlikeourleg? 67
Whydoesourarmdifferfromourleg? 69
Couldcentaurs(orsimilarhexapods)everevolve? 71
Whendidwegaintheupperhand? 72
Howdidevolutionsculptourfoot? 73
CHAPTER5. SEXUALDIMORPHISMS 75
Whydomenandwomendifferinsizeandshape? 75
Whydoesourspecieshaveasmanymalesasfemales? 81
Werewealloncehermaphrodites? 84
Whydomalefetusesmakeincipientoviducts? 86
Howdidwebecometheonly“nakedapes”? 91
Why are men hair ier than women? 92
Whydoonlymengobald? 96
Whydomenhavenipples? 98
Howdidwomenendupwithonlytwobreasts? 101
CHAPTER6. SILLY,STUPID,ANDDANGEROUSQUIRKS 105
Couldchokinghavebeenavoided? 105
Whyareourretinasinsideout? 115
Whyischildbirthsoprecarious? 123
Whyarecrowdedteethsocommon? 125
Isourappendixontheroadtooblivion? 127
Whydowemakeayolksacifwehavenoyolk? 128
Whycan’tweregrowarmsorlegs? 129
CHAPTER7. MINDANDBRAIN 131
Howdidhumansacquireintelligence? 132
Whydidhumansacquireintelligence? 136
Isintelligenceattributabletobrainsize? 140
Didweascendfrombrutetoaesthetesuddenlyorgradually? 142
EPILOGUE 145
APPENDIX. QUIRKSOFHUMANBEHAVIOR 147
References 153
Index 239
Preface
Onceuponatime,wethoughtwewerecreatedintheimageofaperfectdeity.
Then came Darwin. In the very first chapter of his Descent of Man (1871), he
tookpainstopointoutthemanyflawsinouranatomy,includingouruseless—
butdangerous—appendix,ourpatheticcoveringofbodyhair,andthesillylittle
musclesthatletuswiggleourears.Eachoftheseoddtraitsisananachronism
inheritedfromournonhumanancestors.
Of course, the very idea that we had nonhuman ancestors was implicit
in Darwin’s earlier Origin of Species (1859). That book shook the world. The
presentbookhasamuchhumbleraim:tohonorthe150thanniversaryofOri-
ginand,coincidentally,the200thbirthdayofthemanhimself(b.12February
1809).
In Chapter 13 of Origin, Darwin asserted that the evidence from embry-
ology alone was strong enough to convince him of the principle of common
descent. Having taught embryology myself for more than 20 years, I know
what he meant. Human embryos make many structures we don’t need, and
wedestroyothersafterwe’vegonetothetroubleofmakingthem.Noengineer
in his right mind would ever allow such idiocy. Only in the light of evolution
dotheseprocessesmakesense[1265],andonlyinthelanguageofgeneticscan
wecomprehendtheirtortuouslogic[2496].Unfortunately,geneticsblossomed
onlyafterDarwindied(19April1882).InChapter5ofOrigin,helamentedhis
failuretotracethecausesofheritability.
Our ignorance of the laws of variation is profound. Not in one case out of a
hundredcanwepretendtoassignanyreasonwhythisorthatpartdiffers,more
or less, from the same part in the parents. But whenever we have the means
of instituting a comparison, the same laws appear to have acted in producing
the lesser differences between varieties of the same species, and the greater
differencesbetweenspeciesofthesamegenus.[559](p.167)
vii
viii Preface
QuirksofHumanAnatomyexaminesoneofDarwin’sfavoritetopics(oddi-
ties)[965]throughthelensofhismostincisiveapproach(embryology)toinves-
tigateoneofthelastremainingriddlesfromhisresearch—namely,howgenetic
circuitry has facilitated or frustrated anatomical evolution. This puzzle may
nowbesolvablebecausewe’verecentlylearnedsomuchaboutgenomiclogic
[896,1413,1822]. What better way to celebrate Darwin’s life than to revisit his
intellectualplayground withfresheyesandnewtoolsto digforthe treasures
thateludedhim?
Darwin explored everything that caught his fancy, from barnacles to
orchidstoearthworms.Hewasoftenastonishedbywhathefound,andthose
surprisesledhimtowonderwhynatureshouldsooftendefyhisintuition.To
honorhisinquisitivespirit,IhaveapproachedthissubjectasifIwereanalien
seeinghumansforthefirsttime.Hence,Quirksisfullofchildlikequestions.
This book is mainly intended for college classes in which students are
ableandwillingtodoindependentscholarlyresearch—forexample,capstone
courses, honors tutorials, graduate seminars, and journal clubs. For that rea-
son,Ihavethoroughlydocumentedallstatements.Thein-textcitations(num-
bers in brackets) provide essential links to published work, including articles
tooarcanetobelocatedeasilythroughInternetsearches.Stringsofsuchcita-
tionscanserveasready-madereadinglistsfordiscussionsortermpapers.The
References section may seem excessive to some, but not to teachers devis-
ingassignmentsortoresearcherscompilingtheirownbibliographydatabases.
Didactically,oneofthebestpapersforaclasstostudyisPinkerandBloom’s
“NaturalLanguageandNaturalSelection”(pluscommentaries)[2038],butany
articleinthejournalBehavioralandBrainScienceswillservetoillustratethe
fineartofTalmudicdebate.Debatabletopicsarelistedunder“puzzles”inthe
IndexandpostedonTomBrody’sInteractiveFlyWebsite.
Jargonhasbeenpurgedherewhereverpossible,andconceptshavebeen
simplifiedwhereverfeasible.Theintenthasbeentomakethematerialacces-
sibleanddigestible.Evenso,readersmayfindthetexttoughgoingunlessthey
have(1)afamiliaritywithmoleculargenetics(e.g.,[1167]),(2)anacquaintance
withdevelopmentalbiology (e.g., [908]), and (3) someprior exposure to basic
evolutionaryconcepts(e.g.,[815,1336]).
Givenitsbrevity,Quirkscanonlyofferatasteofthemanydiscoveriesgush-
ing from the new field of “evo-devo” (evolutionary–developmental biology)
[1057,1763].Moreriddlescanbefoundin(1)NeilShubin’spopularYourInner
Fish[2384],(2)theblogsofOliviaJudson(WildSide)andP.Z.Myers(Pharyn-
gula),and(3)twotimelessclassics:NaturalSelectionbyGeorgeWilliams[2824]
andTheHumanMachinebyR.McNeillAlexander[50](cf.[2599]).Severalnew