Editing Evolutionary developmental biology (section)

==History==
{{Further|History of evolutionary thought}}

===Early theories===
{{Further|Aristotle's biology}}

Philosophers began to think about how animals acquired form in the [[womb]] in [[classical antiquity]]. [[Aristotle]] asserts in his ''[[Physics (Aristotle)|Physics]]'' treatise that according to [[Empedocles]], order "spontaneously" appears in the developing embryo. In his ''[[The Parts of Animals]]'' treatise, he argues that Empedocles' theory was wrong. In Aristotle's account, Empedocles stated that the [[vertebral column]] is divided into vertebrae because, as it happens, the embryo twists about and snaps the column into pieces. Aristotle argues instead that the process has a predefined goal: that the "seed" that develops into the embryo began with an inbuilt "potential" to become specific body parts, such as vertebrae. Further, each sort of animal gives rise to animals of its own kind: humans only have human babies.<ref name="Leroi 2014">{{Cite book |last=Leroi |first=Armand Marie |author-link=Armand Marie Leroi |title=The Lagoon: How Aristotle Invented Science |title-link=Aristotle's Lagoon |date=2014 |publisher=Bloomsbury |isbn=978-1-4088-3622-4 |pages=181–182}}</ref>

===Recapitulation===
{{Main|Recapitulation theory}}

[[File:Haeckel vs von Baer.svg|thumb|upright=1.5|Embryology theories of [[Ernst Haeckel]], who argued for [[recapitulation theory|recapitulation]]<ref>{{harvnb|Richardson|Keuck|2002|p=516}}</ref> of evolutionary development in the embryo, and [[Karl Ernst von Baer]]'s [[epigenesis (biology)|epigenesis]] ]]

A [[recapitulation theory]] of evolutionary development was proposed by [[Étienne Serres]] in 1824–26, echoing the 1808 ideas of [[Johann Friedrich Meckel]]. They argued that the embryos of 'higher' animals went through or recapitulated a series of stages, each of which resembled an animal lower down the [[great chain of being]]. For example, the brain of a human embryo looked first like that of a [[fish]], then in turn like that of a [[reptile]], [[bird]], and [[mammal]] before becoming clearly [[human]]. The embryologist [[Karl Ernst von Baer]] opposed this, arguing in 1828 that there was no linear sequence as in the great chain of being, based on a single [[body plan]], but a process of [[epigenesis (biology)|epigenesis]] in which structures differentiate. Von Baer instead recognized four distinct animal [[body plan]]s: radiate, like [[starfish]]; molluscan, like [[clam]]s; articulate, like [[lobster]]s; and vertebrate, like fish. Zoologists then largely abandoned recapitulation, though [[Ernst Haeckel]] revived it in 1866.<ref>{{Cite web |last=O'Connell |first=Lindsey |date=10 July 2013 |title=The Meckel-Serres Conception of Recapitulation |url=https://embryo.asu.edu/pages/meckel-serres-conception-recapitulation |access-date=10 October 2016 |website=The Embryo Project Encyclopedia}}</ref><ref>{{Cite book |last=Desmond |first=Adrian J. |author-link=Adrian Desmond |url=https://archive.org/details/politicsofevolut00adri/page/53 |title=The politics of evolution: morphology, medicine, and reform in radical London |publisher=University of Chicago Press |year=1989 |isbn=978-0-226-14374-3 |location=Chicago |pages=[https://archive.org/details/politicsofevolut00adri/page/53 53–53, 86–88, 337–340, 490–491] |url-access=registration}}</ref><ref>{{harvnb|Secord|2003|pp=252–253}}</ref><ref>{{Cite book |last=Bowler |first=Peter J. |url=https://archive.org/details/evolutionhistory0000bowl_n7y8/page/120 |title=Evolution: the history of an idea |publisher=University of California Press |year=2003 |isbn=978-0-520-23693-6 |location=Berkeley |pages=[https://archive.org/details/evolutionhistory0000bowl_n7y8/page/120 120–128, 190–191, 208]}}</ref><ref>{{harvnb|Secord|2003|pp=424, 512}}</ref>

{{anchor|Heterotopy}}<!--until an article on this appears-->

===Evolutionary morphology===
{{Further|Morphology (biology)|Body plan}}

[[File:Comparison of Three Invertebrate Chordates.svg|thumb|left|A. [[Lancelet]] (a chordate), B. Larval [[tunicate]], C. Adult tunicate. [[Alexander Kovalevsky|Kowalevsky]] saw that the [[notochord]] (1) and gill slits (5) are shared by tunicates and vertebrates.]]

From the early 19th century through most of the 20th century, [[embryology]] faced a mystery. Animals were seen to develop into adults of widely differing [[body plan]], often through similar stages, from the egg, but zoologists knew almost nothing about how [[embryogenesis|embryonic development]] was controlled at the [[molecular biology|molecular level]], and therefore equally little about how [[developmental biology|developmental processes]] had evolved.<ref name="CarrollNatHist">{{Cite web |last=Carroll |first=Sean B. |author-link=Sean B. Carroll |title=The Origins of Form |url=http://www.naturalhistorymag.com/features/061488/the-origins-of-form |access-date=9 October 2016 |website=Natural History |quote=Biologists could say, with confidence, that forms change, and that natural selection is an important force for change. Yet they could say nothing about how that change is accomplished. How bodies or body parts change, or how new structures arise, remained complete mysteries.}}</ref> [[Charles Darwin]] argued that a shared embryonic structure implied a common ancestor. For example, Darwin cited in his 1859 book ''[[On the Origin of Species]]'' the [[shrimp]]-like [[larva]] of the [[barnacle]], whose [[Sessility (motility)|sessile]] adults looked nothing like other [[arthropods]]; [[Carl Linnaeus|Linnaeus]] and [[Cuvier]] had classified them as [[mollusc]]s.<ref name="Gilbert2003">{{Cite journal |last=Gilbert |first=Scott F. |author-link=Scott F. Gilbert |date=2003 |title=The morphogenesis of evolutionary developmental biology |url=http://www.chd.ucsd.edu/_files/fall2008/Gilbert.2003.IJDB.pdf |journal=International Journal of Developmental Biology |volume=47 |issue=7–8 |pages=467–477 |pmid=14756322}}</ref><ref>{{Cite book |last=Darwin |first=Charles |author-link=Charles Darwin |url=http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F373&pageseq=457 |title=On the Origin of Species |publisher=John Murray |year=1859 |isbn=978-0-8014-1319-3 |location=London |pages=439–440 |quote=Cirripedes afford a good instance of this: even the illustrious Cuvier did not perceive that a barnacle was, as it certainly is, a crustacean; but a glance at the larva shows this to be the case in an unmistakeable manner.}}</ref> Darwin also noted [[Alexander Kovalevsky|Alexander Kowalevsky]]'s finding that the [[tunicate]], too, was not a mollusc, but in its larval stage had a [[notochord]] and pharyngeal slits which developed from the same germ layers as the equivalent structures in [[vertebrate]]s, and should therefore be grouped with them as [[chordates]].<ref name=Gilbert2003/><ref>{{Cite web |last=Richmond |first=Marsha |date=January 2007 |title=Darwin's Study of the Cirripedia |url=http://darwin-online.org.uk/EditorialIntroductions/Richmond_cirripedia.html |access-date=9 October 2016 |publisher=Darwin Online}}</ref>

19th century zoology thus converted [[embryology]] into an evolutionary science, connecting [[phylogeny]] with [[homology (biology)|homologies]] between the germ layers of embryos. Zoologists including [[Fritz Müller]] proposed the use of embryology to discover [[phylogeny|phylogenetic relationships]] between taxa. Müller demonstrated that [[crustaceans]] shared the [[Crustacean larvae#Nauplius|Nauplius]] larva, identifying several parasitic species that had not been recognized as crustaceans. Müller also recognized that [[natural selection]] must act on larvae, just as it does on adults, giving the lie to recapitulation, which would require larval forms to be shielded from natural selection.<ref name=Gilbert2003/> Two of Haeckel's other ideas about the evolution of development have fared better than recapitulation: he argued in the 1870s that changes in the timing ([[heterochrony]]) and changes in the positioning within the body ([[heterotopy]]) of aspects of embryonic development would drive evolution by changing the shape of a descendant's body compared to an ancestor's. It took a century before these ideas were shown to be correct.<ref name="ReferenceA">{{Cite journal |last=Hall |first=B. K. |date=2003 |title=Evo-Devo: evolutionary developmental mechanisms |journal=International Journal of Developmental Biology |volume=47 |issue=7–8 |pages=491–495 |pmid=14756324}}</ref><ref>{{Cite book |last=Ridley |first=Mark |author-link=Mark Ridley (zoologist) |url=http://www.blackwellpublishing.com/ridley/ |title=Evolution |publisher=Wiley-Blackwell |year=2003 |isbn=978-1-4051-0345-9}}</ref>{{sfn|Gould|1977|pp=221–222}}

[[File:Giant Pufferfish skin pattern detail.jpg|thumb|upright=0.8|Turing's 1952 paper explained mathematically how patterns such as stripes and spots, as in the [[giant pufferfish]], may arise, without molecular evidence.<ref name="Turing 1952"/>]]

In 1917, [[D'Arcy Thompson]] wrote [[On Growth and Form|a book on the shapes of animals]], showing with simple [[Mathematical biology|mathematics]] how small changes to [[parameter]]s, such as the angles of a [[gastropod]]'s spiral shell, can radically alter [[Morphology (biology)|an animal's form]], though he preferred a mechanical to evolutionary explanation.<ref name="Ball">{{Cite journal |last=Ball |first=Philip |author-link=Philip Ball |date=7 February 2013 |title=In retrospect: On Growth and Form |journal=Nature |volume=494 |issue=32–33 |pages=32–33 |bibcode=2013Natur.494...32B |doi=10.1038/494032a |s2cid=205076253 |doi-access=free}}</ref><ref name="Shalizi">{{Cite web |last=Shalizi |first=Cosma |title=Review: The Self-Made Tapestry by Philip Ball |url=http://bactra.org/reviews/self-made-tapestry/ |access-date=14 October 2016 |publisher=University of Michigan}}</ref> But without molecular evidence, progress stalled.<ref name=Gilbert2003/>

In 1952, [[Alan Turing]] published his paper "[[The Chemical Basis of Morphogenesis]]", on the development of patterns in animals' bodies. He suggested that [[morphogenesis]] could be explained by a [[reaction–diffusion system]], a system of reacting chemicals able to diffuse through the body.<ref name="Turing 1952">{{Cite journal |last=Turing |first=Alan M. |author-link=Alan Turing |date=14 August 1952 |title=The Chemical Basis of Morphogenesis |journal=Philosophical Transactions of the Royal Society of London B |volume=237 |pages=37–72 |bibcode=1952RSPTB.237...37T |doi=10.1098/rstb.1952.0012 |s2cid=120437796 |number=641}}</ref> He modelled catalysed chemical reactions using [[partial differential equations]], showing that patterns emerged when the chemical reaction produced both a [[Catalysis|catalyst]] (A) and an [[enzyme inhibitor|inhibitor]] (B) that slowed down production of A. If A and B then diffused at different rates, A dominated in some places, and B in others. The Russian biochemist [[Boris Pavlovich Belousov|Boris Belousov]] had run experiments with similar results, but was unable to publish them because scientists thought at that time that creating visible order violated the [[second law of thermodynamics]].<ref>{{Cite book |last=Gribbin |first=John |author-link=John Gribbin |title=Deep Simplicity |publisher=Random House |year=2004 |page=126}}</ref>

===The modern synthesis of the early 20th century===
{{Main|Modern synthesis (20th century)}}

In the so-called [[Modern synthesis (20th century)|modern synthesis]] of the early 20th century, between 1918 and 1930 [[Ronald Fisher]] brought together Darwin's theory of [[evolution]], with its insistence on natural selection, [[heredity]], and [[genotype|variation]], and [[Gregor Mendel]]'s [[Mendelian inheritance|laws of genetics]] into a coherent structure for [[evolutionary biology]]. Biologists assumed that an organism was a straightforward reflection of its component genes: the genes coded for proteins, which built the organism's body. Biochemical pathways (and, they supposed, new species) evolved through [[mutation]]s in these genes. It was a simple, clear and nearly comprehensive picture: but it did not explain embryology.<ref name=Gilbert2003/><ref>{{Cite journal |last=Bock |first=Walter J. |date=July 1981 |title=Reviewed Work: ''The Evolutionary Synthesis. Perspectives on the Unification of Biology'' |journal=[[The Auk]] |volume=98 |issue=3 |pages=644–646 |jstor=4086148}}</ref> [[Sean B. Carroll]] has commented that had evo-devo's insights been available, embryology would certainly have played a central role in the synthesis.<ref name=Carroll_2008/>

The evolutionary embryologist [[Gavin de Beer]] anticipated evolutionary developmental biology in his 1930 book ''[[Embryos and Ancestors]]'',<ref name="Held">{{Cite book |last=Held |first=Lewis I. |author-link=Lewis I. Held |title=How the Snake Lost its Legs. Curious Tales from the Frontier of Evo-Devo |title-link=How the Snake Lost its Legs |date=2014 |publisher=[[Cambridge University Press]] |isbn=978-1-107-62139-8 |page=67}}</ref> by showing that evolution could occur by [[heterochrony]],<ref>{{harvnb|Gould|1977|pp=221–222}}</ref> such as in [[paedomorphosis|the retention of juvenile features in the adult]].<ref name="ReferenceA" /> This, de Beer argued, could cause apparently sudden changes in the [[fossil record]], since embryos fossilise poorly. As the gaps in the fossil record had been used as an argument against Darwin's gradualist evolution, de Beer's explanation supported the Darwinian position.<ref>{{Cite journal |last=Brigandt |first=Ingo |year=2006 |title=Homology and heterochrony: the evolutionary embryologist Gavin Rylands de Beer (1899-1972) |url=https://www.ualberta.ca/~brigandt/de_Beer.pdf |journal=[[Journal of Experimental Zoology]] |volume=306B |issue=4 |pages=317–328 |bibcode=2006JEZB..306..317B |doi=10.1002/jez.b.21100 |pmid=16506229}}</ref> However, despite de Beer, the modern synthesis largely ignored embryonic development to explain the form of organisms, since population genetics appeared to be an adequate explanation of how forms evolved.<ref name="Gilbert1991">{{Cite journal |last=Gilbert |first=S. F. |last2=Opitz |first2=J. M. |last3=Raff |first3=R. A. |date=1996 |title=Resynthesizing evolutionary and developmental biology |journal=Developmental Biology |volume=173 |issue=2 |pages=357–372 |doi=10.1006/dbio.1996.0032 |pmid=8605997 |doi-access=free}}</ref><ref>{{Cite book |last=Adams, M. |title=New Perspectives in Evolution |date=1991 |publisher=Liss/Wiley |editor-last=Warren, L. |pages=37–63 |chapter=Through the looking glass: The evolution of Soviet Darwinism |editor-last2=Koprowski, H.}}</ref>{{efn|Though [[C. H. Waddington]] had called for embryology to be added to the synthesis in his 1953 paper "Epigenetics and Evolution".<ref name="Smocovitis153">{{harvnb|Smocovitis|1996|page=153}}</ref>}}

===The lac operon===
{{Main|Lac operon}}

[[File:Lac Operon.svg|thumb|275px|The [[lac operon]]. Top: repressed. Bottom: active. (1) [[RNA Polymerase]], (2) [[lac repressor|Repressor]], (3) [[Promoter (genetics)|Promoter]], (4) Operator, (5) [[Lactose]], (6–8) [[structural gene|protein-encoding genes]], controlled by the switch, that cause lactose to be digested.]]

In 1961, [[Jacques Monod]], [[Jean-Pierre Changeux]] and [[François Jacob]] discovered the [[lac operon]] in the [[bacterium]] ''[[Escherichia coli]]''. It was a cluster of [[gene]]s, arranged in a feedback [[control loop]] so that its products would only be made when "switched on" by an environmental stimulus. One of these products was [[β-galactosidase|an enzyme that splits a sugar]], lactose; and [[lactose]] itself was the stimulus that switched the genes on. This was a revelation, as it showed for the first time that genes, even in organisms as small as a bacterium, are subject to precise control. The implication was that many other genes were also elaborately regulated.<ref>{{Cite journal |last=Monod |first=Jacques |author-link=Jacques Monod |last2=Changeux |first2=J.P. |last3=Jacob |first3=François |author-link3=François Jacob |year=1963 |title=Allosteric proteins and cellular control systems |journal=Journal of Molecular Biology |volume=6 |issue=4 |pages=306–329 |doi=10.1016/S0022-2836(63)80091-1 |pmid=13936070}}</ref>

===The birth of evo-devo and a second synthesis===
In 1977, a revolution in thinking about evolution and developmental biology began, with the arrival of [[recombinant DNA]] technology in [[genetics]], the book ''Ontogeny and Phylogeny'' by [[Stephen J. Gould]] and the paper [[Evolutionary tinkering|"Evolution and Tinkering"]]<ref name="Jacob 1977">{{Cite journal |last=Jacob |first=François |date=10 June 1977 |title=Evolution and Tinkering |journal=Science |volume=196 |issue=4295 |pages=1161–1166 |bibcode=1977Sci...196.1161J |doi=10.1126/science.860134 |pmid=860134}}</ref> by [[François Jacob]]. Gould laid to rest Haeckel's interpretation of evolutionary embryology, while Jacob set out an alternative theory.<ref name=Gilbert2003/> 
This led to [[extended evolutionary synthesis|a second synthesis]],<ref>{{Cite journal |last=Gilbert |first=S.F. |last2=Opitz |first2=J.M. |last3=Raff |first3=R.A. |date=1996 |title=Resynthesizing Evolutionary and Developmental Biology |journal=Developmental Biology |volume=173 |issue=2 |pages=357–372 |doi=10.1006/dbio.1996.0032 |pmid=8605997 |doi-access=free}}</ref><ref>{{Cite journal |last=Müller |first=G. B. |author-link=Gerd B. Müller |date=2007 |title=Evo–devo: extending the evolutionary synthesis |journal=Nature Reviews Genetics |volume=8 |issue=12 |pages=943–949 |doi=10.1038/nrg2219 |pmid=17984972 |s2cid=19264907}}</ref> at last including embryology as well as [[molecular genetics]], phylogeny, and evolutionary biology to form evo-devo.<ref>{{Cite journal |last=Goodman |first=C. S. |last2=Coughlin |first2=B. C. |year=2000 |editor2-last=Coughlin B. S. |title=Special feature: The evolution of evo-devo biology |journal=[[Proceedings of the National Academy of Sciences]] |volume=97 |issue=9 |pages=4424–4456 |bibcode=2000PNAS...97.4424G |doi=10.1073/pnas.97.9.4424 |pmc=18255 |pmid=10781035 |doi-access=free |editor1=Goodman, C. S.}}</ref><ref>{{Cite journal |last=[[Gerd Müller (theoretical biologist)|Müller GB]] and [[Stuart Newman|Newman SA]] (Eds.) |year=2005 |title=Special issue: Evolutionary Innovation and Morphological Novelty |url=http://www3.interscience.wiley.com/cgi-bin/jissue/112149101 |journal=Journal of Experimental Zoology Part B |volume=304B |issue=6 |pages=485–631 |doi=10.1002/jez.b.21080 |pmid=16252267 |archive-url=https://archive.today/20121211055145/http://www3.interscience.wiley.com/cgi-bin/jissue/112149101 |archive-date=2012-12-11}}</ref> In 1978, [[Edward B. Lewis]] discovered [[homeosis|homeotic]] genes that regulate embryonic development in ''[[Drosophila]]'' fruit flies, which like all insects are [[arthropods]], one of the major [[Phylum|phyla]] of invertebrate animals.<ref name="Palmer 2004">{{Cite journal |last=Palmer |first=R.A. |year=2004 |title=Symmetry breaking and the evolution of development |journal=[[Science (journal)|Science]] |volume=306 |issue=5697 |pages=828–833 |bibcode=2004Sci...306..828P |citeseerx=10.1.1.631.4256 |doi=10.1126/science.1103707 |pmid=15514148 |s2cid=32054147}}</ref> 
[[Bill McGinnis]] quickly discovered homeotic gene sequences, [[homeobox]]es, in animals in other phyla, in [[vertebrate]]s such as [[frog]]s, [[bird]]s, and [[mammals]]; they were later also found in [[fungi]] such as [[yeast]]s, and in [[plant]]s.<ref name=Winchester/><ref>{{Cite web |last=Bürglin |first=Thomas R. |title=The Homeobox Page |url=http://homeobox.biosci.ki.se/ |access-date=13 October 2016 |publisher=[[Karolinska Institutet]]}}</ref> There were evidently strong similarities in the genes that controlled development across all the [[eukaryote]]s.<ref>{{Cite journal |last=Holland |first=P.W. |date=2013 |title=Evolution of homeobox genes |journal=Wiley Interdiscip Rev Dev Biol |volume=2 |issue=1 |pages=31–45 |doi=10.1002/wdev.78 |pmid=23799629 |s2cid=44396110 |quote=Homeobox genes are found in almost all eukaryotes, and have diversified into 11 gene classes and over 100 gene families in animal evolution, and 10 to 14 gene classes in plants.}}</ref>
In 1980, [[Christiane Nüsslein-Volhard]] and [[Eric Wieschaus]] described [[gap gene]]s which help to create the segmentation pattern in [[Drosophila embryogenesis|fruit fly embryos]];<ref name="Nusslein">{{Cite journal |last=Nüsslein-Volhard, C. |last2=Wieschaus, E. |date=October 1980 |title=Mutations affecting segment number and polarity in ''Drosophila'' |journal=Nature |volume=287 |issue=5785 |pages=795–801 |bibcode=1980Natur.287..795N |doi=10.1038/287795a0 |pmid=6776413 |s2cid=4337658}}</ref><ref name="Arthur2002">{{Cite journal |last=Arthur |first=Wallace |date=14 February 2002 |title=The emerging conceptual framework of evolutionary developmental biology |journal=Nature |volume=415 |issue=6873 |pages=757–764 |bibcode=2002Natur.415..757A |doi=10.1038/415757a |pmid=11845200 |s2cid=4432164}}</ref> they and Lewis won a [[Nobel Prize in Physiology or Medicine|Nobel Prize]] for their work in 1995.<ref name="Winchester">{{Cite journal |last=Winchester |first=Guil |year=2004 |title=Edward B. Lewis 1918-2004 |url=http://www.cell.com/current-biology/pdf/S0960-9822(04)00683-9.pdf |publication-date=Sep 21, 2004 |volume=14 |issue=18 |pages=R740–742 |doi=10.1016/j.cub.2004.09.007 |pmid=15380080 |s2cid=32648995 |doi-access=free |periodical=Current Biology}}</ref><ref>{{Cite web |title=Eric Wieschaus and Christiane Nüsslein-Volhard: Collaborating to Find Developmental Genes |url=https://www.ibiology.org/ibiomagazine/eric-wieschaus-and-christiane-nusselin-volhard.html |url-status=dead |archive-url=https://web.archive.org/web/20161013223611/https://www.ibiology.org/ibiomagazine/eric-wieschaus-and-christiane-nusselin-volhard.html |archive-date=13 October 2016 |access-date=13 October 2016 |publisher=iBiology}}</ref>

Later, more specific similarities were discovered: for example, the [[distal-less]] gene was found in 1989 to be involved in the development of appendages or limbs in fruit flies,<ref>{{Cite journal |last=Cohen, S. M. |last2=Jurgens, G. |date=1989 |title=Proximal-distal pattern formation in Drosophila: cell autonomous requirement for Distal-less activity in limb development |journal=EMBO J. |volume=8 |issue=7 |pages=2045–2055 |doi=10.1002/j.1460-2075.1989.tb03613.x |pmc=401088 |pmid=16453891}}</ref> the fins of fish, the wings of chickens, the [[parapodia]] of marine [[annelid]] worms, the ampullae and siphons of tunicates, and the [[tube feet]] of [[sea urchin]]s. It was evident that the gene must be ancient, dating back to the [[Urbilaterian|last common ancestor of bilateral animals]] (before the [[Ediacaran]] Period, which began some 635 million years ago). Evo-devo had started to uncover the ways that all animal bodies were built during development.<ref>{{Cite book |last=Carroll |first=Sean B. |author-link=Sean B. Carroll |title=Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom |title-link=Endless Forms Most Beautiful (book) |date=2006 |publisher=Weidenfeld & Nicolson [Norton] |isbn=978-0-297-85094-6 |pages=63–70 |orig-date=2005}}</ref><ref>{{Cite journal |last=Panganiban |first=G. |last2=Irvine |first2=S. M. |last3=Lowe |first3=C. |last4=Roehl |first4=H. |last5=Corley |first5=L. S. |last6=Sherbon |first6=B. |last7=Grenier |first7=J. K. |last8=Fallon |first8=J. F. |last9=Kimble |first9=J. |last10=Walker |first10=M. |last11=Wray |first11=G. A. |last12=Swalla |first12=B. J. |last13=Martindale |first13=M. Q. |last14=Carroll |first14=S. B. |year=1997 |title=The origin and evolution of animal appendages |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=94 |issue=10 |pages=5162–5166 |bibcode=1997PNAS...94.5162P |doi=10.1073/pnas.94.10.5162 |pmc=24649 |pmid=9144208 |doi-access=free}}</ref>