Editing Bilateria (section)

=== Common ancestor ===

{{main|Urbilaterian}}

The hypothetical [[most recent common ancestor]] of all Bilateria is termed the '[[urbilaterian]]'. The nature of this first bilaterian is a matter of debate. One side suggests that acoelomates gave rise to the other groups (planuloid–aceloid hypothesis by [[Ludwig von Graff]], [[Elie Metchnikoff]], [[Libbie Hyman]], or {{ill|Luitfried von Salvini-Plawen|nl}}). This means that the urbilaterian had a solid body, and all body cavities therefore secondarily arose later in different groups. The other side poses that the urbilaterian had a coelom, meaning that the main acoelomate phyla ([[flatworm]]s and [[gastrotrich]]s) have secondarily lost their body cavities.<ref>{{cite journal |last1=Knoll |first1=Andrew H. |last2=Carroll |first2=Sean B. |author2-link=Sean B. Carroll |s2cid=8908451 |date=25 June 1999 |title=Early Animal Evolution: Emerging Views from Comparative Biology and Geology |journal=Science |volume=284 |issue=5423 |pages=2129–2137 |doi=10.1126/science.284.5423.2129 |pmid=10381872}}</ref><ref>{{cite journal |last1=Balavoine |first1=G. |last2=Adoutte |first2=Andre |title=The segmented Urbilateria: A testable scenario |journal=Integrative and Comparative Biology |date=2003 |volume=43 |issue=1 |pages=137–147 |doi=10.1093/icb/43.1.137 |pmid=21680418 |citeseerx=10.1.1.560.8727 |s2cid=80975506 }}</ref> 
This is the Archicoelomata hypothesis first proposed by A. T. Masterman in 1899.<ref name="Masterman 1899">{{cite journal |last=Masterman |first=A. T. |title=On the Theory of Archimeric Segmentation and its bearing upon the Phyletic Classification of the Cœlomata |journal=Proceedings of the Royal Society of Edinburgh |volume=22 |date=1899 |doi=10.1017/S0370164600051245 |pages=270–310}}</ref> Variations of the Archicoelomata hypothesis are the [[Gastraea]] by [[Ernst Haeckel]] in 1872<ref name="Levit 2022">{{cite journal |last1=Levit |first1=Georgy S. |last2=Hoßfeld |first2=Uwe |last3=Naumann |first3=Benjamin |last4=Lukas |first4=Paul |last5=Olsson |first5=Lennart |title=The biogenetic law and the Gastraea theory: From Ernst Haeckel's discoveries to contemporary views |journal=Journal of Experimental Zoology Part B: Molecular and Developmental Evolution |volume=338 |issue=1–2 |date=2022 |doi=10.1002/jez.b.23039 |doi-access=free |pages=13–27|pmid=33724681 |bibcode=2022JEZB..338...13L }}</ref> or [[Adam Sedgwick (zoologist)|Adam Sedgwick]], and more recently the Bilaterogastrea by {{Interlanguage link|Gösta Jägersten|sv}},<ref name="Olsson 2007">{{cite journal |last=Olsson |first=Lennart |title=A clash of traditions: the history of comparative and experimental embryology in Sweden as exemplified by the research of Gösta Jägersten and Sven Hörstadius |journal=Theory in Biosciences |volume=126 |issue=4 |date=2007 |doi=10.1007/s12064-007-0008-6 |pages=117–129|pmid=18008099 }}</ref> and the Trochaea by Claus Nielsen.<ref name="Nielsen 1985">{{cite journal |last=Nielsen |first=Claus |title=Animal phylogeny in the light of the trochaea theory |journal=Biological Journal of the Linnean Society |volume=25 |issue=3 |date=1985 |doi=10.1111/j.1095-8312.1985.tb00396.x |pages=243–299}}</ref>

[[File:Xenoturbella japonica.jpg|thumb|One view is that the original bilaterian was a marine worm somewhat like ''[[Xenoturbella]]''.]]

One proposal, by Johanna Taylor Cannon and colleagues, is that the original bilaterian was a bottom dwelling worm with a single body opening, similar to ''[[Xenoturbella]]''.<ref name=Cannon2016>{{Cite journal |last1=Cannon |first1=Johanna Taylor |last2=Vellutini |first2=Bruno Cossermelli |last3=Smith |first3=Julian |last4=Ronquist |first4=Fredrik |last5=Jondelius |first5=Ulf |last6=Hejnol |first6=Andreas |title=Xenacoelomorpha is the sister group to Nephrozoa |journal=Nature |volume=530 |issue=7588 |pages=89–93 |doi=10.1038/nature16520 |pmid=26842059 |year=2016 |bibcode=2016Natur.530...89C |s2cid=205247296 |url=http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-1844 }}</ref> An alternative proposal, by Jaume Baguñà and colleagues, is that it may have resembled the [[planula]] larvae of some [[cnidaria]]ns, which unlike the radially-symmetric adults have some bilateral symmetry.<ref>{{Cite journal |last1=Baguñà |first1=Jaume |last2=Martinez |first2=Pere |last3=Paps |first3=Jordi |last4=Riutort |first4=Marta |date=April 2008 |title=Back in time: a new systematic proposal for the Bilateria |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=363 |issue=1496 |pages=1481–1491 |doi=10.1098/rstb.2007.2238 |pmc=2615819 |pmid=18192186}}</ref> However, [[Lewis I. Held]] presents evidence that it was segmented, as the mechanism for creating segments is shared between vertebrates ([[deuterostome]]s) and arthropods ([[protostome]]s).<ref name="Held 2014">{{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=11 }}</ref>

Bilaterians, presumably including the urbilaterian, share many more [[Hox gene]]s [[Evolutionary developmental biology|controlling the development]] of their more complex bodies, including [[Cephalization|of their heads]], than do the Cnidaria and the Acoelomorpha.<ref name="Hombria 2021">{{cite journal |last1=Hombría |first1=James C.-G. |last2=García-Ferrés |first2=Mar |last3=Sánchez-Higueras |first3=Carlos |title=Anterior Hox Genes and the Process of Cephalization |journal=Frontiers in Cell and Developmental Biology |volume=9 |date=5 August 2021 |pmid=34422836 |pmc=8374599 |doi=10.3389/fcell.2021.718175 |doi-access=free}}<!--paper is CC-by-SA 4.0, could use image(s)--></ref>