Editing Cephalization (section)

==== Vertebrates ====

{{further|Head}}

Cephalization in [[vertebrate]]s, the group that includes [[mammals]], [[birds]], [[reptiles]], [[amphibians]] and [[fishes]], has been studied extensively.<ref name="Cabej2013"/> The heads of vertebrates are complex structures, with distinct sense organs for sight, olfaction, and hearing,<ref name="Schlosser 2006">{{cite journal |last=Schlosser |first=Gerhard |title=Induction and specification of cranial placodes |journal=Developmental Biology |volume=294 |issue=2 |date=2006 |doi=10.1016/j.ydbio.2006.03.009 |doi-access=free |pages=303–351 |quote=Vertebrates are distinguished from other deuterostomes by their specialized head with an elaborate brain encased in a cartilaginous or bony skull and with complex paired sense organs such as nose, eyes, and ears.}}</ref> and a large, multi-lobed brain protected by a skull of bone or [[cartilage]].<ref name="jandzik2014"/> [[Cephalochordate]]s like the [[lancelet]] (''Amphioxus''), a small fishlike animal with very little cephalization, are closely related to vertebrates but do not have these structures.<ref name="D'Aniello Bertrand 2023">{{cite journal |last=D'Aniello |first=Salvatore |last2=Bertrand |first2=Stephanie |last3=Escriva |first3=Hector |title=Amphioxus as a model to study the evolution of development in chordates |journal=eLife |volume=12 |date=2023-09-18 |pmid=37721204 |pmc=10506793 |doi=10.7554/eLife.87028 |doi-access=free |quote=some typical vertebrate characteristics are not present in amphioxus such as paired sensory organs (image-forming eyes or ears), paired appendages and migrating neural crest cells.}}</ref><ref name="Holland 2015">{{cite journal |last=Holland |first=L.Z. |year=2015 |title=The origin and evolution of chordate nervous systems |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=370 |issue=1684 |page=20150048 |doi=10.1098/rstb.2015.0048 |pmid=26554041 |pmc=4650125 |df=dmy-all}}</ref> In the 1980s, the [[new head hypothesis]] proposed that the vertebrate head is an evolutionary novelty resulting from the emergence of [[neural crest]] and cranial [[placode]]s (thickened areas of the embryonic [[ectoderm]] layer), which result in the formation of all sense organs outside the brain.<ref name="Gans Northcutt 1983">{{cite journal |last1=Gans |first1=C. |last2=Northcutt |first2=R. G. |s2cid=39290007 |year=1983 |title=Neural crest and the origin of vertebrates: a new head |journal=Science |volume=220 |issue=4594 |pages=268–273 |doi=10.1126/science.220.4594.268 |pmid=17732898 |bibcode=1983Sci...220..268G }}</ref><ref name="Diogo 2015">{{cite journal |last1=Diogo |first1=R. |display-authors=etal |year=2015 |title=A new heart for a new head in vertebrate cardiopharyngeal evolution |journal=Nature |volume=520 |issue=7548 |pages=466–473 |doi=10.1038/nature14435 |pmid=25903628 |pmc=4851342 |bibcode=2015Natur.520..466D }}</ref> However, in 2014, a transient [[larva]] tissue of the lancelet was found to be virtually indistinguishable from the [[neural crest]]-derived cartilage (which becomes [[bone]] in jawed animals) which forms the vertebrate [[skull]], suggesting that persistence of this tissue and expansion into the entire head space could be a viable evolutionary route to forming the vertebrate head.<ref name="jandzik2014">{{cite journal |doi=10.1038/nature14000 |title=Evolution of the new vertebrate head by co-option of an ancient chordate skeletal tissue |journal=Nature |date=26 February 2015 |last1=Jandzik |first1=D. |last2=Garnett |first2=A. T. |last3=Square |first3=T. A. |last4=Cattell |first4=M. V. |last5=Yu |first5=J. K. |last6=Medeiros |first6=D. M. |volume=518 |issue=7540 |pages=534–537 |pmid=25487155 |bibcode=2015Natur.518..534J |s2cid=4449267 |quote=a pronounced head that is supported and protected by a robust cellular endoskeleton}}  ''For lay summary see:'' {{cite journal |journal=[[Nature (journal)|Nature]] |department=Research |page=171 |type=paper |date=11 December 2014 |title=Evolution: How vertebrates got a head |volume=516 |issue=7530}}</ref> Advanced vertebrates have increasingly elaborate brains.<ref name="Cabej2013"/>

[[File:Vertebrate body plan.svg|thumb|center|upright=3|Idealised vertebrate body plan, showing brain and sense organs at the head end]]