Editing Head (section)

=== Vertebrates and the "new head hypothesis" ===
Though [[invertebrate]] [[chordate]]s – such as the [[tunicate]] larvae or the [[lancelet]]s – have heads, there has been a question of how the vertebrate head, characterized by a bony skull clearly separated from the main body, might have evolved from the head structures of these animals.<ref name="jandzik2014" />

According to Hyman (1979), the evolution of the head in the [[vertebrates]] has occurred by the fusion of a fixed number of anterior segments, in the same manner as in other "heteronomously segmented animals". In some cases, segments or a portion of the segments disappear. The head segments also lose most of their systems, except for the nervous system. With the progressive development of cephalization, "the head incorporates more and more of the adjacent segments into its structure, so that in general it may be said that the higher the degree of cephalization the greater is the number of segments composing the head".<ref name="Hyman1992">{{cite book|first=Libbie Henrietta|last=Hyman|title=Hyman's Comparative Vertebrate Anatomy|url=https://books.google.com/books?id=VKlWjdOkiMwC&pg=PA4|date=1979|publisher=University of Chicago Press|isbn=978-0-226-87013-7 |edition=3rd |pages=4. [1st ed., 1922; 2nd ed., 1942.; 3rd ed., 1979, reprinted in 1992.]}}</ref>

In the 1980s, the "new head hypothesis" was proposed, suggesting that the vertebrate head is an evolutionary novelty resulting from the emergence of [[neural crest]] and cranial [[placode]]s.<ref>{{Cite journal |last1=Gans |first1=Carl |last2=Northcutt |first2=R. Glenn |date=1983-04-15 |title=Neural Crest and the Origin of Vertebrates: A New Head |url=https://www.science.org/doi/10.1126/science.220.4594.268 |journal=Science |language=en |volume=220 |issue=4594 |pages=268–273 |doi=10.1126/science.220.4594.268 |pmid=17732898 |bibcode=1983Sci...220..268G |s2cid=39290007 |issn=0036-8075|url-access=subscription }}</ref><ref>{{Cite journal |last1=Diogo |first1=Rui |last2=Kelly |first2=Robert G. |last3=Christiaen |first3=Lionel |last4=Levine |first4=Michael |last5=Ziermann |first5=Janine M. |last6=Molnar |first6=Julia L. |last7=Noden |first7=Drew M. |last8=Tzahor |first8=Eldad |date=2015-04-23 |title=A new heart for a new head in vertebrate cardiopharyngeal evolution |journal=Nature |language=en |volume=520 |issue=7548 |pages=466–473 |doi=10.1038/nature14435 |issn=0028-0836 |pmc=4851342 |pmid=25903628|bibcode=2015Natur.520..466D }}</ref> In 2014, a transient [[larva]] tissue of the lancelet was found to be virtually indistinguishable from the neural crest-derived [[cartilage]] which forms the vertebrate skull, suggesting that persistence of this tissue and expansion into the entire headspace could be a viable evolutionary route to formation of the vertebrate head.<ref name="jandzik2014" />