Editing Cephalization (section)

=== Complex active bodies ===

The philosopher Michael Trestman noted that three bilaterian phyla, namely the arthropods, the molluscs in the shape of the cephalopods, and the chordates, were distinctive in having "complex active bodies", something that the acoels and flatworms did not have. Any such animal, whether predator or prey, has to be aware of its environment—to catch its prey, or to evade its predators. These groups are exactly those that are most highly cephalized.<ref name="Trestman2013">{{cite journal |last=Trestman |first=Michael |s2cid=84629416 |title=The Cambrian Explosion and the Origins of Embodied Cognition |journal=Biological Theory |volume=8 |issue=1 |date=April 2013 |doi=10.1007/s13752-013-0102-6 |pages=80–92 }} Available [https://michaeltrestman.files.wordpress.com/2013/11/trestman-cambrian-pub.pdf on Trestman's website]</ref><ref>{{cite book |last=Godfrey-Smith |first=Peter |title=Other Minds: The Octopus and the Evolution of Intelligent Life |url=https://books.google.com/books?id=GeEZDQAAQBAJ  |year=2017 |publisher=HarperCollins Publishers |isbn=978-0-00-822628-2 |page=38}}</ref> These groups, however, are not closely related: in fact, they represent widely separated branches of the Bilateria, as shown on the [[phylogenetic tree]]; their lineages split hundreds of  millions of years ago. Other (less cephalized) phyla are omitted for clarity.<ref>{{cite journal |last1=Peterson |first1=Kevin J. |last2=Cotton |first2=James A. |last3=Gehling |first3=James G. |last4=Pisani |first4=Davide |date=27 April 2008 |title=The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records |journal=Philosophical Transactions of the Royal Society of London B: Biological Sciences  |volume=363 |issue=1496 |pages=1435–1443 |doi=10.1098/rstb.2007.2233 |pmid=18192191 |pmc=2614224}}</ref><ref>{{cite Q |Q24614721}}</ref><ref>{{cite web |title=Raising the Standard in Fossil Calibration |url=http://fossilcalibrations.org/ |website=Fossil Calibration Database |access-date=3 March 2018 |url-status=dead |archive-url=https://web.archive.org/web/20180307054141/http://fossilcalibrations.org/ |archive-date=7 March 2018}}</ref>

{{Clade |style= font-size:90%; line-height:90%
|label1=[[Planulozoa]] |sublabel1=680 mya
|1={{Clade
   |1= [[Cnidaria]] [[File:Aurelia aurita NASA.png|50 px]]
   |label2= [[Bilateria]] |sublabel2=''''' Cephalization '''''
   |2={{Clade
      |1= [[Acoela]] etc [[File:Neochildia fusca.jpg|60 px]]
      |label2= [[Nephrozoa]] |sublabel2=650 mya
      |2={{clade
         |label1=[[Deuterostomia]]
         |1={{clade
            |label1=[[Echinoderm]]s
            |1=[[File:Portugal 20140812-DSC01434 (21371237591).jpg|55 px]]
            |label2=[[Chordates]]
            |2={{clade
               |1={{clade
                  |label1=[[Tunicata|Tunicates]] (larva)
                  |1= [[File:Larval tunicate drawing.png|70 px]]
                  |label2='''[[Vertebrate]]s''' |sublabel2=''''' Large brains'''''
                  |2= [[File:White shark (Duane Raver).png|100 px]]
                  }}
               }}
            }}
         |label2=[[Protostomia]] |sublabel2=610 mya
         |2={{Clade
            |label1=[[Ecdysozoa]] 
            |1={{clade
               |label1=[[Nematode]]s etc
               |1= [[File:Female Phasmarhabditis hermaphrodita (cropped).jpg|70 px]]
               |label2='''[[Arthropod]]s''' |sublabel2='''''Running legs''''' 
               |2= [[File:Long nosed weevil edit.jpg|100 px]]
               }}
            |label2=[[Spiralia]]
            |2={{Clade
               |label1=[[Flatworm]]s etc
               |1= [[File:Sorocelis reticulosa.jpg|60 px]]
               |label2='''[[Cephalopod]]s''' |sublabel2='''''Active arms''''' 
               |2= [[File:Octopusvulgaris.jpg|70 px]]
               }}
            }}
         }}
      }}
   }}
}}

==== Arthropods ====

{{further|Arthropod head problem}}

In [[arthropod]]s, cephalization progressed with the gradual incorporation of trunk segments into the head region. This was advantageous because it allowed for the [[evolution]] of more effective mouth-parts for capturing and processing food. [[Insect]]s are strongly cephalized, their brain made of three fused [[ganglia]] attached to the [[ventral nerve cord]], which in turn has a pair of ganglia in each segment of the [[Thorax (arthropod anatomy)|thorax]] and [[Abdomen (insect anatomy)|abdomen]], the parts of the trunk behind the head. The [[insect head]] is an elaborate structure made of several [[Segmentation (biology)|segment]]s fused rigidly together, and equipped with both simple and [[compound eye]]s, and multiple [[appendage]]s including sensory [[Antenna (biology)|antennae]] and complex [[insect mouthparts|mouthparts]] (maxillae and mandibles).<ref name="Cabej2013"/>

[[File:Cuttlefish eye.jpg|thumb|upright=0.9|[[Cephalopod]]s like this [[cuttlefish]] have [[cephalopod eye|advanced 'camera' eyes]]. The cuttlefish has a W-shaped [[pupil]].]]

==== Cephalopods ====

{{further|Cephalopod}}

[[Cephalopod]]s including the [[octopus]], [[squid]], [[cuttlefish]] and [[nautilus]] are the most intelligent of [[mollusc]]s.<ref>{{cite web |title=The cephalopoda |url=https://ucmp.berkeley.edu/taxa/inverts/mollusca/cephalopoda.php |publisher=University of California Museum of Paleontology |access-date=19 February 2025}}</ref> They are highly cephalized,<ref>{{cite web |last1=Tublitz |first1=Nathan |title=Neural plasticity: a window into the complexity of the brain |url=http://www.ruf.rice.edu/~eivs/sympo/papers/Tublitz.pdf |publisher=University of Oregon |access-date=19 February 2025 |page=29 |date=March 2008 |quote=to the highly cephalized brain of cephalopods with functionally discrete regions}}</ref> with well-developed senses, including [[cephalopod eye|advanced 'camera' eyes]] and large brains.<ref>{{cite journal |last1=Nilsson |first1=Dan-E. |last2=Johnsen |first2=Sönke |last3=Warrant |first3=Eric |title=Cephalopod versus vertebrate eyes |journal=Current Biology |volume=33 |issue=20 |year=2023 |pages=R1100-R1105. |url=https://www.cell.com/current-biology/pdf/S0960-9822(23)00988-0.pdf |quote=Vertebrates and cephalopods are the two major animal groups that view the world through sophisticated camera-type eyes... also devote major parts of their brains to the processing of visual information.|doi=10.1016/j.cub.2023.07.049|doi-access=free}}</ref><!--<ref>{{cite book |last1=Solomon |first1=Eldra |last2=Berg |first2=Linda |last3=Martin |first3=Diana W. |title=Biology |url=https://books.google.com/books?id=qdQ8AAAAQBAJ&pg=PA884 |year=2010 |publisher=Cengage Learning |isbn=978-1-133-17032-7 |page=884}}</ref>-->

==== 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]]