Cephalization

File:Bilaterian body plan.svgn body plan. With a cylindrical body (in the main clade, the nephrozoa) and a direction of travel, the animal has head and tail ends, favouring cephalization by natural selection. Sense organs, brain, and mouth form the basis of the head.]]

{{further|Bilateria}}

Cephalization is both a characteristic feature of any animal that habitually moves in one direction, thereby gaining a front end, and an evolutionary trend which created the head of these animals. In practice, this primarily means the bilaterians, a large group containing the majority of animal phyla.{{cite web |title=Trends in evolution |url=https://evolution.berkeley.edu/evolibrary/article/evo_54 |publisher=University of California Museum of Paleontology |access-date=10 January 2019}} These have the ability to move, using muscles, and a body plan with a front end that encounters stimuli first as the animal moves forwards, and accordingly has evolved to contain many of the body's sense organs, able to detect light, chemicals, and gravity. There is often a collection of nerve cells able to process the information from these sense organs, forming a brain in several phyla and one or more ganglia (clusters of nerve cells) in others.{{Cite book |url=http://www.sinauer.com/media/wysiwyg/samples/Brusca3e_Chapter_9.pdf |chapter=Introduction to the Bilateria and the Phylum Xenacoelomorpha: Triploblasty and Bilateral Symmetry Provide New Avenues for Animal Radiation |title=Invertebrates |last=Brusca |first=Richard C. |date=2016 |publisher=Sinauer Associates |pages=345–372 |isbn=978-1605353753}}

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.{{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]{{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}} 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.{{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}}{{cite Q |Q24614721}}{{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}}

{{Clade |style= font-size:90%; line-height:90%

|label1=Planulozoa |sublabel1=680 mya

|1={{Clade

|1= Cnidaria 50 px

|label2= Bilateria |sublabel2= Cephalization

|2={{Clade

|1= Acoela etc 60 px

|label2= Nephrozoa |sublabel2=650 mya

|2={{clade

|label1=Deuterostomia

|1={{clade

|label1=Echinoderms

|1=55 px

|label2=Chordates

|2={{clade

|1={{clade

|label1=Tunicates (larva)

|1= 70 px

|label2=Vertebrates |sublabel2= Large brains

|2= 100 px

}}

}}

}}

|label2=Protostomia |sublabel2=610 mya

|2={{Clade

|label1=Ecdysozoa

|1={{clade

|label1=Nematodes etc

|1= 70 px

|label2=Arthropods |sublabel2=Running legs

|2= 100 px

}}

|label2=Spiralia

|2={{Clade

|label1=Flatworms etc

|1= 60 px

|label2=Cephalopods |sublabel2=Active arms

|2= 70 px

}}

}}

}}

}}

}}

}}

{{further|Arthropod head problem}}

In arthropods, 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. Insects 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 and abdomen, the parts of the trunk behind the head. The insect head is an elaborate structure made of several segments fused rigidly together, and equipped with both simple and compound eyes, and multiple appendages including sensory antennae and complex mouthparts (maxillae and mandibles).

File:Cuttlefish eye.jpgs like this cuttlefish have advanced 'camera' eyes. The cuttlefish has a W-shaped pupil.]]

{{further|Cephalopod}}

Cephalopods including the octopus, squid, cuttlefish and nautilus are the most intelligent of molluscs.{{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}} They are highly cephalized,{{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}} with well-developed senses, including advanced 'camera' eyes and large brains.{{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}}

{{further|Head}}

Cephalization in vertebrates, the group that includes mammals, birds, reptiles, amphibians and fishes, has been studied extensively. The heads of vertebrates are complex structures, with distinct sense organs for sight, olfaction, and hearing,{{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.}} and a large, multi-lobed brain protected by a skull of bone or cartilage. Cephalochordates like the lancelet (Amphioxus), a small fishlike animal with very little cephalization, are closely related to vertebrates but do not have these structures.{{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.}}{{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}} 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 placodes (thickened areas of the embryonic ectoderm layer), which result in the formation of all sense organs outside the brain.{{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 }}{{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 }} 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.{{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 |department=Research |page=171 |type=paper |date=11 December 2014 |title=Evolution: How vertebrates got a head |volume=516 |issue=7530}} Advanced vertebrates have increasingly elaborate brains.

File:Vertebrate body plan.svg

Bilaterians have many more Hox genes controlling the development, including of the front of the body than do the less cephalized Cnidaria (two Hox clusters) and the Acoelomorpha (three Hox clusters). In the vertebrates, duplication resulted in the four Hox clusters (HoxA to HoxD) of mammals and birds, while another duplication gave teleost fishes eight Hox clusters. Some of these genes, those responsible for the front (anterior) of the body, helped to create the heads of both arthropods and vertebrates. However, the Hox1-5 genes were already present in ancestral arthropods and vertebrates that did not have complex head structures. The Hox genes therefore most likely assisted in cephalization of these two bilaterian groups independently by convergent evolution, resulting in similar gene networks.{{cite journal |last=Hombría |first=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}}

File:Yellow papillae flatworm (Thysanozoon nigropapillosum) (cropped) (cropped).jpg, is somewhat cephalized, with a distinct head end (at right) which has pseudotentacles and an photoreceptive eyespot.]]

The Acoela are basal bilaterians, part of the Xenacoelomorpha. They are small and simple animals with flat bodies. They have slightly more nerve cells at the head end than elsewhere, not forming a distinct and compact brain. This represents an early stage in cephalization.{{cite book |last=Çabej |first=Nelson |chapter=Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution |title=Building the most complex structure on Earth: an epigenetic narrative of development and evolution of animals |publisher=Elsevier |year=2013 |isbn=978-0-12-401667-5 |pages=239–298|doi=10.1016/B978-0-12-401667-5.00005-5}}

Also among the bilaterians, Platyhelminthes (flatworms) have a more complex nervous system than the Acoela, and are lightly cephalized, for instance having an eyespot above the brain, near the front end.

Among animals without bilateral symmetry, the Cnidaria, such as the radially symmetrical (roughly cylindrical) Hydrozoa, show some degree of cephalization. The Anthomedusae have a head end with their mouth, photoreceptor cells, and a concentration of nerve cells.{{cite book |last1=Satterlie |first1=Richard |editor1-first=John H |editor1-last=Byrne |title=The Oxford Handbook of Invertebrate Neurobiology |chapter=Cnidarian Neurobiology |volume=1 |pages=184–218 |publisher=Oxford University Press |doi=10.1093/oxfordhb/9780190456757.013.7 |date=February 2017 |quote=Ocelli located at the base of the many tentacles represent one input to the B system, whereas the neurons of the O system are directly photosensitive. Many hydromedusae have ocelli of different levels of complexity (Singla, 1974). In addition, other marginal sensory structures associated with the outer nerve ring include statocysts (Singla, 1975), and mechanoreceptors, such as the tactile combs of Aglantha, which are located at the tentacle bases, and can activate the escape swimming circuitry (Arkett & Mackie, 1988; Mackie, 2004b). |isbn=9780190456757}}

• Organogenesis
• Phylogenetics

{{reflist|30em}}

Category:Evolutionary biology

Category:Evolutionary biology terminology