Cephalization is an evolutionary trend in animals that, over a sufficient number of generations, concentrates the special sense organs and nerve ganglia towards the front of the body where the mouth is located, often producing an enlarged head. This is associated with the animal's movement direction and bilateral symmetry. Cephalization of the nervous system has led to the formation of a brain with varying degrees of functional centralization in three phyla of bilaterian animals, namely the arthropods, cephalopod molluscs, and vertebrates. Hox genes organise aspects of cephalization in the bilaterians.
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. 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.
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. 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.
| Planulozoa |
| |||||||||||||||||||||||||||||||||||||||||||||||||||
| 680 mya |
Arthropods
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).
Cephalopods
Cephalopods including the octopus, squid, cuttlefish and nautilus are the most intelligent of molluscs. They are highly cephalized, with well-developed senses, including advanced 'camera' eyes and large brains.
Vertebrates
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, 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. 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. 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. Advanced vertebrates have increasingly elaborate brains.
Anterior Hox genes
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.
Partly cephalized phyla
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.
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.
See also
- Organogenesis
- Phylogenetics
References
- Brusca, Richard C. (2016). "Introduction to the Bilateria and the Phylum Xenacoelomorpha: Triploblasty and Bilateral Symmetry Provide New Avenues for Animal Radiation". Invertebrates (PDF). Sinauer Associates. pp. 345–372. ISBN 978-1605353753.
- "Trends in evolution". University of California Museum of Paleontology. Retrieved 10 January 2019.
- Trestman, Michael (April 2013). "The Cambrian Explosion and the Origins of Embodied Cognition". Biological Theory. 8 (1): 80–92. doi:10.1007/s13752-013-0102-6. S2CID 84629416. Available on Trestman's website]
- Godfrey-Smith, Peter (2017). Other Minds: The Octopus and the Evolution of Intelligent Life. HarperCollins Publishers. p. 38. ISBN 978-0-00-822628-2.
- Peterson, Kevin J.; Cotton, James A.; Gehling, James G.; Pisani, Davide (27 April 2008). "The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records". Philosophical Transactions of the Royal Society of London B: Biological Sciences. 363 (1496): 1435–1443. doi:10.1098/rstb.2007.2233. PMC 2614224. PMID 18192191.
- Laura Wegener Parfrey; Daniel J G Lahr; Andrew H Knoll; Laura A Katz (16 August 2011). "Estimating the timing of early eukaryotic diversification with multigene molecular clocks" (PDF). Proceedings of the National Academy of Sciences of the United States of America. 108 (33): 13624–9. Bibcode:2011PNAS..10813624P. doi:10.1073/PNAS.1110633108. ISSN 0027-8424. PMC 3158185. PMID 21810989. Wikidata Q24614721.
- "Raising the Standard in Fossil Calibration". Fossil Calibration Database. Archived from the original on 7 March 2018. Retrieved 3 March 2018.
- Çabej, Nelson (2013). "Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution". Building the most complex structure on Earth: an epigenetic narrative of development and evolution of animals. Elsevier. pp. 239–298. doi:10.1016/B978-0-12-401667-5.00005-5. ISBN 978-0-12-401667-5.
- "The cephalopoda". University of California Museum of Paleontology. Retrieved 19 February 2025.
- Tublitz, Nathan (March 2008). "Neural plasticity: a window into the complexity of the brain" (PDF). University of Oregon. p. 29. Retrieved 19 February 2025.
to the highly cephalized brain of cephalopods with functionally discrete regions
- Nilsson, Dan-E.; Johnsen, Sönke; Warrant, Eric (2023). "Cephalopod versus vertebrate eyes" (PDF). Current Biology. 33 (20): R1100-R1105. doi:10.1016/j.cub.2023.07.049.
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.
- Schlosser, Gerhard (2006). "Induction and specification of cranial placodes". Developmental Biology. 294 (2): 303–351. doi:10.1016/j.ydbio.2006.03.009.
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.
- Jandzik, D.; Garnett, A. T.; Square, T. A.; Cattell, M. V.; Yu, J. K.; Medeiros, D. M. (26 February 2015). "Evolution of the new vertebrate head by co-option of an ancient chordate skeletal tissue". Nature. 518 (7540): 534–537. Bibcode:2015Natur.518..534J. doi:10.1038/nature14000. PMID 25487155. S2CID 4449267.
a pronounced head that is supported and protected by a robust cellular endoskeleton
For lay summary see: "Evolution: How vertebrates got a head". Research. Nature (paper). 516 (7530): 171. 11 December 2014. - D'Aniello, Salvatore; Bertrand, Stephanie; Escriva, Hector (2023-09-18). "Amphioxus as a model to study the evolution of development in chordates". eLife. 12. doi:10.7554/eLife.87028. PMC 10506793. PMID 37721204.
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.
- Holland, L.Z. (2015). "The origin and evolution of chordate nervous systems". Philosophical Transactions of the Royal Society B: Biological Sciences. 370 (1684): 20150048. doi:10.1098/rstb.2015.0048. PMC 4650125. PMID 26554041.
- Gans, C.; Northcutt, R. G. (1983). "Neural crest and the origin of vertebrates: a new head". Science. 220 (4594): 268–273. Bibcode:1983Sci...220..268G. doi:10.1126/science.220.4594.268. PMID 17732898. S2CID 39290007.
- Diogo, R.; et al. (2015). "A new heart for a new head in vertebrate cardiopharyngeal evolution". Nature. 520 (7548): 466–473. Bibcode:2015Natur.520..466D. doi:10.1038/nature14435. PMC 4851342. PMID 25903628.
- Hombría, James C.-G.; García-Ferrés, Mar; Sánchez-Higueras, Carlos (5 August 2021). "Anterior Hox Genes and the Process of Cephalization". Frontiers in Cell and Developmental Biology. 9. doi:10.3389/fcell.2021.718175. PMC 8374599. PMID 34422836.
- Satterlie, Richard (February 2017). "Cnidarian Neurobiology". In Byrne, John H (ed.). The Oxford Handbook of Invertebrate Neurobiology. Vol. 1. Oxford University Press. pp. 184–218. doi:10.1093/oxfordhb/9780190456757.013.7. ISBN 9780190456757.
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).
Cephalization is an evolutionary trend in animals that over a sufficient number of generations concentrates the special sense organs and nerve ganglia towards the front of the body where the mouth is located often producing an enlarged head This is associated with the animal s movement direction and bilateral symmetry Cephalization of the nervous system has led to the formation of a brain with varying degrees of functional centralization in three phyla of bilaterian animals namely the arthropods cephalopod molluscs and vertebrates Hox genes organise aspects of cephalization in the bilaterians A lobster is heavily cephalized with eyes antennae multiple mouthparts and the brain inside the armoured exoskeleton all concentrated at the animal s head end BilateriaIdealised bilaterian 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 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 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 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 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 Planulozoa CnidariaBilateria Acoela etcNephrozoa Deuterostomia EchinodermsChordates Tunicates larva Vertebrates Large brainsProtostomia Ecdysozoa Nematodes etcArthropodsRunning legsSpiralia Flatworms etcCephalopodsActive arms610 mya650 mya Cephalization 680 mya Arthropods 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 Cephalopods like this cuttlefish have advanced camera eyes The cuttlefish has a W shaped pupil Cephalopods Cephalopods including the octopus squid cuttlefish and nautilus are the most intelligent of molluscs They are highly cephalized with well developed senses including advanced camera eyes and large brains Vertebrates 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 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 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 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 Advanced vertebrates have increasingly elaborate brains Idealised vertebrate body plan showing brain and sense organs at the head endAnterior Hox genes 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 Partly cephalized phylaThe gold speckled flatworm Thysanozoon nigropapillosum 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 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 See alsoOrganogenesis PhylogeneticsReferencesBrusca Richard C 2016 Introduction to the Bilateria and the Phylum Xenacoelomorpha Triploblasty and Bilateral Symmetry Provide New Avenues for Animal Radiation Invertebrates PDF Sinauer Associates pp 345 372 ISBN 978 1605353753 Trends in evolution University of California Museum of Paleontology Retrieved 10 January 2019 Trestman Michael April 2013 The Cambrian Explosion and the Origins of Embodied Cognition Biological Theory 8 1 80 92 doi 10 1007 s13752 013 0102 6 S2CID 84629416 Available on Trestman s website Godfrey Smith Peter 2017 Other Minds The Octopus and the Evolution of Intelligent Life HarperCollins Publishers p 38 ISBN 978 0 00 822628 2 Peterson Kevin J Cotton James A Gehling James G Pisani Davide 27 April 2008 The Ediacaran emergence of bilaterians congruence between the genetic and the geological fossil records Philosophical Transactions of the Royal Society of London B Biological Sciences 363 1496 1435 1443 doi 10 1098 rstb 2007 2233 PMC 2614224 PMID 18192191 Laura Wegener Parfrey Daniel J G Lahr Andrew H Knoll Laura A Katz 16 August 2011 Estimating the timing of early eukaryotic diversification with multigene molecular clocks PDF Proceedings of the National Academy of Sciences of the United States of America 108 33 13624 9 Bibcode 2011PNAS 10813624P doi 10 1073 PNAS 1110633108 ISSN 0027 8424 PMC 3158185 PMID 21810989 Wikidata Q24614721 Raising the Standard in Fossil Calibration Fossil Calibration Database Archived from the original on 7 March 2018 Retrieved 3 March 2018 Cabej Nelson 2013 Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution Building the most complex structure on Earth an epigenetic narrative of development and evolution of animals Elsevier pp 239 298 doi 10 1016 B978 0 12 401667 5 00005 5 ISBN 978 0 12 401667 5 The cephalopoda University of California Museum of Paleontology Retrieved 19 February 2025 Tublitz Nathan March 2008 Neural plasticity a window into the complexity of the brain PDF University of Oregon p 29 Retrieved 19 February 2025 to the highly cephalized brain of cephalopods with functionally discrete regions Nilsson Dan E Johnsen Sonke Warrant Eric 2023 Cephalopod versus vertebrate eyes PDF Current Biology 33 20 R1100 R1105 doi 10 1016 j cub 2023 07 049 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 Schlosser Gerhard 2006 Induction and specification of cranial placodes Developmental Biology 294 2 303 351 doi 10 1016 j ydbio 2006 03 009 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 Jandzik D Garnett A T Square T A Cattell M V Yu J K Medeiros D M 26 February 2015 Evolution of the new vertebrate head by co option of an ancient chordate skeletal tissue Nature 518 7540 534 537 Bibcode 2015Natur 518 534J doi 10 1038 nature14000 PMID 25487155 S2CID 4449267 a pronounced head that is supported and protected by a robust cellular endoskeleton For lay summary see Evolution How vertebrates got a head Research Nature paper 516 7530 171 11 December 2014 D Aniello Salvatore Bertrand Stephanie Escriva Hector 2023 09 18 Amphioxus as a model to study the evolution of development in chordates eLife 12 doi 10 7554 eLife 87028 PMC 10506793 PMID 37721204 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 Holland L Z 2015 The origin and evolution of chordate nervous systems Philosophical Transactions of the Royal Society B Biological Sciences 370 1684 20150048 doi 10 1098 rstb 2015 0048 PMC 4650125 PMID 26554041 Gans C Northcutt R G 1983 Neural crest and the origin of vertebrates a new head Science 220 4594 268 273 Bibcode 1983Sci 220 268G doi 10 1126 science 220 4594 268 PMID 17732898 S2CID 39290007 Diogo R et al 2015 A new heart for a new head in vertebrate cardiopharyngeal evolution Nature 520 7548 466 473 Bibcode 2015Natur 520 466D doi 10 1038 nature14435 PMC 4851342 PMID 25903628 Hombria James C G Garcia Ferres Mar Sanchez Higueras Carlos 5 August 2021 Anterior Hox Genes and the Process of Cephalization Frontiers in Cell and Developmental Biology 9 doi 10 3389 fcell 2021 718175 PMC 8374599 PMID 34422836 Satterlie Richard February 2017 Cnidarian Neurobiology In Byrne John H ed The Oxford Handbook of Invertebrate Neurobiology Vol 1 Oxford University Press pp 184 218 doi 10 1093 oxfordhb 9780190456757 013 7 ISBN 9780190456757 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 amp Mackie 1988 Mackie 2004b
