Brain Evolution and Comparative Neuroanatomy


Evolution shapes the brain, as it does the body, to allow an organism to adapt to its ecological niche by building upon inherited, conserved traits and developing new, divergent ones. Recently, some investigators have proposed that brains may not have evolved from a single common ancestor, but rather may have evolved more than once. Therefore, molecules, cells and genes that evolved earlier for other purposes may have been co‐opted for use in building nervous systems independently. Prior to the emergence of bilaterians, invertebrate nervous systems displayed radial symmetric nerve net formations, but the beginnings of nervous system condensation were there. In bilaterian nervous systems we see great variety; invertebrates of this group display simple ganglion/nerve cord formations as well as highly elaborated brains. Vertebrates have the most elaborated brains of all metazoans with complex subdivisions, some of which are highly conserved and others highly variable.

Key Concepts

  • Brains have evolved to enable organisms to compete successfully in different environmental niches, and the adaptations they show reflect the demands of those environments.
  • Recently, some investigators have proposed that brains may not have evolved from a single common ancestor, but rather may have evolved more than once.
  • In this view, which is controversial, the common ancestor of all invertebrates did not have a nervous system, and the nervous system developed separately in Ctenophora, on the one hand, and Cnidaria and Bilateria on the other.
  • Some evidence suggests that, even within Bilateria, nervous systems may have evolved separately in different groups.
  • Attempts to understand the origins of the nervous system must take into account the possibility that similar characteristics in present‐day nervous systems are not homologous as derivatives of a structure in the common ancestor, but rather, they have arisen separately in different lineages because of exaptation of pre‐existing traits in a common ancestor.
  • Invertebrate brains show a wide variety of structures from nerve nets to condensed ganglia to fully elaborated brains.
  • All vertebrates have the same subdivisions of the brain: the hindbrain, midbrain and forebrain. The hindbrain is relatively conservative, retaining a recognisable structure in spite of many variations, whereas the forebrain shows more variability among vertebrate groups.
  • Elaborate nervous systems and the capacity for complex learning have developed independently in some molluscs, arthropods and vertebrates.
  • The presence of elaborated brains in disparate groups illustrates that evolution of the brain has not shown a trend from simple to complex across metazoan taxa, but rather has occurred as a result of diverse environmental pressures that have led to the same elaborate structures more than once.

Keywords: bilateria; definition of nervous system; evolution of nervous system; exaptation; homology; Hox genes; invertebrate brain; metazoans; radial glia; vertebrate brain

Figure 1. Phylogenetic relationship of metazoans based on Ryan et al., 2013. Figure by Annelysia Napoli. PhyloPic: Porifera. Source: From Mali'o Kodis, photograph by Derek Keats, under a CC BY 3.0. license. Public Domain., Platyhelminthes: Source: From Matthew Hooge (vectorized by T. Michael Keesey), under a CC3.0 Share‐Alike license. Public Domain.‐sa/3.0/, Mollusca: Source: From Stanton F. Fink (vectorized by T. Michael Keesey), under a CC 3.0 Share‐Alike license. Public Domain.‐sa/3.0/, Annelida: Source: From B. Duygu Özpolat under a Attribution‐NonCommercial‐ShareAlike 3.0. Public Domain.‐nc‐sa/3.0/, Crustacea: Source: From Almandine (vectorized by T. Michael Keesey), under a CC BY 3.0 Share‐like license. Public Domain.‐sa/3.0/
Figure 2. A cross section through the brain of an insect. The protocerebrum, deutocerebrum, tritocerebrum and optic lobes can be seen. The gnathal ganglia comprise the mandibular, maxillary and labial ganglia. Source: Adapted from Ito, K., Shinomiya, K., Ito, M., Armstrong, J. D., Boyan, G., Hartenstein, V., … Vosshall, L. B. (2014). A Systematic Nomenclature for the Insect Brain. Neuron, 81(4), 755–765. doi:10.1016/j.neuron.2013.12.017.
Figure 3. Lateral view of the brain of a generalised ray‐finned fish. Rostral is towards the left and dorsal is towards the top. The optic tract forms the lateral surface of the diencephalon, and the optic tectum, which is called the superior colliculus in mammals, is a major component of the mesencephalon. In this view, the optic tectum conceals the auditory relay part of the midbrain roof, called the torus semicircularis in many nonmammals and the inferior colliculus in mammals. The hindbrain includes the cerebellum and the brainstem caudal to the midbrain.


Albuixech‐Crespo B, Lopez‐Blanc L and Burguera D (2017) Molecular regionalization of the amphioxus neural tube challenges major partitions of the vertebrate brain. PLoS Biology 15 (4): e2001573.

Atwood HL and Klose MK (2010) Comparative biology of invertebrate neuromuscular junctions. In: Squire L (ed.) Encylopedia of Neuroscience, pp 1185–1209. Elsevier: Amsterdam.

Babonis LS and Martindale MQ (2014) Old cell, new trick? Cnidocytes as a model for the evolution of novelty. Integrative and Comparative Biology 54 (4): 714–722.

Bozorgmehr T, Ardiel EL, McEwan AH and Rankin CH (2013) Mechanisms of plasticity in a Caenorhabditis elegans mechanosensory circuit. Frontiers in Physiology 4 (88): 1–11.

Bruce LL and Neary TJ (1995) The limbic system of tetrapods: a comparative analysis of cortical and amygdalar populations. Brain Behavior and Evolution 46 (4–5): 224–234.

Butler AB and Hodos W (2005) Comparative Vertebrate Neuroanatomy: Evolution and Adaptation, 2nd edn. Wiley: Hoboken, NJ.

Butler AB, Reiner A and Karten HJ (2011) Evolution of the amniote pallium and the origins of mammalian neocortex. Annals of the New York Academy of Sciences 1225: 14–27.

Davies R, Gagen MH, Bull JC and Pope EC (2019) Maze learning and memory in a decapod crustacean. Biology Letters 215: 20190417.

Dugas‐Ford J, Rowell JJ and Ragsdale CW (2012) Cell‐type homologies and the origins of the neocortex. Proceedings of the National Academy of Sciences of the United States of America 109 (42): 16974–16979.

Firth BT, Christian KA, Belan I and Kennaway DJ (2010) Melatonin rhythms in the Australian freshwater crocodile (Crocodylus johnstoni): a reptile lacking a pineal complex? Journal of Comparative Physiology B 180 (1): 67–72.

Freudenmacher L, Schauer M, Walkowiak W and von Twickel A (2019) Refinement of the dopaminergic system of anuran amphibians based on connectivity with habenula, basal ganglia, limbic system, pallium, and spinal cord. Journal of Comparative Neurology. DOI: 10.1002/cne.24793.

Gans C and Northcutt RG (1983) Neural crest and the origin of vertebrates: a new head. Science 220 (4594): 268–274.

Hartenstein V (2019) Development of the nervous system of invertebrates. In: Byrne JH (ed.) The Oxford Handbook of Invertebrate Neurobiology, pp 1–88. Oxford University Press: Oxford.

Hartline DK (2011) The evolutionary origins of glia. Glia 59 (9): 1215–1236.

Hawkins RD, Kandel ER and Bailey CH (2006) Molecular mechanisms of memory storage in Aplysia. Biological Bulletin 210 (3): 174–191.

Helm C, Karl A, Beckers P, et al. (2017) Early evolution of radial glial cells in Bilateria. Proceedings of the Royal Society B: Biological Sciences 284 (1859): 20170743.

Hikosaka O (2010) The habenula: from stress evasion to value‐based decision‐making. Nature Reviews Neuroscience 11 (7): 503–513.

Hochner B and Glanzman DL (2016) Evolution of highly diverse forms of behavior in mdolluscs. Current Biology 26 (9): R965–R971.

Ito K, Shinomiya K, Ito M, et al. (2014) A systematic nomenclature for the insect brain. Neuron 81 (4): 755–765.

Katsuki T and Greenspan RJ (2013) Jellyfish nervous systems. Current Biology 23 (14): R592–R594.

Laberge F, Mühlenbrock‐Lenter S, Grunwald W and Roth G (2006) Evolution of the amygdala: new insights from studies in amphibians. Brain, Behavior, and Evolution 67 (4): 177–187.

Laberge F, Muhlenbrock‐Lenter S, Dicke U and Roth G (2008) Thalamo‐telencephalic pathways in the fire‐bellied toad Bombina orientalis. Journal of Comparative Neurology 508 (5): 806–823.

Mallatt J and Chen J‐Y (2003) Fossil sister group of craniates: predicted and found. Journal of Morphology 258: 1–31.

Medina L, Abellan A, Vicario A and Desfilis E (2014) Evolutionary and developmental contributions for understanding the organization of the basal ganglia. Brain, Behavior, and Evolution 83 (2): 112–125.

Menzel R (2012) The honeybee as a model for understanding the basis of cognition. Nature Reviews Neuroscience 13 (11): 758–768.

Montgomery JC, Bodznick D and Yopak KE (2012) The cerebellum and cerebellum‐like structures of cartilaginous fishes. Brain, Behavior, and Evolution 80 (2): 152–165.

Moroz LL (2009) On the independent origins of complex brains and neurons. Brain, Behavior, and Evolution 74 (3): 177–190.

Moroz LL (2015) Convergent evolution of neural systems in ctenophores. Journal of Experimental Biology 218 (4): 598–611.

Mueller T, Wullimann MF and Guo S (2008) Early teleostean basal ganglia development visualized by zebrafish Dlx2a, Lhx6, Lhx7, Tbr2 (eomesa), and GAD67 gene expression. Journal of Comparative Neurology 507 (2): 1245–1257.

Nauta WJH and Karten HJ (1970) A general profile of the vertebrate brain with sidelights on the ancestry of the cerebral cortex. In: Schmitt FO (ed.) The Neurosciences: Second Study Section, pp 1–26. Rockefeller University Press: New York, NY.

Northcutt RG (2011) Do teleost fishes possess a homolog of mammalian isocortex? Brain, Behavior, and Evolution 78 (2): 136–138.

Northcutt RG and Gonzalez A (2011) A reinterpretation of the cytoarchitectonics of the telencephalon of the Comoran coelacanth. Frontiers in Neuroanatomy 5 (9): 1–7.

Northcutt RG (2012) Evolution of centralized nervous systems: two schools of evolutionary thought. Proceedings of the National Academy of Sciences of the United States of America 109 (supplement 1): 10626–10633.

Perez‐Fernandez J, Stephenson‐Jones M, Suryanarayana SM, Robertson B and Grillner S (2014) Evolutionarily conserved organization of the dopaminergic system in lamprey: SNc/VTA afferent and efferent connectivity and D2 receptor expression. Journal of Comparative Neurology 522 (17): 3775–3794.

Puelles L, Kuwana E, Bulfone A, et al. (2000) Pallial and subpallial derivatives in the embryonic chick and mouse telencephalon, traced by the expression of the genes Dlx‐2, Emx‐1, Nkx‐2.1, Pax‐6, and Tbr‐1. Journal of Comparative Neurology 424 (3): 409–438.

Reiner A (2002) Functional circuitry of the avian basal ganglia: implications for basal ganglia organization in stem amniotes. Brain Research Bulletin 57 (3/4): 513–528.

Reiner O, Coquelle FM, Peter B, et al. (2006) The evolving doublecortin (DCX) superfamily. BMC Genomics 7 (188): 1–16.

Rodríguez F, López JC, Vargas JP, et al. (2002) Conservation of spatial memory function in the pallial forebrain of reptiles and ray‐finned fishes. Journal of Neuroscience 22 (7): 2894–2903.

Ryan JF, Pang K, Schnitzler CF, et al. (2013) The genome of the ctenophore Mnemiopsis leidl and its implications for cell type evolution. Science 342 (1684): 1242592.

Ryan JF and Chiodin M (2015) Where is my mind? How sponges and placozoans may have lost neural cell types. Philosophical Transactions of the Royal Society B: Biological Sciences 370 (1684): 20150059.

Satterlie RA (2015) Cnidarian nerve nets and neuromuscular efficiency. Integrative and Comparative Biology 55 (6): 1050–1057.

Stephenson‐Jones M, Floros O, Robertson B and Grillner S (2012) Evolutionary conservation of the habenular nuclei and their circuitry controlling the dopamine and 5‐hydroxytryptophan (5‐HT) systems. Proceedings of the National Academy of Sciences of the United States of America 109 (3): E164–E173.

Striedter GF (1997) The telencephalon of tetrapods in evolution. Brain, Behavior, and Evolution 49 (4): 179–213.

Sugahara F, Pascual‐Anaya J, Oisi Y, et al. (2016) Evidence from cyclostomes for complex regionalization of the ancestral vertebrate brain. Nature 531 (7592): 97–100.

Swanson LW and Petrovich GD (1998) What is the amygdala? Trends in Neurosciences 21 (8): 323–331.

Tan DX, Hardeland R, Manchester LC, et al. (2010) The changing biological roles of melatonin during eveolution: from an antioxidant to signals of darkness, sexual selection and fitness. Biological Reviews 85 (3): 607–623.

Turchetti‐Maya A, Shomrat T and Hochner B (2019) The vertical lobe of cephalopods: a brain structure ideal for exploring the mechanisms of complex forms of learning and memory. In: Byrne JH (ed.) The Oxford Handbook of Invertebrate Neurobiology, pp 1–25. Oxford University Press: Oxford.

Westfall JA (2004) Neural pathways and innervation of cnidocytes in tentacles of sea anemones. Hydrobiologia 530/531 (1–3): 117–121.

White JG, Southgate E, Thomson JN and Brenner S (1986) The structure of the nervous system of the nematode Caenorhabditis elegans. Philosophical Transactions of the Royal Society of London B Biological Sciences 314 (1165): 1–340.

Further Reading

Byrne JH (ed.) (2019) The Oxford Handbook of Invertebrate Neurobiology. Oxford University Press: Oxford.

Demski L (2013) Evolution of brain complexity and animal minds. 24th Annual Karger Workshop. Brain, Behavior, and Evolution 82 (1): 1–79.

Kaas JH (ed.) (2009) Evolutionary Neuroscience. Elsevier: Oxford.

Papini MR (2008) Comparative Psychology: Evolution and Development of Behavior, 2nd edn. Psychology Press: New York, NY.

Roth G (2013) The Long Evolution of Brains and Minds. Springer: Berlin.

Shepherd S (ed.) (2016) The Wiley Handbook of Evolutionary Neuroscience. Wiley: Hoboken, NJ.

Smarandache‐Wellman CR (2016) Arthropod neurons and nervous system. Current Biology 26 (20): R960–R965.

Strausfeld NJ (2012) Arthropod Brains: Evolution, Functional Elegance, and Historical Significance. Harvard University Press: Cambridge, MA.

Striedter GF, Avise JC and Ayala FJ (2012) In the light of evolution. VI. Brain and behavior. Proceedings of the National Academy of Sciences of the United States of America 109 (supplement 1): 10607–10740.

Striedter GF and Northcutt RG (2020) Brains Through Time. Oxford University Press: Oxford.

Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite close
Napoli, Amalia J, and Powers, Alice S(Jul 2020) Brain Evolution and Comparative Neuroanatomy. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000088.pub4]