Brain Evolution and Comparative Neuroanatomy

Abstract

Brains have many diverse forms in different animal groups, from complex layered structures to simple nerve nets. The range of different brain forms is reflective of the huge variety of environmental niches and lifestyles in which organisms have been successful. Both simple and elaborate brains are adaptive, depending on the selective pressures to which organisms are subjected. In vertebrates, a basic plan can be recognised across groups, especially in the brainstem, but the forebrain of birds and mammals is much more elaborate than those of other vertebrates. Elaborate nervous systems have developed independently in some molluscs, arthropods and vertebrates. Correlated with this elaboration is the capacity for complex learning, which is highly developed in some molluscs, arthropods and birds and mammals. The presence of elaborated brains in disparate groups illustrates that evolution of the brain has not shown a trend from simple to complex, but rather has occurred independently in different lineages.

Key Concepts:

  • Two structures are homologous if they can be traced to a common ancestor. Because brains do not fossilise, establishment of brain homologies requires as much information as possible about the similarities between brains of different groups.

  • There is no trend in the evolution of the brain. Specialisations in brain traits develop in response to the demands of various environmental selective pressures. Many animals with simple nervous systems are very successful.

  • Two major groups of invertebrates can be distinguished: those with radial symmetry and those with bilateral symmetry. Those with radial symmetry have simple nervous systems with a ring of neurons, while bilaterally symmetrical organisms have concentrations of neurons in the head region.

  • Octopi, insects and crustaceans have elaborate brains, well‐developed eyes and extensive learning ability, rivalling that of mammals.

  • The brains of the earliest chordate ancestors of vertebrates had bilateral eyes, a diencephalon and hindbrain.

  • All vertebrates have the same subdivisions of the brain: the hindbrain, midbrain and forebrain (the diencephalon and telencephalon). The hindbrain is relatively conservative, retaining a recognisable structure in spite of many variations, whereas the forebrain shows more variability among vertebrate groups.

  • Twenty‐five cranial nerves can be recognised in vertebrates, innervating muscles of the head and specialized sense organs, including lateral line organs and taste buds on the body surface.

  • The vertebrate telencephalon includes a dorsal pallial area which differentiates into the cerebral cortex in mammals, but there is disagreement about the homologous areas in nonmammals.

  • Brains have evolved to enable organisms to compete successfully in different environmental niches and the adaptations they show reflect the demands of those environments.

Keywords: evolution of brain; insect brain; nerve net; octopus; vertebrate brain; telencephalon; fish; reptile; bird; pallium

Figure 1.

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. Adapted from Ito et al. (), courtesy of Laura Powers.

Figure 2.

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.

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Powers, Alice S(Nov 2014) Brain Evolution and Comparative Neuroanatomy. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000088.pub3]