Brain Evolution and Comparative Neuroanatomy

Abstract

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.