Implications of the “initial brain” concept for brain evolution in Cetacea

Abstract

We review the evidence for the concept of the “initial” or prototype brain. We outline four possible modes of brain evolution suggested by our new findings on the evolutionary status of the dolphin brain. The four modes involve various forms of deviation from and conformity to the hypothesized initial brain type. These include examples of conservative evolution, progressive evolution, and combinations of the two in which features of one or the other become dominant. The four types of neocortical organization in extant mammals may be the result of selective pressures on sensory/motor systems resulting in divergent patterns of brain phylogenesis. A modular “modification/multiplication” hypothesis is proposed as a mechanism of neocortical evolution in eutherians. Representative models of the initial ancestral group of mammals include not only extant basal Insectivora but also Chiroptera; we have found that dolphins and large whales have also retained many features of the archetypal or initial brain. This group evolved from the initial mammalian stock and returned to the aquatic environment some 50 million years ago. This unique experiment of nature shows the effects of radical changes in environment on brain-body adaptations and specializations. Although the dolphin brain has certain quantitative characteristics of the evolutionary changes seen in the higher terrestrial mammals, it has also retained many of the conservative structural features of the initial brain. Its neocortical organization is accordingly different, largely in a quantitative sense, from that of terrestrial models of the initial brain such as the hedgehog.

Mirror self-recognition in the bottlenose dolphin: A case of cognitive convergence

“…Bottlenose dolphins, great apes, and humans all possess high degrees of encephalization and neocortical expansion (26,33,34). Yet the brains of dolphins are markedly different from those of primates on many levels, including cortical cytoarchitecture and organization (33,34), reflecting the fact that the cetacean (dolphin, whale, and porpoise) and primate ancestral lines diverged at least 65-70 million years ago. The present findings imply that the emergence of self-recognition is not a byproduct of factors specific to great apes and humans but instead may be attributable to more general characteristics such as a high degree of encephalization and cognitive ability.…”

A Neurological Comparative Study of the Harp Seal (Pagophilus groenlandicus) and Harbor Porpoise (Phocoena phocoena) Brain

“…Also, the regional localization of different centers in the neocortex has changed. Compared with the visual and auditory cortices of the terrestrial mammalian brain, the two cortices lie next to each other in the parietal lobe (Supin et al, 1978) separated from their association cortices (Supin et al, 1978;Glezer et al, 1988). Whether this provides an advantage in processing auditory and visual input is unknown.…”

Neuroanatomy of the common dolphin (Delphinus delphis) as revealed by magnetic resonance imaging (MRI)

“…the lobular formations in the dolphin brain are organized in a pattern fundamentally different from that seen in the brains of primates or carnivores." As there is a 55-60 million year divergence between cetaceans and the phylogenetically closest group (the artiodactyls), odontocete brains represent a blend of early mammalian and uniquely derived features (Ridgway, 1986;Glezer et al, 1988;Ridgway, 1990;Manger et al, 1998). The differences between cetacean and other mammalian brains of similar size have been noted at the level of cortical cytoarchitecture and histochemistry (Garey et al, 1985;Garey and Leuba, 1986;Glezer et al, , 1992aGlezer et al, , b, 1993Glezer et al, , 1998Hof et al, 1992Hof et al, , 1995, cortical surface configuration (Jacobs et al, 1979;Morgane et al, 1980;Haug, 1987), and subcortical structural morphology (Tarpley and Ridgway, 1994;Glezer et al, 1995a, b).…”