Perspectives on brain evolution in primates

Abstract

Evolutionary Anatomy of the Primate Cerebral Cortexedited by Dean Falk and Kathleen R. Gibson, Cambridge University Press, 2001.£50 (344 pages)ISBN 0 521 64271 XThis timely volume comprises a series of papers that were presented at a symposium in honor of Harry J. Jerison at an annual meeting of the American Association of Physical Anthropologists. Jerison's classic monograph, Evolution of the Brain and Intelligence 1xJerison, H.J. See all References, established the precedent for subsequent studies of brain evolution by concentrating on one variable that is easily measured in both living and extinct mammals, namely brain size. We know from this early work that independent lines of evolution commonly lead to larger brains. But are separately evolved larger brains bigger and ‘better’ in similar or different ways? In this volume, arguments for both alternatives are convincingly presented, suggesting that brains have often evolved in similar ways while also specializing differently for specific functional roles.Larger brains probably resemble each other in overall organization for two basic reasons. First, there might be limited ways in which the course of development can be altered to produce differences in brain size. Thus, we repeatedly see the same types of enlargements. Most notably, larger brains have proportionately more neocortex, apparently because late-maturing areas grow even more significantly compared with early-maturing areas. Second, larger brains have more neurons and longer connection distances. Thus, it becomes increasingly difficult to maintain the connections of each neuron with the same proportion of other neurons, and maintain action potential conduction timing over longer distances. Therefore, similar methods of reorganizing brains to reduce these connection problems are probable.However, there is also convincing evidence from comparative studies of the brains of living mammals, and from brain casts of fossil skulls, that brains vary greatly both in global features and in local structural detail. At the global level, the two cerebral hemispheres of humans and chimpanzees differ in shape, supporting other types of evidence that there are functional differences between the hemispheres. At the local level, many differences in cortical microanatomy in homologous subdivisions of neocortex, such as primary visual cortex, are apparent across primate species. The chapters that address these issues convince the reader that further studies of both the overall trends and the many divergent specializations in brain evolution would be productive.This book will be of value to those interested in the evolution of different types and sizes of neocortex, the structure most responsible for making humans and mice what they are. But, more importantly, the book gives us a sense of where we are today and how far we need to go in understanding the evolution of primate brains from the small but still impressive brains of the earliest primates to those, including our own, extant today. As Jerison concludes in the book's last chapter, ‘more research is needed’.