THE MAMMALIAN BRAIN AND THE ADAPTIVE ADVANTAGE OF CEREBRAL ASYMMETRY

During recent years there has been a remarkable resurgence of interest in questions relating t o the evolution of behavior. Biologists have always assumed that evolutionary considerations are essential for understanding the structure and function of living creatures. But psychologists, and particularly American psychologists, have, a t best, ignored the fact that animals evolved, and, a t worst, they have attempted t o demonstrate that an animal’s genotype (and particularly a human genotype, and thus the evolutionary history) is totally irrelevant for studying behavior. Such a position must necessarily argue that all the genetic variation for brain organization from which natural selection created the modern human brain suddenly disappeared some third of a million years ago when Homo sapiens first appeared, or else it must argue that genetic variations in neurologic structure have n o implications for behavior. The former argument not only is a t variance with known genetic principles, but also is contradicted by even the most cursory inspection of a few fetal brain specimens. The latter argument is mystical and has been refuted by observed co-variances between patterns of cerebral and behavioral organization, as for example, the association between handedness and the direction and degree of asymmetry in the size of the parietal o p er cu lum . I However, even if n o empirical data were available t o support the view that genetic and evolutionary factors are important for understanding behavior, theoretical considerations would force the conclusion. For any animal, behavior stands at the top of a hierarchy of organization (extending through the molecular, organic, and organismic) that serve t o preserve the animal’s life and assure the continuation of the species. Without a finely adapted behavioral organization t o coordinate the activities of lower-level systems, even if these systems had the most perfect mechanical and biochemical designs, survival and procreation would not be possible. Further, and most importantly, the more highly evolved is an animal, the more dependent is his biological fitness on behavior. For example, although the rate of heat loss from the body is slower for people with light than dark skins, a native Kenyan is as capable of surviving in Alaska as a Swede through the behavioral compensations he can and will make. In shaping the human family, the forces of natural selection have acted much more strongly in selecting adaptable behavior than in selecting uniquely human fixed structural traits. If information coded in the human genome guides the development of any aspect of man, then it surely guides those aspects that define his humanity. As Piaget has emphasized, in the primates and man, intelligence itself, and all the cognitive operations and behavioral patterns it implies, is a biological adaptation that allows its possessors t o construct schemas that encompass the entire spatiotemporal dimension of reality.2 It should be obvious, then, that those neuroanatomic organizations especially relevant t o the unique cognitive dynamics of man are the consequence of a bio-