Brains, maturation times, and parenting

Abstract

Finch and Sapolsky propose that the slow development of human infants and their consequent long period of dependency on their parents have favored the evolution of genes that retard brain senescence, specifically recently evolved variants of the apolipoprotein E gene. We examine here the probable reasons why human maturation is so slow, and the influence of this slow development on parental dependence and patterns of survival. Large brains are expensive in terms of energy, anatomic complexity, and the time required to reach particular stages of postnatal maturation. We hypothesize that the maturational time costs arise from the fact that the brain is unique among the organs of the body in requiring a great deal of interaction with the environment (learning experience) to achieve adult competence, and thus that the brain serves as a rate-limiting factor governing the maturation of the entire body. Although the brain achieves its adult size at an earlier age than the other organs of the body, it does not become structurally and functionally mature until some point after sexual maturity [30]. The classical studies of developmental myelination by Flechsig [14], [15] indicate that the brain matures slowly in stepwise hierarchies proceeding, for example, from the thalamus to the primary cortical sensory areas to the higher cortical areas of the temporal, parietal, and frontal lobes. Quartz and Sejnowski [33] have proposed that the brain builds sequentially from one level to the next on the basis of experience, and thus larger brains may require more time to mature, in part because they have more levels. We have examined the time costs associated with enlarged brains by analyzing the relationships between average brain size and the average times required to reach various stages of postnatal maturation, such as the eruption of various classes of teeth and reproductive maturity, in different primate species. Because both brain and developmental timing variables are related to body mass, we have first extracted the statistical effect of mass for each variable and then compared the residual values related to brain weight and maturation times (Fig. 1). The near identity of the five maturation timing relationships as a function of relative brain size illustrate the consistent, clock-like nature of these relationships (Fig. 2). It is remarkable that the times required to attain each of these maturational stages, which range from events occurring in infancy to the threshold of adulthood, are so similarly influenced by relative brain size. However, although the absolute times required by humans to reach any particular stage of maturation are longer than for any other primate, humans actually mature somewhat faster than would be expected for a primate of our brain size. We will return to this interesting point later in our discussion.