The model of energetics and pubertal timing outlined above suggests that, in the case of humans, energetic constraints on the simultaneous development of the brain and reproductive maturation acted as the primary selective force in the development of the maturation process beginning with adrenarche and marked by continuing increases in DHEA/S. It is difficult to be sure when changes in human life history evolved. In the scenario suggested here, changes in life history patterns associated increased size, slowed childhood growth, and changes in the relative metabolic costs of the gut versus the brain would have started with the origins of Homo ergaster (Aiello and Wells, 2002). Initially, the increased metabolic demands of the brain may have been met by an increase in caloric intake, involving dietary shift, changes in relative size of the gut, and slower childhood growth.
With further increases in brain size subsequent to Homo ergaster full development of a larger brain would have required additional energy and time. In order to allow for continued brain development without engendering the energetic costs of reproductive maturation, increases in DHEA/S production would have promoted development of the prefrontal cortex, while suppressing GnRH production and the onset of pubertal maturation. At younger ages, reproductive suppression may have been complete, creating a juvenile phase in which the brain could develop without the competing demands of reproductive maturation. However, continued increases in DHEA/S would allow for further brain maturation even as reproductive maturation and sexual behavior commenced, thus allowing the integration of sexual impulses and their behavioral expression.
Bogin (1994) has argued that the adolescent growth spurt in human males reflects the results of male-male competition. He suggests that, once males exhibit the somatic cues of maturity, they enter into a competitive world of adult males that inevitably carries costs, including the risk of injury. Thus it would be beneficial to delay development of full secondary sexual characteristics, including growth, until one is as competitive as possible so that the benefits of engaging in male-male competition are more likely to outweigh the costs. Bercovitch and Ziegler (2002) make a similar argument for primates.
Among orangutans the presence of adult males does directly suppress the development of secondary sexual characteristics in developing males without suppressing reproductive function (Maggioncalda et al., 2002), suggesting that the presence of older males is a critical social signal of