weight during reproductive maturation (Leigh and Shea, 1996). Thus, the distinctive element of the adolescent growth spurt in humans is not increased growth per se but, more specifically, the dramatic increases in stature. This is easily attributable to our bipedality, which makes height a more salient dimension of size.
Unfortunately, definitive fossil evidence for the origins of the adolescent growth spurt in height during human evolution is not available. Current speculations focus on Homo ergaster when increases in size and loss of sexual dimorphism suggest important changes in the timing of maturation (Hawkes, 2002). However, interpretation of the almost complete skeleton of the Nariokotome boy at 1.8MYA does not provide conclusive evidence for the existence of a growth spurt during this time period. Smith (1993) argues that the relationship between dental and postcranial developmental timing demonstrated by the specimen is at variance with that seen in modern humans, while Clegg and Aiello (1999) argue that it is, in fact, within the range of variation found in modern humans. Thus, the temporal origin of the human growth spurt remains an open issue.
In addition to the pubertal growth spurt, the existence of adrenarche as part of human maturation deserves attention as a distinctive clue to the human life course. Again, adrenarche is not uniquely a human attribute but rather one we share with other great apes (Collins et al., 1981; Smail et al., 1982) that may take on a distinctively human pattern. Importantly, adrenarche follows the completion of brain growth around the age of 7 and precedes the onset of puberty (Bogin, 1999), suggesting that it plays some role between the energetic demands of brain growth and those of puberty.
While adrenarche was once thought to be directly related to the onset of puberty, there is no evidence for an obligate role of adrenarche in the onset of pubertal maturation, and adrenarche and gonadarche are considered functionally separate processes. Instead it appears that adrenarche may be more directly related to processes of brain maturation. DHEA/S has an impact on Gamma-aminobutyric acid (GABA(a)) neurons (Majewska, 1995) that act to inhibit dopaminergic neurons in the prefrontal cortex that regulate judgment and the control of behavioral impulses (Niehoff, 1999). In addition, the impact of DHEA/S on GABA(a) neurons in the hypothalamus may act to inhibit GnRH production and retard the onset of puberty (Genazzani et al., 2000). Thus, while adrenarche and gonadarche are fundamentally separate processes, they are not unrelated, a point that comes home when we consider the role of energetics in reproductive maturation.
For males, increases in stature and muscle mass come well after the onset of testicular growth and the onset of spermatogenesis (Tanner, 1978).