Once we stop trying to force Nariokotome into a human mold, we can explore a more evolutionary approach. Comparative study of living mammals has long suggested that important aspects of maturation evolve in parallel with brain size (Sacher, 1959; Smith, 1989 and references therein). The strongest evidence for such a correspondence is found in primates, where the age of emergence of the first permanent molar (M1) correlates with cranial capacity at r = 0.98 or higher (Smith, 1989; Smith et al., 1995). Even if both are ultimately responses to some third factor (e.g., mortality), cranial capacity may be a window on growth and development of extinct hominids. If we use regression to predict age of tooth emergence from cranial capacity, early Homo erectus (with a cranial capacity of 810 cc) is expected to erupt M1 at 4.5 years, with M2 following at approximately 9 years of age (Smith, 1993; Smith and Tompkins, 1995; Smith et al., 1994). Such timing is not typical for any living primate, but would describe a maturation rate intermediate between living apes and humans. These predictions are remarkably close to those derived from enamel and dentine histology in Homo erectus.
Smith (1993) and Smith and Tompkins (1995) have also discussed the issue of the adolescent growth spurt in Homo erectus and questioned the likelihood of it being present 1.6 million years ago, although this has generated considerable debate. The human adolescent growth spurt - or rather the very slow period of growth between weaning and puberty that precedes the spurt, is unique. The spurt has variously been seen as a kind of catch-up growth following a period of prolonged and intensive calorific investment in brain growth, or alternatively as the end of a period when it is socially advantageous to be small during the long human learning process (Bogin, 1990, 1999). Gurven and Walker (2006), however, argue that the very slow period of growth between weaning and puberty in modern humans enables mothers to support more infants at any one time because their combined body mass is small and their energy requirements are thus lower for as long as possible before sexual maturity. For a slowed late childhood growth to be favored, we suspect that stacking multiple dependents in a family unit must have increased survival for both toddler and adolescent, while moderating energy demands. Either way, the adolescent growth spurt is arguably intricately involved with prolonged childhood dependency. If Homo erectus offspring were energetically independent of their mothers to a greater degree at an earlier age than in modern humans, then the advantage of the slow growth period between weaning and puberty becomes less obvious.
Aiello and Key (2002) have previously argued that the energetic costs of being a Homo erectus mother with a body size ~50% greater than a female australopith mother during lactation, gestation and non-reproductive periods would have been considerably higher and would have required "a revolution" in the way in which females obtained and utilised energy. Lieberman et al. (2008, 2009) have argued that the transition from Ausralopithecus to Homo was indeed characterized by a new strategy for acquiring and using energy in open habitats, and that this transition was almost certainly related to a profound behavioral shift characterized by an increase in meat acquisition through scavenging and/or hunting and the regular manufacture of stone tools designed for food extraction and food processing. These lines of evidence, together with those set out in this review, suggest a strategy of co-operative provisioning and food sharing where Homo erectus offspring were able to contribute from an early age to their own energetic requirements.
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