I’ve already discussed at length my thoughts about Dettwyler and her so-called “natural” range of weaning, derived largely from primate and other animal studies. However, I’ve recently come across an interesting article (.pdf file) by G.E. Kennedy, an anthropologist from UCLA, which makes an interesting argument in this regard: that humans are evolutionarily programmed to wean earlier than other primates. The reasoning being that evolution is “pressing” not towards survival of the individual child, but towards accelerated human brain development, enabled by no longer subsisting merely on mother’s milk and adding varied foods, specifically protein-rich meat. The article also looks at the ‘expected’ (i.e, according to primate status and size) weaning age vs. the actual weaning age in most human societies – which it estimates is between 2-3 years of age. Their conclusion:
If selection had targeted only the survival of the child, then humans would have continued to nurse long and wean late, like the other hominoids. Yet doing so would have prevented not only the development of our large brain, but would also have prevented increasing population numbers and the vital genetic diversity that accompanies population growth. It has been argued here that selection favored early weaning because the early shift to adult foods, despite the hazards to the weanling, supports the child during a critical period of neurological development; thus, selection in humans has favored not merely the survival of the child, but the intellectual potential of that child. The evolutionary tradeoff is harsh, but it was the only way we could have attained our large brain.
I don’t know if Kennedy’s right or not, but it seems like a plausible explanation of the observed phenomenon, and much better than trying to make the data fit the theory, the way Dettwyler has done.