A comment on an evolutionary lecture at New Paltz, via facebook, Holly Dunsworth's revision of the hip-and-brain hypothesis of human neoteny:
Abstract of Holly Dunsworth's paper:
The classic anthropological hypothesis known as the “obstetrical dilemma” is a well-known explanation for human altriciality, a condition
that has significant implications for human social and behavioral evolution. The hypothesis holds that antagonistic selection fo a larg neonatal brain and a narrow, bipedal-adapted birth canal poses a problem for childbirth; the hominin “solution” is to truncate gestation, resulting in an altricial neonate. This explanation for human altriciality based on pelvic constraints persists despite data linking human life history to that of other species. Here, we present evidence that challenges the importance of pelvic morphology and mechanics in the evolution of human gestation and altriciality. Instead, our analyses suggest that limits to maternal metabolism are the primary constraints on human gestation length and fetal growth. Although pelvic remodeling and encephalization during hominin evolution contributed to the present parturitional difficulty, there is little evidence that pelvic constraints have altered the timing of birth.
Well it's not a matter of either or... there's much to be said I think for the "traditional" theory about the brain size and pelvis, but metabolism must be an issue too, and brain size is an issue in thar respect as well - after all it must take lots of extra energy to house two brains instead of one!
Perhaps further evidence may be available in the future about the relative altriciality of bipedal apes (autralopithecines), which would help give more relative weight to one side or the other.
The issue of altriciality, linked to neoteny, is a key one regarding the evolution of language. It ensures that the brain is not mature enough at the time of birth, and that therefore the child is an overgrown (altricial) foetus, finishing gestation and maturity in a linguistic environment. This is the way the symbolic networks and and grammatical patterns of language are imprinted on the brain, ensuring that the human development takes place in a cultural, linguistic atmosphere—our way into the symbolic realm which is specifically human.