Re: Breast Size (Was: Re: Homosexuality and genetic determinism)
Gerold Firl (geroldf@sdd.hp.com)
2 Jun 1995 13:38:29 -0700
In article <3qh80d$gm@news.cc.utah.edu> pb1487@u.cc.utah.edu (Paul Bernhardt) writes:
>Upright walking narrows the birth canal. The narrowed birth canal
>necessitates an underdeveloped infant, particularly the brain (the head
>is the largest part of the infant).
According to leakey and lewin, in _the people of the lake_,
australopithecus had a fully upright posture *and* a significantly smaller
birth canal than modern humans. (of course, australopithicus was a much
smaller animal than h. sapiens too.) A wider birth canal results in a less
efficient stride, so hominids have been squeezed between two competing
selection pressures: efficient stride for females vs. maximum cranial
capacity at birth. The compromise has not been an entirely happy one, since
human maternal birth mortality is probably the highest of any animal. (does
anyone have data on this?)
But I would stress the maximization of cranial capacity over the effects of
bipedalism. The transition to bipedalism seems to have beed accomplished
very quickly, being already complete as far back as the hominid fossil
record is availible. The cranial increase, on the other hand, has steadily
increased over the last few million years. It appears that cranial increase
has been the driving factor, with all others following as secondary
effects.
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