SeeBelow at SeeBelow.Nut
SeeBelow at SeeBelow.Nut
Mon Mar 22 21:18:50 EST 2004
B Gilmour wrote:
> Big Brains and Bipedalism
> One of the reasons often quoted for the limit on human brain size, is the
> restriction placed on head size due to childbirth. In bipedal humans,
> childbirth poses a serious risk for both mother and child (mortality rates
> increasing for both), because the newborn must now pass out of a narrow
> birth canal between the narrower pelvis hips (which have evolved for an
> upright posture). This is a problem not experienced to the same degree by
> quadrupeds. It is now well established that bipedalism evolved first
> followed by large brains. To me this seems something of a paradox because we
> have a trait, bipedalism (with its associated narrower birth canal and
> increased mortality) which would seem to place selective pressure on smaller
> heads (and associated brains), not larger. The adaptive value of larger
> brains must now be explained as being very significant in order to overcome
> this reverse trend which would have been expected.
> I would now like to suggest an alternate hypothesis, which is, that larger
> brains are a direct result of bipedalism and difficult child birth rather
> than the opposite.
> Let me explain.
> Because of bipedalism and its associated difficult childbirth, evolution has
> come up with a partial solution for the big head problem, (neoteny being
> one), and also a softer, (less rigid) more plastic or malleable skull during
> childbirth. Humans at birth have not only a hole in the top of the skull but
> also the skull cap is extremely soft with radial unhealed fractures running
> in all directions. (If you have every seen a newborn immediately after
> birth, their heads look like squashed prunes). Only bipedal humans have this
> trait, quadrupeds have much more hard and rigid skulls with little
> deformation (its not needed), their skulls size (while they still grow) are
> more set at birth.
> Could it be that the physiology of brain growth is such that they simply
> grow to their container size? and that the pressure of a growing brain in a
> softer more plastic skull has resulted in a larger brain before the skull
> matures (heals) into a more rigid one. Skulls do not grow to accommodate a
> predetermined brain size, the opposite is true, brains grow to fit a
> genetically determined skull size. When you think about it, brains exactly
> fit skulls. This may seem intuitively obvious, but wait, other body organs
> cannot follow this growing strategy, because they are not enclosed (and
> subsequently restrained) in a rigid structure like a boney skull. This is
> why (as any surgeon will attest) organs can and do vary in size constrained
> only by an inherited genetic growth limit. Is it possible that brains follow
> a different growth strategy. The physiology of brain growth may be different
> from other organs, relying more on the pressure of the container size to
> restrict growth rather than having a preset genetic size, (the folds in the
> brain may have something to do with this pressure growth relation). One
> observation seems to confirm this, which is, have you ever heard of a person
> with a large skull but a small brain, the answer is of course no, brains
> always grow to container size. If brain size of an individual is
> predetermined by inherited genetics (skull size most certainly is) we should
> expect statistically to see varying sizes of brains to skulls (small brain,
> large skull, would be one) and of course we don't. If this idea is correct,
> large Brains would now seem to be a physiological development phenomenon
> rather than a naturally selected adaptive trait.
> The traits usually quoted as the adaptive reasons for big brains, speech,
> mental maps, group hunting, social situation manipulators, heat radiators,
> tool use, dexterity (opposable thumbs) etc. can now be viewed as exaptations
> (which are not the reasons for large brains but a consequence of them),
> which have found utility within the confines of a larger brain. Of course
> once any or all of the above traits become incipient, a positive feedback
> loop would set in reinforcing the value of the larger brain.
> So there it is,
> 1- Bipedalism evolves for some reason,
> 2- Childbirth with rigid skulls increases mortality rates,
> 3- Soft, plastic more malleable skulls are naturally selected for,
> 4- Neotenist brain growth restricted by the pressure of the container now
> pushes on a more malleable skull,
> 5- Large brains. .... Could it be this simple ....
> Gould would have liked this, he favors more random, contingent processes.
> Are there any stats to indicate larger [or smaller] brains in c sections?
> Any correlations with IQ? (I believe there is).
> A good Grad thesis would be to remove mouse embryos and place radial cuts in
> skull caps, replace to term and measure brain growth to size.
> Anyway, this is all off the top of my hard rigid head.
I like your idea. It makes sense to me.
"Many are stubborn in pursuit of the path they have chosen, few in
pursuit of the goal." - Friedrich Nietzsche
http://annevolve.sourceforge.net is what I'm into nowadays.
Humans may write to me at this address: zenguy at shaw dot ca
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