wgilmour at I-zoom.net
Mon Mar 22 14:00:26 EST 2004
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.
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