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 reinforceing 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 probably like this, he favors more random, contingent
processes.
Maybe there's some good brain people out there that could comment.
Anyway, this is all off the top of my hard rigid head, and as I'm out of my
area of expertise on this, there's probably some reason its all wrong.
Cheers; gilmour at interlynx.net
[Extinction events are an evolutionary pump for diversity]