Big Brains

B Gilmour said:

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

...<snip>...

I suppose experiments could be done on animals to remove
parts of the skulls at birth (perhaps enlarge them by
replacing them with flexible plates?) to see if the brains
grew larger. (I must say I find the idea kind of gruesome;
I'm not fond of vivisection, but worse things are done to
animals' brains in the name of scientific research.)

Your position in a nutshell is that the ability of the human
head to deform allowed large brains, rather than large
brains caused (required) human skulls to be able to deform
during childbirth.

I would firstly be interested to know if other animals had
skulls which were soft during childbirth. A Google search
showed only human references

You identfy that there would be considerable evolution
pressure against big brains, hence it is hard to see how the
intermediate species has big brains without soft skulls, and
hence why soft skulls would have to have developed first.

There is additional, indirect support for your view. All
animals in development as embryos (and later) repeat there
evolutionary history - we look like fish at three weeks
after conception ,("ontogeny recapitulates phylogeny" as it
is known) , and hence by this measure our development of
soft skulls probably preceeded our big brains, because we
have soft skulls before we understand calculus .

You cannot however then say soft skulls caused bgi brains;
there is a lot more to making us smart than making our
brains big, or the IBM 370 series would be the most powerful
computers in the world. Certainly soft skulls occurred in
parallel with evoltuion of the speech cortex , and other
imprtanr changes ...

"B Gilmour" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

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

Quoted message said:

have a trait, bipedalism (with its associated narrower
birth canal and increased mortality) which would seem to
place selective pressure on

smaller

Quoted message said:

heads (and associated brains), not larger. The adaptive
value of larger brains must now be explained as being very
significant in order to

overcome

Quoted message said:

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

Quoted message said:

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

Quoted message said:

childbirth. Humans at birth have not only a hole in the
top of the skull

but

Quoted message said:

also the skull cap is extremely soft with radial unhealed
fractures

running

Quoted message said:

in all directions. (If you have every seen a newborn
immediately after birth, their heads look like squashed
prunes). Only bipedal humans have

this

Quoted message said:

trait, quadrupeds have much more hard and rigid skulls
with little deformation (its not needed), their skulls
size (while they still grow)

are

Quoted message said:

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

Quoted message said:

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

Quoted message said:

only by an inherited genetic growth limit. Is it possible
that brains

follow

Quoted message said:

a different growth strategy. The physiology of brain
growth may be

different

Quoted message said:

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

Quoted message said:

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

Quoted message said:

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

Quoted message said:

expect statistically to see varying sizes of brains to
skulls (small

brain,

Quoted message said:

large skull, would be one) and of course we don't. If
this idea is

correct,

Quoted message said:

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

Quoted message said:

(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 for both). A good Grad thesis would be to remove mouse
embryos and place radial cuts

in

Quoted message said:

skull caps, replace to term and measure brain growth
to size.

Anyway, this is all off the top of my hard rigid head.

For Further Speculations visit; home.i-home.i-Open ↗
zoom.net/~wgilmour/Cosmology%20and%20Evolution.html