[ExI] Human birth was extinction

[hkhenson]

Re the subject of pain in childbirth and maternal deaths, Dr. William 
Calvin goes into some detail.

It helps to remember that humans evolved huge brains over the past 
2.5 million years.  The process of adjusting is not complete.

http://williamcalvin.com/bk5/bk5.htm

The Ascent of Mind
Ice Age Climates and
the Evolution of Intelligence

Chapter <http://williamcalvin.com/bk5/bk5ch7.htm>7
WHIDBEY ISLAND: Ratcheting Up Brain Size

snip

Parts, Process, Product: the legislative process illustrates a 
transformational process at work, and illustrates what a detailed 
understanding of human evolutionary processes might involve. Beach 
walk to an old Indian fishing village buried by a mud slide on 
Whidbey Island. And a mother-and-unborn-child burial. Relatively big 
heads occur with early puberty. Then stature re-enlarges in other 
ways. When the resulting big heads kill their mothers, there is 
selection for slower-than-average body growth rates, so the survivors 
are born premature-looking. Given selection pressure resulting in 
juvenilization, one gets a cycle that can be repeated many times to 
enlarge the brain fourfold.


WE TEND TO TAKE OUR BIG BRAINS for granted. Even those with a working 
knowledge of evolution often make the mistake of assuming that big 
brains would naturally evolve by slow increments: we assume that a 
bigger brain is a smarter brain. And since a smarter brain is surely 
a better brain, then it is not surprising that, analogous to compound 
interest, we should have bootstrapped ourselves up to a much bigger 
brain. After all, some people naturally have somewhat bigger heads 
than others, so all it takes is some natural selection for the 
obviously useful variant.

       There is something very wrong with this commonplace 
explanation: it ignores the enormous natural selection against bigger 
heads. Maybe bigger brains are indeed better for something, but it 
would have been bought at an enormous price, extorted over and over 
again at each little increment along the way to a brain four times 
larger than that of our presumed ancestors, the australopithecines.

       Actually, it isn't clear that bigger brains are even 
necessary; an ape-sized brain reorganized to facilitate language and 
plan-ahead might work equally well. Yet the truly horrendous problem 
with bigger-heads-are-better should have been obvious long before 
anyone got around to noticing that someone's hat size didn't 
correlate with how smart he was: big heads cause a lot of trouble at 
childbirth. Big heads not only kill themselves but, moreover, others 
carrying similar gene combinations: their mothers. Thus all potential 
siblings (and occasionally some of the still-dependent prior children 
of that mother as well), many likely to carry those same gene 
combinations, will also be eliminated from the surviving gene pool.

       It is hard to imagine any form of natural selection that is 
more powerfully negative; modern genetic diseases such as hemophilia 
pale by comparison. Big heads are a candidate for the worst genetic 
disease of all time. By all rights, any straightforward tendency 
toward bigger heads should have been promptly squelched.

       Those who nonetheless argue bigger-is-smarter-is-better should 
realize that a small increment in intelligence would have had to be 
overwhelmingly better ever to establish a somewhat larger brain. The 
next increment would have had to be overwhelmingly better than the 
previous miracle, and so on. While perhaps anything is possible given 
a long enough time and compound interest, bigger-brain cleverness per 
se seems unlikely as a source for the fastest encephalization on 
record, fourfold in a mere 2.5 million years.

       It makes you wonder how bigger-brains-are-better ever became 
established in the first place as the dominant explanation for human 
evolution. If women had been the scientists doing the theorizing, I 
suspect that we would have long ago abandoned the notion and gone in 
search of a better idea.

       Big heads, however, nonetheless happened. And so there is 
presumably some way around this problem. Something else must have 
been under frequent selection pressure, with big heads as an unwanted 
side effect that was dragged along. This suggests that big heads were 
achieved by some decoupled backdoor route, rather than via 
straightforward selection for variants in brain size. And indeed big 
heads come as part of a package, a panoply of linked features called 
juvenilization (or paedomorphosis or, in even older literature, 
fetalization) that has been a repeated theme of vertebrate evolution.

snip

       So it is hard to imagine why brain size would be under natural 
selection for its advantages -- especially when the disadvantages of 
an increased brain/body ratio are so immediate and so horrendous. For 
it is the bigger head relative to the smaller body that gets us into 
so much trouble: If hip size had increased commensurately, no birth 
canal bottleneck would have developed.

       Yet it is precisely brain/body ratio that increases with 
juvenilization. And so an adult woman has to give birth with (by the 
standards of earlier generations) the narrow-hipped body of an 
adolescent girl. True, hip size in women does increase with 
childbearing; true, short adult women cannot find something that fits 
in the children's section of a clothing store, thanks to the hip size 
disproportion. But whatever the hip size compensation has been, it 
has been insufficient: it cannot explain the fourfold larger brain of 
modern humans compared to apes and the australopithecines. So if the 
boom time physiology of the ice ages produced juvenilizations, 
selection against big heads would surely have followed.

snip

THE BIRTH CANAL BOTTLENECK comes next because, without further 
changes, bigger-headed fetuses are going to start getting stuck 
during childbirth (if they hadn't already had trouble at the smaller 
stature). This in turn will start selection operating on another 
common variation-on-a-theme, somatic developmental rate -- just due 
to their genes, some children gain height and weight more slowly than others.

       We knew that some more changes were going to be necessary 
because juvenilization by itself tends to suggest a shorter childhood 
-- indeed, its truncation by early sexual maturity. But the 
monkey-to-ape and ape-to-human transitions show exactly the opposite: 
a lengthening of childhood. This paradox is resolved if we assume 
that a slowing of general body development (selected from that 
variation-on-a-theme that Boas observed) has been superimposed on 
juvenilization, moving the earlier menarche back out to its original 
year and even beyond. It's the relative rates of somatic and sexual 
development that control childhood's tempo and the resulting adult 
shape, just as it is the relative rate of growth in the north and 
south sides of a flower stem that cause it to bend south toward more sunshine.

       The main reason to believe that slowing has actually happened 
is that slowed development is more general than just childhood. Most 
life phase durations (conception-to-birth, birth-to-weaning, 
weaning-to-menarche, adult span) have been nearly doubled in going 
from monkey to ape. And nearly doubled again in going from ape to 
human. Though human gestation would at first appear to constitute an 
exception (it is only several weeks longer than in apes), this 
doubling rule seems to apply there too: human infants do not attain 
the same developmental landmarks as newborn apes until many months 
after birth, for a total internal-plus-external "gestation time" 
about twice that of chimpanzees.

       This halving of the rate of the somatic developmental clock 
throughout pre- and postnatal life also needs explaining; I'm surely 
not the first to suggest that it was the solution to the childbirth 
problem presented by that big head that came along with 
juvenilization. If there had been a way of slowing only prenatal 
development without concomitant slowing of postnatal development, it 
might have done the job too -- but the more generalized slowing may 
have been the only variant available.

       Because juvenilization makes the adult head relatively larger 
and the adult pelvis relatively smaller, repeated juvenilizations 
will eventually run into trouble when the baby's head can no longer 
get through the pelvic outlet. The gene combinations that result in 
early puberty and normal somatic developmental rates will then be 
edited out, unfortunately via maternal mortality rather than merely 
unsuccessful fetuses (but therefore at a much faster rate, because of 
the kin selection practiced by the unsuccessful fetus). The same 
would be true for faster-than-average somatic development genes. The 
gene combinations of precocity and slowed somatic developmental rates 
will get by, provided parturition is not equally delayed.

       So long as the surviving mother can cope with raising a 
relatively fragile premature infant, the gene pool would soon come to 
be dominated by the genes for slower-than-average somatic 
development. This escape route for big baby heads would seem to 
require slowed somatic development superimposed upon the accelerated 
sexual maturity; our longer life spans after birth may be largely a 
side effect of the slowing of somatic development needed to work 
around the birth canal bottleneck.

       Thus we get the sequence of 1) juvenilization via 
faster-than-average sexual development, 2) re-enlarged stature via 
other taller-than-average genes, and 3) slower-than-average somatic 
developmental rate. And because of the carryover of slowed 
development into postnatal life, the usual time scale is stretched; 
the number of years that it takes to get to puberty may have moved 
back out beyond what it was before the changes started to take place. 
Body size is also potentially back to the norm. Only head size is 
still increased, along with a few other uncorrected side effects such 
as reduced tooth size, flatter faces, and other such juvenile features.
       Eureka? Only if the three-part cycle can be repeated quite a 
few times. And body style doesn't backslide.

****************

I advise you to read the whole book.  It's on line, in many libraries 
and for sale if you want hard copy.

Keith