How We Started Losing 7/8ths of Our Brain
Ever Getting Born
by Win Wenger
Photo courtesy of
Steven C. Hall
I may have found - or guessed - why human gestation, 30,000
to 60,000 years ago, increased from seven months long to nine months long.
Yes, there are numerous advantages to leaving the cookies in the oven a
little longer - but one remarkable consequence is that seven-eighths of the
brain cells you had at seven months in the womb, are dead and gone by the
time you are born.
(The figure which researchers admitted to at the time I wrote my prior article on this topic, "The Pre-Natal Curriculum", was the one I used there: "Over half" of the brain cells alive at seven months are dead and gone by time of birth. Only later did the data develop, according to the researchers, that the mortality figure for brain cells in the seventh month was seven-eighths dead and gone by time of birth.)
That is an enormous biological investment, just thrown away. "Nature's way
of pruning things down," the researchers are saying.
- Why did those "excess" brain cells die?
Well, they weren't used.
- If they had been used, they'd still be alive?
- If they had been used and were still alive, they would be useful?
Certainly, useful in regard that if they were still around for that reason
they'd be recruited into at least some of the things that are going on now
in their brain. But nature prunes down what's not needed, and those weren't needed.
That was something of why I wrote "Pre-Natal Curriculum And Why", proposing easy ways to provide the fetus consistent feedback on some of his actions while in the womb, so he would develop some sense of
control over aspects of his environment and develop his brain accordingly.
This was and is a far stronger way of engaging his brain than just the
pre-natal stimulation without the feedback, which has become popular in some
quarters and which at least is better than nothing.
All things considered: to throw away seven-eighths of the brain seems
remarkable. That proliferation of brain cells in the first place is an
enormous biological investment. That peak comes at the seventh month
because, for a long time through human pre-history, that was how long it
took before humans got born. We were designed so we'd be born with a maximum
of brain cells with which to encounter the world. Then something drastic
must have changed.
Why we changed from seven to nine months:
There were and are a lot of advantages to extending the period of human
pregnancy, but enough to offset losing seven-eighths of your brain? There
had to be a major bioevolutionary driver, a powerful survival advantage for
babies kept longer before being born.
I think we are looking at the immune system here, in one specific regard, as
that major bioevolutionary driver for this was also about the same time in pre-history that humans started
living in larger tribal groupings and began some trade and commerce and
exogamy (intermarriage) between those larger tribal groups. The colds and
diseases of the rare individual, more and more became the common property of
We see this very same effect today, when small groups of people are
isolated for a long time by extremes of weather or geography. First thing
that happens when they rejoin larger society: they all catch colds. Their
immune systems have to catch up on what's been going on.
Before people started mingling on a larger scale, the incidence of colds and
diseases had to be very low. A baby whose immune system was as yet immature,
had a very good chance of coasting on immunities in his mother's milk while
his immune system developed for the first few months after birth, without
encountering any new disease bacteria or viruses. There was no great
pressure or premium for survival on having a matured immune system until
some months past birth.
Once people were mingling on a large scale and transmitting their diseases
around, that changed. A lot of newborns must have died, throwing unusual
survival advantage to the freakish few who took a little longer to get born,
who had their immune systems much better developed by the time they had to
face the world. And so here we are.
Yes, there are other advantages - sturdier respiratory system for one thing
for the full-term nine-monther; better body-temperature control (often under
desert and/or Ice Age conditions), among other factors. The immune system of
the seven-month preemie is still challenged.
Also, after 30,000 to 60,000 years of enjoying those two extra months, we've
accumulated a lot of "gotten-by-withs" which would be hard to do without any
more, it seems unlikely we could go back to a seven-month gestation without
a lot of casualties. The hardship which a preemie birth now entails, seems
more than enough to make up for whatever brain advantages might once have
pertained to being born in the seventh month.
Yet. we've paid quite a price for those advantages, such as they are, of
going for the full nine months. We do pretty well have to keep the full nine
months of full-term pregnancies. But do we have to pay all of that price
for it? Do we have to lose all of that seven-eighths of our brains? We may
not have a choice in the length of pregnancies, but we definitely have a
choice in terms of how much brain we (at least our children and
grandchildren) get to keep.
Pre-Natal Curriculum And Why
for how we can improve that.
See the article beginning at Feeding the Loop for the crucial importance of
feedback on one's own activities, instead of mere stimulus, in development
of learning and of the brain and nervous system. This applies to everyone,
not only to fetuses.
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