by Win Wenger, Ph.D.
Winsights No. 105 (October/November 2008)
Photograph courtesy of Steven C. Hall
| Please also see these relevant articles: Winsights No. 45, “Pre-Natal Curriculum And Why” and Feeding the Loop |
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?
—Probably.
- 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.
Hm-m-m.
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 all.
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.
See 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.