How We Started Losing 7/8ths of our Brain Before Getting Born

by Win Wenger, Ph.D.
published in The Stream, April 2005
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This is a speculation, but I think I may know 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 admitted to at the time I wrote my article, “The Pre-Natal Curriculum,” Winsights No. 45, was the one I used: “over half” of the braincells 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 braincells 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,” researchers say.

  • Why did those die?
  • Well, they weren’t used.
  • If they had been used, they’d still be alive?
  • Probably.
  • If they had been used and still alive, they would be useful? – Certainly, useful in that they were still around for that reason they’d be recruited into at least some of the things that are going on. But nature prunes down what’s not needed, and those weren’t needed.


That was something of why I wrote Winsights No. 45, proposing in “The Pre-Natal Curriculum” easy ways to provide the fetus consistent feedback on some of its actions while in the womb, so it would develop some sense of control over aspects of its environment and develop its brain accordingly. This was and is a far stronger way of engaging its brain than just the pre-natal stimulation that has become popular in some quarters and which is certainly 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 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, and in a specific way.

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 intermarriage (exogamy) 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 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 again is that 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 its mother’s milk while its immune system developed for the first few months after birth, without encountering any 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 temperature control, other things. The immune system of the seven-month preemie is still challenged. Also, after 30-60,000 years of enjoying those two extra months, we’ve accumulated a lot of “gotten-bys” 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, and 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 pretty well have to keep the full nine months of full-term pregnancies. Do we have to pay all of that price for it? Do we have to lose all that seven-eighths of our brains? I don’t think we have a choice in the length of pregnancies, but I think we do have some choice in terms of how much brain we (at least our children and grandchildren) get to keep.

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