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The Great Eviction: Speculations on Why Our Babies Can’t
Stay in the Womb for a Fourth Trimester
Upon thee I was cast out from the womb.
Psalms 22:10
During the past century, archaeologists have pieced together a clearer
picture of how humans evolved over the past five million years. They
have studied such issues as why we switched from knuckle-walking to
running upright and when we began using language and tools. However,
what has not been fully appreciated until now is that over millions of
years evolutionary changes gradually forced our ancestral mothers to
deliver babies who were more and more immature. I believe that
eventually, prehistoric human mothers had to evict their newborns three
months early because their brains got so big!
In the very distant past, our ancestors likely had tiny-headed babies
who didn’t need to be evicted early from the womb. However, a few
million years ago, our babies began going down a new branch of the
evolutionary tree—the branch of supersmart people with big-brained
babies. Pregnant mothers began stuffing new talents into their unborn
babies’ brains, filling them up like Christmas stockings. Eventually, their
heads must have gotten so large that they began to get stuck during
birth.
Perhaps that would have ended the evolution of our big brains, but
four adaptations occurred that allowed our babies’ brains to continue
growing:
1. Our fetuses began to develop no-frills brains, containing only the
most basic reflexes and skills needed to survive after birth (like
sucking, pooping, and keeping the heart beating).
2. An ultrasleek head design slowly evolved to keep the big brain

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from getting wedged in the birth canal. On the outside it had
slippery skin, squishable ears, and a tiny chin and nose. On the
inside it had a compressible brain and a soft skull that could
elongate and form itself into a narrower, easier to deliver cone
shape.
3. Their big heads began to rotate as they exited the womb. (You’ve
probably noticed it’s easier to get a tight cork out of a bottle if you
twist it as you pull it out.)
These three modifications helped tremendously. However, the
crowning change that allowed the continued growth of our babies’
brains was the fourth change—“eviction.”
4. I believe that over hundreds of thousands of years, big-brained
babies were less likely to get stuck in the birth canal—and more
likely to survive—if they were born a little prematurely. In other
words, if they were evicted.
Today, mothers give birth to their babies about three months before
they’re fully mature in order to guarantee a safe delivery.
However, as any mother can tell you, even with all these adaptations,

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giving birth is still a very tight squeeze. At eleven and a half centimeters
across, our fetuses’ heads have to compress quite a bit to get pushed
through a ten centimeter, fully dilated cervix. No wonder midwives call
the cervix at delivery the “ring of fire”!
Childbirth has always been a hazardous business occasionally putting
both children and mothers in mortal peril. That’s why, through the ages,
many societies have honored childbirth as a heroic act. The Aztecs
believed women who died giving birth entered the highest level of
heaven, alongside courageous warriors who lost their lives in battle.
Imagine giving birth to a baby half the height or weight of an adult.
Of course, a three-foot-long, eighty-pound newborn would be
ridiculous. Now imagine giving birth to a baby with a head half the
size of an adult’s. That sounds even more absurd, but the fact is that
such a head would be small for a new baby. At birth, our babies’
noggins are almost two-thirds as big around as an adult head.
(Ouch!)
Early eviction lessened that risk and was made possible by the ability
of prehistoric parents to protect their immature babies. Thanks to their
upright posture and highly developed manual dexterity, early humans
could walk while carrying their infants to keep them warm and cuddled.
And our ancestors used their hands for more than holding. They created
warm clothing and slinglike carriers that mimicked the security of the
womb.
The hard work of imitating the uterus was the price our Stone Age
relatives accepted in exchange for having safer early deliveries.
However, in recent centuries, many parents have tried to wiggle out of
this commitment to their babies.
They still wanted their babies to have big smart brains and be born

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early, but they didn’t want to feed them so frequently or carry them
around all day. Some misguided experts even insisted that newborns
should be expected to sleep through the night and calm their own
crying. Like kangaroos refusing their babies’ entrance to the pouch,
parents who subscribed to these theories denied what mothers and
fathers for hundreds of thousands of years had promised to give their
new infants.