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 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, 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 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.