A baby gazelle struggles to climb on its legs straight after being born. A newborn monkey clings tightly to its mother’s fur. And then you look at human babies, feeble and vulnerable creatures. And you can’t help but wonder how our species managed to survive and make it so far. Human newborns have the least developed brains of all primates. A human newborn brain is less than 30% of its adult size while for a newborn chimpanzee it’s about 40%. The comparison is particularly striking as chimpanzees are our close cousins. Why are we so different?
You’ve probably heard, like me, that it’s because we walk on two feet and we have big heads. As we evolved we got bigger brains but the bones of the pelvis could only become so much wider before it impeded our bipedal walking so Nature had to make a compromise: keep the hips narrower but get the baby out before its head becomes too big. And that’s why we have helpless babies, unlike chimpanzees. Because pregnangy has been shortened during the course of human evolution. Our babies are born early, before they’ve had the time to develop fully. At least that’s the classical explanation. But, according to a recent paper, it’s wrong.
First, a wider pelvis does not hinder locomotion. If it was the case, women’s walking should be mechanically and energetically less efficient than men’s as they have broader hips. However, studies have found no difference between sexes or shown that women’s walking required less energy. Recent research demonstrated that the force produced by our muscles to prevent our hips from rotating while we stand on one leg does not depend on the size of the pelvis so this force does not increase with the width of the pelvis as it was previously thought. Furthermore, to deliver a baby with a brain 40% of its adult size as in chimpanzees the human pelvis would need to be 3cm wider, which is in the range of pelvic size seen in modern women and has no effect on locomotion.
Also, pregnancy hasn’t shortened during human evolution, rather it has increased. When comparing the duration of pregnancy to the mother’s body size for different primates, it comes out that human pregnancy is 37 days longer than what would be expected for a primate of a similar size. And it’s not because human mothers give less resources to their babies that they have a less developed brain at birth. Human newborns have larger brains and are much bigger than can be expected from the mother’s body size.
So if it’s not because of our bipedalism and our big brains, why are we giving birth to underdeveloped babies compared to other primates? The answer from the paper goes back to the resources human mothers provide for their babies growth. Pregnancy costs a lot of energy. By the 6th month the amount of energy spent daily by human mothers is 2 times greater than the amount before pregnangy. And that’s close to the maximum the human metabolism can reach. By the 9th month the energy required by a human baby is close to the limit the mother’s body can provide and extending pregnancy by one month would go beyond this limit. Human babies are born underdeveloped because human mothers just cannot sustain their growth any longer.
Throughout human evolution, pregnancy duration and the amount of resources given by the mother to its baby have increased but only so much as was possible. Nature had indeed to compromise but it wasn’t between brain size and bipedalism, rather it was between the energy a baby demands and the one a mother’s body can supply.
Dunsworth HM et al. (2012). Metabolic hypothesis for human altriciality. PNAS, 109 (38), 15212-15216 DOI: 10.1073/pnas.1205282109