Eli5: Why are human babies born so ill prepared for the world, compared to some other mammals who are born much more well equipped?


Eli5: Why are human babies born so ill prepared for the world, compared to some other mammals who are born much more well equipped?

In: 2

Humans have large heads (because we’ve got large brains) and narrow hips (because we’re bipedal). These factors make birth very difficult. Babies can’t spend any longer developing in the womb or birth would become impossible, so they have to come out “half finished”.


Primates have (relatively) very large brains, which means relatively large heads. If we tried to be born with full sized heads we’d kill the mother because our heads are too big to fit through the pelvis. So we’re born early, developmentally speaking, because that’s the longest we can stay “in” without drastically increasing the chance of death for both mom and baby.

Then we do the rest of our developing on the outside while our giant melons grow to full size.

Humans are limited in terms of development because of our massive brains (and therefore massive skulls). If you stayed in the uterus any longer than 9 months, your skull wouldn’t have fit through your mom’s hips. So humans don’t develop as long in the uterus because we simply can’t. However, we do have a very finely honed skill that is fsr superior to any similar skill in the animal kingdom: socialization. We can more than compensate for this lack of in utero development through the amount of knowledge we can bestow through socialization.

Because we walk upright, our pelvises can be only so wide. That puts an upper limit to how large-headed babies women can give birth to which is an issue because the defining human trait is our enormous brain.

The compromise is that human babies are born “too early” in order to fit through the birth canal, and then finish their development outside of the womb.

This requires a lot of effort on the parents’ part to keep their offspring alive, but that effort also means that there’s a high chance that any one child makes it to adulthood. Other species (say, salmon) swing the other way and instead pump out a huge amount of offspring with little oversight so that at least some survive.

We evolved a compromise between a big brain and walking upright. So the baby needs to come out while it’s still small enough to go through the birth canal. Because of the trade off, human babies come out less developed and more reliant on other people.

Awesome! Thank you everyone.

This is something called altriciality, which most marsupials, rodents, primates, cats and dogs have, and several species of bird.
And the reason for why, is cause birth is easier that way, imagine if pregnancy lasted for 2-3 years instead in humans.

Follow up question.

Because of widely availability of c sections will we develop even weaker and bigger brained babies?