This is one of those topics that evolutionary biologists love to play with. And of course the overall answer is - given all the factors involved, those who have pregnancies of this length do best and have the most descendants. But of course that begs the question.
So what are the pressures?
Re head size - the article points out that "
Human babies are born underdeveloped compared with other primates: Our brains are less than 30 percent their adult size at birth, compared with around 40 percent for chimpanzees, our closest living ape relative. In fact, it would take a gestation length of 18 to 21 months instead of nine months for human babies' brains to reach that level of development, according to zoologist Adolf Portmann's book "A Zoologist Looks at Humankind" " So what I remembered totally underestimated how early our babies are. And for relatives to compare us with, chimpanzees (both species) are by far our closest relatives, so they are the ones we should look at.
Brain size is part of it - young female chimpanzees (i.e. young, not fully developed pelvic girdle) can usually deliver a baby fine if it is term. If it is 2 weeks late, and the baby is just that much bigger, they often die during delivery. That same baby would not be a problem for an older female. We see the same thing in our own livestock, farmers debate how early to breed a heifer, and it is often dependent on the breed of bull (how big will that calf be?). We also see it in dogs - dogs with normal skull shapes have easy deliveries, dogs with flat faces and round skulls often require C-sections. Um, we have those awkward round skulls, shape matters too. So when we think of head size we should be thinking of first pregnancy, and age and level of development at first pregnancy.
Metabolism - sure food supply will be limiting. So is space. We know twins are normally born earlier than singletons.
Other factors, off the top of my head - predation - how mobile is the mother, and how much protection does she get from her group? Even if she can have a bigger baby, it doesn't do much good if she gets eaten first. Parasite load - how is her food intake given that she is feeding herself, her baby, and her parasites (internal and external, biting flies and fleas/lice can take a lot of blood when they are numerous)?
Trade-offs - the last few weeks of pregnancy the fetus is not growing that fast compared to earlier, it is putting on fat to survive the first few days post-partum (mother has to get milk flow going, colostrum is not that nourishing, etc., she is probably not eating well those first few days). Early babies are scrawny. A bigger baby would need that much more protective fat, and that is more demand on the mother.
Not really arguing with the article, its just that in Evolutionary Biology there are usually so many factors that "
this is the cause of whatever" is leaving out all the other factors - yes, it's a messy historical science.
And hormones, oh my, you look at your baby with love, and wonder how much of it is your brain's oxytocin making sure you love this baby and your genes get passed on (because you are the daughter of a long line of mothers whose oxytocin made them good loving mothers). We are all puppets!
Weird fact - our babies are all born way too early. Based on our body size and what our relatives (chimpanzees) do, our gestation period should be 11-12 months. Of course then the baby's head would be so big the mother would die, every time. So we make up for our big brains by being born too soon. So that blob of baby really is a blob, because it shouldn't even be here yet. Once they get to be a few months old they are ready for the world.
Actually, this has been shown not to be true. It's not the head size, it's due to metabolism of the mother. We just can't take in/expend enough energy to keep a fetus growing at the rate it should. And when overall human size is taken into account, we actually have large babies. Bigger than gorilla babies, on average.
As someone who has gestated a child, thank the glorious Flying Spaghetti Monster that babies aren't in the womb for longer. Even aside from the discomfort, not being in control of my own emotions or body was a very bizarre experience. That was what forced me to realize how much we are at the mercy of our hormones.
I am not one of those women who gushes about loving every second of being pregnant.
