The classic anthropological hypothesis on why humans have so much pain in childbirth compared to other animals, even our closest relatives, is the obstetrical dilemma (OD) hypothesis. That says that human babies have large brains, and human birth canals aren’t really big enough because we walk on two legs instead of four. It’s also why human babies are born so helpless and underdeveloped compared to apes. That’s the conventional wisdom, although if you look closely, there’s not much empirical evidence that even wider hips and a bigger birth canal would hinder a woman’s ability to walk. Biological anthropologist Holly Dunsworth gives us a different hypothesis, called EGG (energetics, growth, gestation) to explain the length of human gestation, which controls the other factors.
The EGG hypothesizes that mothers give birth when they do because they cannot possibly give any more energy into gestation and fetal growth. And when you look at the data available on pregnancy and lactation metabolism in our species, it suggests that right around 9 months of gestation, mothers reach the metabolic ceiling for most humans.
Here’s Herman’s Figure 3 from our paper, showing the EGG for humans, plotted with real metabolic data. Circles are the offspring, squares are the mother. Notice how fetal energy demands increase exponentially as the end of a normal human gestation period approaches. To keep it in any longer, mother would have to burst through her normal metabolic ceiling. Instead, she gives birth and remains in a safe and feasible metabolic zone.
After explaining the EGG hypothesis that she published with several colleagues, Dunsworth switches gears and talks about how this hypothesis relates to any readers who may be pregnant.
You worrying that you gestated too long or too little compared to the species average is a bit like you worrying that you’re shorter or taller than average, have a larger or smaller head than average, have more saliva than average, or that you can’t intentionally fart. Stop worrying about your normal variation. Variation exists because it works. There’s safe wiggle room around most traits and sometimes there’s even full-on spasmodic dancing room. We’d be extinct if there wasn’t any room for variation in how to survive and reproduce. Celebrate your weirdness, your slightly long healthy gestation, your slightly short healthy gestation, your big healthy baby, your small healthy baby, your freckles, your asymmetrical face, your hairy knuckles, your lack of wisdom teeth, your pterodactyl toes. Who cares! If life’s getting on with your weird ass, then you can certainly get on with life.
The article at Scientific American is delightful in that it makes a difficult scientific concept easy to understand, with plenty of extra information about how evolutionary science really works. -via Metafilter