Evolution and humans: our huge childhood (Evolution)

by David Turell @, Wednesday, July 12, 2023, 00:23 (278 days ago) @ David Turell

Longer than almost any other species:


"Our babies require an intense amount of investment, and as a species we have gone to staggering lengths to give it to them. As placental mammals, we solved the limitations placed on babies who are gestated in eggs with a fixed amount of resources by capturing the code of an RNA virus in our DNA to create the placenta: a temporary organ that allows our embryos and foetuses to draw sustenance directly from our bodies. As humans, however, we have gone a step further and altered the signalling mechanisms that maintain the delicate balance between our voracious young and the mothers they feed off. Our species’ pregnancies – and only our species’ pregnancies – have become life-threatening ordeals specifically to deal with the outrageous demands of our babies. Gestational diabetes and preeclampsia are conditions virtually unknown in the animal kingdom, but common killers of pregnant humans thanks to this subtle alteration. Babies grow to an enormous size and plumpness, and they’re so demanding that the resources in one body aren’t enough to sustain them. They emerge into the world with large brains and a hefty 15 per cent lard, but still unripe and unready.


"Our babies have very large heads, and our mothers quite narrow pelvises, and what seems a trivial question about furniture logistics is in fact a huge impediment to the successful reproduction of our species: this makes human birth dangerous, and mothers die giving birth at a far higher rate than any other species.

"Classically, this was viewed as an acceptable trade-off between competing evolutionary demands. This is what the anthropologist Sherwood Washburn in 1960 called the ‘obstetrical dilemma’: the dangerous trip down the birth canal is necessitated by our upright posture and the tight fit required by our big brains. (My bold)


"...human babies really do have a terrible time coming into the world, above and beyond other species, due to that tight fit. So what gives?

"The answer may be in that glorious pinchable baby fat. Having precision-engineered our offspring to siphon resources from their mothers in order to build calorifically expensive structures like our big brains and our chubby cheeks, we have, perhaps, become victims of our own success. Our babies can build themselves up to an impressive size in the womb, one that comes near to being unsurvivable. But the truly fantastic thing is that, having poured so much into our pregnancies, after we hit the limit of what our babies can catabolise from their mothers’ bodies, they are forced to emerge into the world still fantastically needy. For any mammal, survival after birth calls for the magic of milk, and our babies are no different, but here we find another very unusual feature of humans: our long childhood starts with cutting off infancy early.


"we kick off our babies from the breast quick – but, once they’ve moved from infancy into childhood, there is yet another surprise: we let them stay there longer than any other species on the planet.

"Childhood in humans is extended, by any measure you care to use. We can look at the 25-odd years it takes to get to physical maturity (in fact, the tiny end plate of your clavicle where it meets the sternum doesn’t fully finish forming until your early 30s) and compare it with our nearest relatives, to see that we have slowed down by a decade or more the time it takes to build something great-ape sized.


"There is one more adaptation at play in the support of our needy offspring that should be accounted for: the utter unlikeliness that is a grandmother. Specifically, it is the almost unheard-of biological process of menopause, and the creation of a stage of life for half of our species where reproduction just stops. This is outrageous in evolutionary terms and it occurs only in humans (and a handful of whales).


"A long childhood is our greatest evolutionary adaptation. It means that we have created needy offspring, and this has surprising knock-on effects in every single aspect of our lives, from our pair bonds to our dads to our boring genitals to our dangerous pregnancies and births and our fat-cheeked babies and even that unlikely creature, the grandmother. The amount of time and energy required to grow a human child, and to let it learn the things it needs to learn, is so great that we have stopped the clock: we have given ourselves longer to do it, and critically, made sure there are more and more investors ready to contribute to each of our fantastically expensive children."

Comment: this is an old story. Low reproduction rates, a dangerous way to give birth, lengthy childhood, and yet we are the most successful species on Earth. This is the way it had to be.
The obstetrical dilemma was mentioned before. The DNA of Mother, Father and baby have to evolve in coordinated fashion for 'tight fit' pregnancies to work. Not by chance evolution.

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