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Good read, Thanks. My question is----did the baby carrier cause the better brain or did a better brain come up with an idea to improve life and allow for caring for the baby instead of leaving it under a bush while daily work was performed?
"The invention of the baby sling, which allowed more babies to successfully mature outside the female body, instantly removed the barrier to increased head and brain size."
Someone invents the sling, so the babies are then born earlier because there's a sling? Where's the mechanism for that?
As for this - "the invention of the baby sling, dated by Dr Taylor to at least 2.2 million years ago" - well, where's his evidence for that? It wouldn't be surprising if women (or on occasion older children) used such devices, but Taylor can't have found any. Organic material rots over such long spaces of time.
Oh, wait, here we are:
"... the invention of the baby sling, dated by Dr Taylor to at least 2.2 million years ago, when human ancestor head size suddenly began to increase"
In other words he has a circular argument. The head size proves there were slings, and then the "fact" that there were slings accounts for the head size.
My personal opinion is that the sling was invented to carry a baby so that our hands would be free. And that the increase in brain size, do to added fat and protein caused us to bear children earlier before the head became too large for the female pelvis. The need to carry around an increasingly more 'premature' infant would certainly lead to such handy inventions.
This has the unexpected and certainly unplanned consequence that it is no longer important for it to be able to hang on as chimps do. Although, due to the bio-mechanical constraints of a bipedal pelvis, the hominin child cannot be born with a big head (thus large initial brain capacity) it can now be born underdeveloped. That is to say, the sling frees fetuses to be born in an ever more ontogenically retarded state.
Because they don't have to hang on they can be born less capable but with greater potential capacities (neotony). Since some individuals would tend that way* those individuals would be, when fully grown, "Favoured" individuals in the "Struggle for Life". So they'd be more likely to reproduce and pass on their genes, and the degree of neotony could become more pronounced over many many generations.
Fair enough. However speculative that is, it's not kooky.
* Before the invention of the sling, individuals like that might, he's presumably thinking, have dropped off and broken their heads long before they could reach reproductive age.