Is it because of the whole big head/bipedalism thing? Other mammals can pop out babies in no time and it seems like that would be way more advantageous for multiple reasons (less stress on mom and baby, not in a vulnerable position for hours in the wild, etc). Why does the norm skew towards a day rather than hours?
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Lots of good explanations for physically why but the core issue really is a lack of natural selection.
We have modern medicine that saves lives. Other animals in nature typically do not. Only those able to pass genes down that promote traits that allow for a smooth birth reproduce.
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