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Comment by rolleiflex | Hacker News Reader

One of the evolutionary theses about why our brains are at the size they are, and no larger / smaller is that we’ve hit the hard limit on what can exit a female body without killing either the parent or the baby. This paper adds a lot of credence to that theory.

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Sounds plausible. I have an extended family member who was born after a multi-day labor [in the 40s] where the doctor asked of the father “do you want the baby or your wife to go home, because they’re not both going to make it”. He picked his wife; his wife picked the baby; both went home, but my aunt had life-long deficits from the multi-day traumatic labor and a forceps extraction.

thank god they picked both

> This paper adds a lot of credence to that theory.

This is a nice, high-resolution illustration of a phenomenon that has been known for a very long time. This data may inform an analysis of the detailed anatomical mechanics, but it doesn't tells us anything about the evolutionary theory that wasn't already known.

Yes, that is true, this is well known and fairly established. I was using ‘adds a lot of credence’ in a colloquial sense.

Size incompatibility is thought to have killed neanderthal women who paired up with non-neanderthal men.

Our brain size has been getting smaller in last 20k years, accelerating in the last 10.

10k you mean? Although it sometimes feels like people have smaller brains just lately, too...

It’s plausible that early humans broke into the “success through smarts” niche by a sudden expanded brain mutation that left them with a horrendous birth mortality rate, and that since then selection for mutations for smaller heads (and thus better birth safety) has been very strong (though hopefully not as strong as overall selection for intelligence- essentially evolution finding ways to pack more into a smaller space.)

Those huge Neanderthal brains were just the clunky and unstable Minimum Viable Product; ours are the more refined version.

Absent selective pressure for wider-hipped women.

This might explain why we see a significant number of birth deformation disabilities in East Africa. Recent migrations (past 0.5k-3k years) might have an impact. For example, it is thought that some parts of East Africa were originally occupied by a pygmy population. They inter married with the migrating Bantu (typically 1.5-1.7m) who also inter married with the Nilotes (Very tall. Masai, Samburu, Luo...). So you now have a descendant population with large variances in height, waist size and so on.

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