As the world grows wealthier, births by Caesarian section are growing common.

This sends human evolution in a new direction.

Obviously the human species is optimized for intelligence.  That is its competitive advantage.  Intelligence requires brain capacity, which requires a big head.  The correlation is not perfect, but a strong predictor of an adult’s intelligence is head size at birth.

Head size has not changed much for millennia.  This, in turn, implies that human head size has been at an equilibrium.  If the head got much smaller, offspring would tend to be too dumb to survive and/or compete for mates.  If the head got much bigger, the baby wouldn’t fit through the birth canal, and mother and child would both die at birth.

The evolutionary advantage of large head size is implicit in the high death rates that prevailed in childbirth before modern medicine.  Big heads were so competitively valuable that they were worth a material risk of the mother dying on every single birth.

With widespread C-sections, that risk is no longer material, and human head size is now unbounded.  If high intelligence is still reproductively valuable (I don’t know the answer to that), then we should now expect human heads to grow larger and larger.

If that happens, then we also take on a new civilizational instability.  If humans adapt to produce babies that do not fit through the birth canal, and if C-sections for some reason became unavailable for a period of decades…