There has been incredibly strong selection pressure for babies being able to get down the birth canal - the death rate in child birth pre-modern times was something like 10%. The human brain is about as big as you can make it and still reproduce. What we would expect when you have strong selection for two competing forces (smaller head size and greater intelligence) is for the brain to shrink in size and become more asymmetrical - basically you get rid of system redundancy by having each side of the brain specialise in one thing. This is exactly what we have seen in humans.
One of the more interesting aspects of recent human evolution is how much has occurred in the the last 10,000 years. Evolutionary speed is basically proportional to the population size and the massive expansion that agriculture allowed has caused the rate of evolution in humans to increase around 1000 fold over the neolithic rate.
One other factor looking at eurasian brain size is gene competition resulting from the neanderthal / ancient african hybridisation event that occurred around 50,000 years ago. When you have a hybridisation event the genes from the two populations often don’t get along too well and it takes a while for selection to remove the incompatibilities. I would not be surprised if the genes controlling brains size in neanderthals and ancient africans were not very compatible (there is no reason that they should be given selection for brain size increase took place in parallel in both species) and that hybridisation resulted in an brain size excessively large given the birth constraints. It would be really great to know the change in childbirth death rate over the last 50,000 years.
When we look at the baby-in-a-birth-canal problem, you can obviously go two ways: smaller baby head (and torso) or wider birth canal.
Apparently Neanderthals had wider pelvis _and_ larger heads/brains. So they kind of picked the #2 route.
The problem with pelvis expansion is it is hard to select for widening of the pelvis without upsetting walking and running. Development is hierarchical and bones are lower down in the heirachy than brain size.
In regards Neanderthals I would not be surprised if it was wider pelvis that allowed for them to have larger brained babies. This is a fascinating area.
One of the more interesting aspects of recent human evolution is how much has occurred in the the last 10,000 years. Evolutionary speed is basically proportional to the population size and the massive expansion that agriculture allowed has caused the rate of evolution in humans to increase around 1000 fold over the neolithic rate.
One other factor looking at eurasian brain size is gene competition resulting from the neanderthal / ancient african hybridisation event that occurred around 50,000 years ago. When you have a hybridisation event the genes from the two populations often don’t get along too well and it takes a while for selection to remove the incompatibilities. I would not be surprised if the genes controlling brains size in neanderthals and ancient africans were not very compatible (there is no reason that they should be given selection for brain size increase took place in parallel in both species) and that hybridisation resulted in an brain size excessively large given the birth constraints. It would be really great to know the change in childbirth death rate over the last 50,000 years.