There doesn’t seem to be a clear explanation as to why:
Based on measurements from 122 populations, modern adult brains range from 900 to 2,100 mL, with a global average of 1,349 mL, which is smaller than our Stone Age predecessors. However, we can’t reach meaningful conclusions from these species-wide global averages, in part because methods of skull measurement differ between datasets.
More convincing evidence for cranial decline comes from studies that applied the same measuring technique to hundreds or even thousands of skulls from a particular region across the millennia. Bridget Alex, “The Human Brain Has been Getting Smaller Since the Stone Age” at Discover Magazine
Apparently, a 1988 study of 12,000 European and North African skulls showed a 10% decrease in males and 17% in females in the last 10,000 years.
But what does it all mean? Alex surveys a variety of theories and favors the one that suggests that brain size decreased as we domesticated ourselves and grew more social. The same pattern is found in domestic animals.
The trouble is, it’s not clear what the impact of the size of a human brain really is, vs. its organization. Then there’s the question of what opportunities a
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See also: Do big brains matter to human intelligence?
Just a few of the attempts to explain how the human brain came to be what it is:
Eating fat, not meat, led to bigger human type brains, say researchers. Theories of the evolution of the human brain are a war of trivial explanations that no one dare admit are too trivial for what they purport to explain. It’s like blaming World War II on indigestion, only monstrously bigger.
Earlier discussion of the fat theory.
Starchy food may have aided human brain development
Do big brains matter to human intelligence?
Human evolution: The war of trivial explanations
What great physicists have said about immateriality and consciousness