Further to the claim that ecology, not social challenges drove the huge increase in human brain size, Ashley Yeager offers some alternative views at The Scientist:
“González-Forero and Gardner are on the right track,” David Geary of the University of Missouri in Columbia tells New Scientist. But he questions whether the model accurately calculated just how challenging it is to live in groups. “Their conclusion that human brain evolution was largely driven by ecological pressures, and only minimally by social pressures, is surprising and likely premature.”
Language is another missing link in the model, Dean Falk, a brain-evolution expert at Florida State University, tells The Washington Post. González-Forero admits that the model falls short in addressing the influence cultural factors, such as language, had on expansion of brain size, but he and Gardner plan to incorporate those human traits in future work. More.
Put another way, no one has any better theories.
See also: Human evolution researchers: Social challenges decreased brain size Mathematical models are probably the closest thing to literary fiction that science offers. Ferrero tells us, “[o]ur approach offers a new way to understand brain evolution using little more than some maths.” It is good that no more resources were wasted on a thesis so inconclusive.
Can we talk? Language as the business end of consciousness
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The Science Fictions series at your fingertips: Human evolution