Human brains have tripled in size over the past 2 million years,  growing much faster than those of other mammals.

What might the reasons be for such dramatic brain expansion?

University of Missouri researchers studied three hypotheses for brain growth: ecological demand,  social competition and climate change.

Yes, climate change.   They're not stupid.   An entire presidential cabinet is stuffed with carbon dioxide true believers so it's good diplomacy to at least consider global warming may make us devolve - that would be terrific marketing for a carbon trading scheme.   Luckily, the much more likely social competition was determined in their analysis as the major cause of increased cranial capacity.

To test their hypotheses, they collected data from 153 hominid (humans and ancestors) skulls from the past 2 million years.  They examined the locations and made a best guess at the global climate changes at the time the fossil was dated, the number of parasites in the region and estimated population density in the areas where the skulls were found.

They determined that population density had the biggest effect on skull size and thus cranial capacity.  Sounds vague?  Well, there are few rules in psychosocial sciences and a lot of assumptions.

"Our findings suggest brain size increases the most in areas with larger populations and this almost certainly increased the intensity of social competition," said David Geary, Curator's Professor and Thomas Jefferson Professor of Psychosocial Sciences in the MU College of Arts and Science. "When humans had to compete for necessities and social status, which allowed better access to these necessities, bigger brains provided an advantage."

Climate change isn't completely out of the question tangentially, since global climate change and migrations away from the equator resulted in humans becoming better at coping with diverse environments. But the importance of coping with a changing climate was much smaller than the importance of coping with other people.

"Brains are metabolically expensive, meaning they take lots of time and energy to develop and maintain, making it so important to understand why our brains continued to evolve faster than other animals," said Drew Bailey, MU graduate student and co-author of the study. "Our research tells us that competition, whether healthy or not, sets the stage for brain evolution." 

Article:  "Hominid Brain Evolution," Human Nature, co-authored by Geary and Bailey.