University of Missouri scientists say that social competition is the major cause of a three-fold increase in the size of the human brain in the past two million years.
The researchers collected data from 153 hominid (humans and our ancestors) skulls from the past 2 million years, and examined the locations and 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 found that population density had the biggest effect on skull size and thus cranial capacity.
"Our findings suggest brain size increases the most in areas with larger populations and this almost certainly increased the intensity of social competition. When humans had to compete for necessities and social status, which allowed better access to these necessities, bigger brains provided an advantage," said David Geary, Curator's Professor and Thomas Jefferson Professor of Psychosocial Sciences in the MU College of Arts and Science.
According to the researchers, they also found some credibility to the climate-change hypothesis, which assumes that global climate change and migrations away from the equator resulted in humans becoming better at coping with climate change.
They, however, added that the importance of coping with 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. Our research tells us that competition, whether healthy or not, sets the stage for brain evolution," said Drew Bailey, MU graduate student and co-author of the study.
The study has been published in Human Nature.