The reason? Life on the veldt was tough, and prehistoric humans’ genes were constantly subjected to selective pressure in an environment where the species’ survival depended on it. For humans, that meant getting smarter. ”The development of our intellectual abilities and the optimization of thousands of intelligence genes probably occurred in relatively non-verbal, dispersed groups of peoples before our ancestors emerged from Africa,” Crabtree said in a news release.

The urbanization that followed the development of agriculture simplified survival by removing some of its challenges, which likely weakened natural selection’s ability to eliminate mutations associated with deficiencies in intelligence. Crabtree estimates that over the last 3,000 years (about 120 generations), humans have sustained at least two mutations that have eroded our intellectual and emotional intelligence.