The Brain Explosion

The human brain is about three times larger than expected for a primate our size, and that tripling happened over just 2 million years — extraordinarily fast in evolutionary terms. The brain burns about 20% of our energy while making up only 2% of body weight, a steep cost that demanded big changes in diet.

Two theories explain the investment. The social brain hypothesis says managing relationships in growing groups demanded more processing power. The cooking hypothesis points to fire: heat pre-digests food, extracting far more calories and freeing energy for the brain. Both likely reinforced each other.