The Bipedal Revolution

Walking on two legs is far more energy-efficient than the way chimps move. It also freed the hands for carrying food, tools, and infants — cascading advantages that reshaped social life and the brain over millions of years.

Homo erectus, appearing about 2 million years ago, was built for distance. Tall and lean, with far more sweat glands than a chimp — roughly ten times as many per patch of skin — it could shed heat while staying active in the open. Many researchers think this cooling ability supported persistence hunting, the idea that early humans chased prey at a steady jog until the animal overheated. This endurance-running hypothesis is debated — there is no direct archaeological proof, and the method is rare among recent hunter-gatherers — but it highlights something not in doubt: humans are exceptional endurance runners in the heat.