Aiello and Wheeler observed that the size of the human brain is more than four times as large, relative to its body weight, as the brain of the average mammal. The human brain weighs about three pounds—about 2 percent of the average adult’s total body weight. Yet when it is active, the human brain can consume as much as 20 percent of the body’s available energy—roughly ten times as much energy, pound for pound, as is consumed by the human body as a whole.
