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. Furthermore, the brain is not the only “expensive tissue” in the body. Other tissues with similarly high energy demands include the heart, liver, kidneys, and digestive organs. Taken together, the brain and these vital organs make up less than 7 percent of the body’s weight, yet, when the body is in
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The ability to cook through the use of fire allowed us to spend less time eating/digesting raw food, which in turn helped us conserve much needed energy to be applied elsewhere. These other endeavors ultimately freed us from the restraints of a long hunter/gatherer past.
