A human adult's brain weighs about 3 pounds (1.5 kg). Scientists believe that the modern human adult's brain is about 10% smaller than it was 10,000 years ago, which is thought to be due to the introduction of agriculture and less of a need to be hunter-gatherers. A human adult's brain is about three times as large in relation to the rest of the body when compared with the brains of other similarly sized mammals. It also uses up a large amount of energy, requiring about 20% of the body’s energy supply even though the brain is just 2% of the body’s mass.
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