Hair for Brain Trade-Off: A Metabolic Bypass for Encephalization

The document hypothesized that human hair loss was an evolutionary trade-off to support brain development, known as encephalization. It proposed that the metabolic energy saved from reduced hair growth could have been redirected to support the energy-intensive brain, explaining the unique pattern of human hairlessness compared to other primates. Simulations suggested that hair loss preceded brain size increase around 2.2-2.4 million years ago, facilitated by a sialic acid deletion mutation. The study estimated that a large-brained, hairy hominid would lose 21-56 grams of protein daily into hair, limiting brain growth unless hair density was reduced. This trade-off likely occurred in an equatorial African savannah, where insulation from hair was less critical. The authors concluded that the reduction in body hair might have provided a metabolic advantage by reducing energy expenditure on thermoregulation, aligning with the "expensive tissue hypothesis."