In physical anthropology, the genetic adaptation of human beings to different environmental conditions, such as extreme cold, humid heat, desert habitat, and high altitudes.
Extreme cold favours short, round bodies with short arms and legs, flat faces with fat pads over the sinuses, narrow noses, and a heavy layer of body fat. These adaptations provide minimum surface area in relation to body mass for minimum heat loss and protect the lungs and base of the brain against cold air in the nasal passages. In conditions of humid heat, where body heat must be dissipated, selection favours tall and thin bodies with maximum surface area for heat radiation. A wide nose prevents warming of the air in the nasal passages, and dark skin protects against harmful solar radiation. The desert-adapted person must compensate for water loss through sweating. A thin but not tall body minimizes both water needs and water loss; skin pigmentation is moderate, since extreme pigmentation is good protection from the sun but allows absorption of heat from direct sunlight, which must be lost by sweating. Adaptation to night cold, often part of a desert environment, provides increased metabolic activity to warm the body during sleep. High altitudes demand, in addition to cold adaptation, adaptation for low air pressure and the consequent low oxygen, usually by an increase in lung tissue. See also acclimatization ; race .