years ago. Bipedal hominin body hair may have disappeared gradually to allow better heat dissipation through sweating. The emergence of skin pigmentation dates to about 1.2 million years ago, under conditions of a megadrought that drove early humans into arid, open landscapes. Such conditions likely caused excess UV-B radiation. This favored the emergence of skin pigmentation in order to protect from folate depletion due to the increased exposure to sunlight. A theory that the pigmentation helped counter xeric stress by increasing the epidermal permeability barrier has been disproved. With the evolution of hairless skin, abundant sweat glands, and skin rich in melanin, early humans could walk, run, and forage for food for long periods of time under the hot sun without brain damage due to overheating, giving them an evolutionary advantage over other species. By 1.2 million years ago, around the time of Homo ergaster, archaic humans (including the ancestors of Homo sapiens) had exactly the same receptor protein as modern sub-Saharan Africans. This was the genotype inherited by anatomically modern humans, but retained only by part of the extant populations, thus forming an aspect of human genetic variation. About 100,000–70,000 years ago, some anatomically modern humans (Homo sapiens) began to migrate away from the tropics to the north where they were exposed to less intense sunlight. This was possibly in part due to the need for greater use of clothing to protect against the colder climate. Under these conditions there was less photodestruction of folate and so the evolutionary pressure working against the survival of lighter-skinned gene variants was reduced. In addition, lighter skin is able to generate more vitamin D (cholecalciferol) than darker skin, so it would have represented a health benefit in reduced sunlight if there were limited sources of vitamin D. Hence the leading hypothesis for the evolution of human skin color proposes that: From about 1.2 million years ago to less than 100,000 years ago, archaic humans, including archaic Homo sapiens, were dark-skinned. As Homo sapiens populations began to migrate, the evolutionary constraint keeping skin dark decreased proportionally to the distance north a population migrated, resulting in a range of skin tones within northern populations. At some point, some northern populations experienced positive selection for lighter skin due to the increased production of vitamin D from sunlight and the genes for darker skin disappeared from these populations. Subsequent migrations into different UV environments and admixture