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.
Terminology
A number of terms are used to describe critical illness polyneuropathy, partially because there is often neuropathy and myopathy in the same person, and nerve and muscle degeneration are difficult to distinguish from each other in this condition. Terms used for the condition include: critical illness polyneuromyopathy, critical illness neuromyopathy, and critical illness myopathy and neuropathy (CRIMYNE). Boltons neuropathy is an older term, which is no longer used. See also
Chronic critical illness

References


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