Dark skin in the context of Folate

Human skin color ranges from the darkest brown to the lightest hues. Differences in skin color among individuals is caused by variation in pigmentation, which is largely the result of genetics (inherited from one's biological parents), and in adults in particular, due to exposure to the sun, disorders, or some combination thereof. Differences across populations evolved through natural selection and sexual selection, because of social norms and differences in environment, as well as regulation of the biochemical effects of ultraviolet radiation penetrating the skin.

Human skin color is influenced greatly by the amount of the pigment melanin present. Melanin is produced within the skin in cells called melanocytes; it is the main determinant of the skin color of darker-skin humans. The skin color of people with light skin is determined mainly by the bluish-white connective tissue under the dermis and by the hemoglobin circulating in the veins of the dermis. The red color underlying the skin becomes more visible, especially in the face, when, as a consequence of physical exercise, sexual arousal, or the stimulation of the nervous system (e.g. due to anger or embarrassment), arterioles dilate. Color is not entirely uniform across an individual's skin; for example, the skin of the palm and the soles of the feet is lighter than most other skin; this is more noticeable in darker-skinned people.

Black is a racial classification of people, usually a political and skin color-based category for specific populations with a mid- to dark brown complexion. Often in countries with socially based systems of racial classification in the Western world, the term "black" is used to describe persons who are perceived as darker-skinned in contrast to other populations. It is most commonly used for people of sub-Saharan African ancestry, Indigenous Australians, Melanesians, and Negritos, though it has been applied in many contexts to other groups, and is no indicator of any close ancestral relationship whatsoever. However, not all people considered "black" have dark skin and often additional phenotypical characteristics are relevant, such as certain facial and hair-texture features. Indigenous African societies do not use the term black as a racial identity outside of influences brought by Western cultures.

Contemporary anthropologists and other scientists, while recognizing the reality of biological variation between different human populations, regard the concept of a "Black race" as social construct. Different societies apply different criteria regarding who is classified "black", and these social constructs have changed over time. In a number of countries, societal variables affect classification as much as skin color, and the social criteria for "blackness" vary. Some perceive the term 'black' as a derogatory, outdated, reductive or otherwise unrepresentative label, and as a result neither use nor define it, especially in African countries with little to no history of colonial racial segregation.

Australo-Melanesians (also known as Australasians or the Australomelanesoid, Australoid or Australioid race) is an outdated historical grouping of various people indigenous to Melanesia and Australia. Controversially, some groups found in parts of Southeast Asia and South Asia were also sometimes included.

While most authors included Papuans, Aboriginal Australians and Melanesians (mainly from Fiji, New Caledonia, Solomon Islands, and Vanuatu), there was controversy about the inclusion of the various Southeast Asian populations grouped as "Negrito", or a number of dark-skinned tribal populations of the Indian subcontinent.

Light skin is a human skin color that has a low level of eumelanin pigmentation as an adaptation to environments of low UV radiation.Due to migrations of people in recent centuries, light-skinned populations today are found all over the world. Light skin is most commonly found amongst the native populations of Europe, East Asia, West Asia, Central Asia, South Asia, Siberia, and North Africa as measured through skin reflectance. People with light skin pigmentation are often referred to as "white", but the majority of countries officially categorize people by ethnic or national origin and not by perceived skin tone. Furthermore, definitions and perceptions of "ethnicity" or "race" vary greatly from country to country.

Humans with light skin pigmentation have skin with low amounts of eumelanin, and possess fewer melanosomes than humans with dark skin pigmentation. Light skin provides better absorption qualities of ultraviolet radiation, which helps the body to synthesize higher amounts of vitamin D for bodily processes such as calcium development. On the other hand, light-skinned people who live near the equator, where there is abundant sunlight, are at an increased risk of folate depletion. As a consequence of folate depletion, they are at a higher risk of DNA damage, birth defects, and numerous types of cancers, especially skin cancer. Humans with darker skin who live further from the tropics may have lower vitamin D levels, which can also lead to health complications, both physical and mental, including miscarriage and a greater risk of developing schizophrenia. These two observations form the "vitamin D–folate hypothesis", which attempts to explain why populations that migrated away from the tropics into areas of low UV radiation evolved to have light skin pigmentation.

Black hair is the darkest and most common of all human hair colors globally, due to large populations with this trait. This hair type contains a much more dense quantity of eumelanin pigmentation in comparison to other hair colors, such as brown, blonde and red. In English, various types of black hair are sometimes described as soft-black, raven black, or jet-black. The range of skin colors associated with black hair is vast, ranging from the palest of light skin tones to dark skin. Black-haired humans can have dark or light eyes, but more commonly dark.