This might provide some answers...a bit long but very informative....
Melanin comes in two types: pheomelanin (red) and eumelanin (very dark brown). Both amount and type are determined by four to six genes which operate under incomplete dominance. One copy of each of those genes is inherited from each parent. Each gene comes in several alleles, resulting in the great variety of different skin tones.
The evolution of the different skin tones is thought to have occurred as follows:[who?] the haired primate ancestors of humans, like modern great apes, had light skin under their hair. When Hominids evolved relative hairlessness (the most likely function of which was to facilitate perspiration), they evolved dark skin, which was needed to prevent low folate levels since they lived in sun-rich Africa. (The skin cancer connection is probably of secondary importance, since skin cancer usually kills only after the reproductive age and therefore does not exert much evolutionary selection pressure.) When humans migrated to less sun-intensive regions in the north, low vitamin D3 levels became a problem and light skin color re-emerged. Sexual selection and diet may have played a part in the evolution of skin tone diversity, as well.
The Inuit and Yupik are special cases: even though they live in an extremely sun-poor environment, they have retained their relatively dark skin. This can be explained by the fact that their traditional fish-based diet provides plenty of vitamin D.
Brown skin is the likely ancestral (or original) skin color among modern humans (Harding et al 2000). This is due to modern humanity's common origin in equatorial Africa ~200,000 years ago (Tishkoff, 1996). Dark skin was crucial in this UV rich context given that a thick coat of UV protective body hair had long been selected against by this time (Rogers et al 2004) most likely in order to facilitate the evaporation of perspiration (ie the cooling of the body). This trait (dark skin) continues to be under strong selection in equatorial regions such as Africa, India, and New Guinea (Harding 2000 p 1355). Geneticists estimate that a relatively small group of humans left Africa ~60,000 years ago, and that the descendants of this group went on to populate the entire non-sub-Saharan African world. Those migrants that settled in non-African equatorial regions (such as the mentioned India, New Guinea, and/or Australia) retained most of the ancestral sequence at the MC1R locus (Harding 2000 p 1355), a gene strongly a**ociated with determining skin color. Specifically, Harding et al (2000 p 1355) found that the haplotype sequences for Indians and New Guineans are virtually identical to those of continental sub-Saharan Africans (except for a small number of variants at silent sites).
The retention of the ancestral trait at the equator is due to natural selection for melanin pigment production which serves to protect the body from harmful UV rays (Jablonski 2006). Notably, given that hair is a part of the skin, the retention is also analogous to that which occurred for Natural afro-hair prior to pre-Holocene admixture events among people who settled in India and Australia. However, certain evidence suggests that, unlike skin color, Afro hair ceased to be under strong selection once dark skin arose ~1 million years ago (Harding 2000) (rather, it remained as a vestigial trait among Africans, Andamanese, and Melanesians and changed to straight in the north for adaptive reasons--see hair texture). In fact, dark skin is so selectively advantageous at the equator that initially light skinned native Americans who migrated to Mexico and/or South America experienced renewed selective pressure towards the evolution of dark skin.
According to (Norton et al., 2006), light skin observed in Europeans (with deep red and/or yellowish skin tones), non-Indian Southeast Asians, East Asians and North Africa (Maghreb) is due to independent genetic mutations in at least three loci. They concluded that light pigmentation is at least partially due to sexual selection, however Jablonski postulates that the predominant reason revolved around the facilitation of vitamin D production in northern Eurasia (see hair texture).
Health related effects
Dark skin (melanin) protects against ultraviolet light; this light causes mutations in skin cells, which in turn may cause skin cancers. Light-skinned persons have about a tenfold greater risk of dying from skin cancer under equal sunlight exposure, with redheads having the greatest risk. Furthermore, dark skin prevents radiation of UV-A rays from destroying the essential folic acid, derived from B vitamins. Folic acid (or folate) is needed for the synthesis of DNA in dividing cells and folate deficiency in pregnant women are a**ociated with birth defects.
While dark skin better preserves vitamin B, it can also lead to vitamin D deficiency at higher latitudes which in turn can cause fatal cancers affecting the colon, lung and prostate. Dark-skinned people are also at higher risk for rickets, cardiovascular disease, diabetes and multiple sclerosis.An American study by the USDA found 87% of African Americans to be Vitamin D deficient.To address this issue, some countries have programs to ensure fortification of milk with vitamin D.
The advantage of light skin at high latitudes is that it allows more sun absorption, leading to increased production of vitamin D3, necessary for calcium absorption and bone growth. The lighter skin of women at higher latitudes most likely results from the higher calcium needs of women during pregnancy and lactation. However, some have postulated that it may also derive from sexual selection.
Albinism is a condition characterized by the absence of melanin, resulting in very light skin, eyes, and hair; it is caused by an inability to synthesize tyrosine, and has a genetic basis.
Cultural effects
Sexual preference of paleness in women by men is a cultural universal[citation needed], and has been reported from medieval Japan or Aztec Mexico[citation needed], even before there was significant contact with Western societies. The effect has even been found in Moorish Spain, where the ruling cla** was of darker complexion than the conquered natives.Also, preference of lighter-skinned women by black men is reported both in sub-Saharan Africa and in the black diaspora.In his foreword to Peter Frost's 2005 Fair Women, Dark Men, U. of Washington sociologist Pierre L. van den Berghe summarizes:
"Although virtually all cultures express a marked preference for fair female skin, even those with little or no exposure to European imperialism, and even those whose members are heavily pigmented, many are indifferent to male pigmentation or even prefer men to be darker."
A consequence of this is that, since higher-ranking men get to marry the more attractive women, the upper cla**es of a society generally tend to develop a lighter complexion than the lower cla**es by sexual selection (see also Fisherian runaway).
Differences in skin tone are the most readily perceptible phenotypical distinction of human populations, and hence has historically lent itself to color terminology for race, often to the effect of darker skin being seen as being of lowest social value, and lighter skin of highest. However, according to cla**ical scholar Frank Snowden, the Egyptians and Greeks (et al.) a**igned relatively neutral connotations to skin color variation because conquest rather than skin color was the major determinant of slave status.
Skin tone variability
The tone of human skin can vary from a dark brown to nearly a colorless pigmentation, which may appear reddish due to the blood in the skin. Europeans generally have lighter skin, hair, and eyes than any other group on Earth, although this is not always the case. For practical purposes, such as exposure time for sun tanning,
source-wikipedia
