Around 6 million years ago, human like apes started walking on two legs. Hot under the African sun they lost their hairy bodies, but started to burn. Only those with darker pigmentation in their skin could continue to roam comfortably in the midday sun.
An excess of UV can not only burn and cause melanomas, but will also strip the body of folic acid. This is essential for foetus development and this susceptibility could be a main factor in driving natural selection towards darker skin.
Six million years later, humans living across the equator, where the sun is strongest, retain this dark layer of protection against the sun’s UV. But the skin doesn’t block it all. It needs to absorb a certain amount to convert into Vitamin D. This is used in processing calcium for bone growth and maintenance, a lack of which can cause disorders such as rickets.
When humans migrated north, the sun disappeared. As well as being cold, rainy and sad, it was harder to absorb the amount of UV needed when there was well evolved protection in the way. Lower amounts of melanin in skin spread through the sun deprived population and UV was now more easily absorbed. The further north, the less sun was available, the less pigmentation people needed and the lighter the average skin tone became, until Northern Europe where the palest skin is found.
A new diet rich in cereals from agricultural societies which were low in vitamin D concentration, as opposed to a diet largely consisting of hunted meat; common amongst sub Saharan African diets in the Palaeolithic, further exasperated this condition.
One exception to this are the communities in Northern Canada and Alaska. While in very weak sunshine for most of the year, they retain darker skin due to the food they consume. A diet high in seal and other marine sources, it contains all the Vitamin D they need. They do not need to absorb UV, so their melanin composition does not matter. Instead, evolution has focussed on creating a protective barrier against the harmful effects.
Interestingly, women are often paler than their male counterparts. Women need more Vitamin D for pregnancy and lactation and are more at risk of osteoporosis in old age than men. The cost of the dangers of UV are outweighed by this need to produce milk while retaining enough nutrients to support their own body.
Albinism results in no pigmentation at all, including the hair and eyes. It is caused by a recessive allele and creates an “all or nothing” response, as opposed the sliding scale of skin pigmentation usually seen. It affects 1 in 5000 in sub-Saharan Africa and 1 in 20,000 in Europe and North America and it varies between other countries too.
However, globalisation means people are no longer confined to the areas their ancestors lived in. Short term precautions can render thousands of years of adaptations redundant. Sunscreen can protect almost as well as extra melanin when properly applied. Vitamin D is not naturally found in common food items, but now it is artificially added to cereals, soy milk and other products. There is no reason for different skin tones in the modern world, other than a way to express our heritage.