Humans evolved in Africa, the most tropical continent on Earth. When your ancestors lost their primate fur and moved out onto the African savannas, their skin was exposed to a tremendous amount of sunlight. In order to survive this assault from damaging solar radiation, their skin became darker through the production of melanin. This is the ancestral state for our species — every person alive 200,000 years ago had darkly pigmented skin, like sub-Saharan Africans today. During our migration out of the tropics into Eurasia beginning 60,000 years ago, we lost some of this pigmentation as an adaptation to the lower sun intensity at higher latitudes. It turns out that sunlight is necessary for the deeper layers of your skin to produce vitamin D — without this critical nutrient your bones become weak and brittle, and your ancestors’ diets didn’t include much of it. Mutations that reduced the amount of pigmentation in the skin of people living in northern latitudes were advantageous, and became widespread. One of the genes regulating pigmentation is melanocortin receptor 1, or MC1R. Modern humans have several mutations in this gene that are associated with fair or red hair and lighter skin, and it appears that your Neanderthal cousins had similar (but unique) changes in the MC1R gene. One of these Neanderthal MC1R variants was introduced into modern human population when humans interbred with Neanderthals, and the advantage it conferred to its carriers caused it to increase in frequency, such that today is widespread in modern non-African populations, particularly in East Asians and Native Americans. Another gene responsible for regulating pigmentation is oculocutaneous albinism 2, or OCA2. It is also associated with fair or red hair and lighter skin and like MC1R has a unique Neanderthal version. Today, OCA2 is widespread in modern Eurasian populations, particularly in East Asia.