Why do darker skinned people live nearer to the equator?

Well; mankind first originated and evolved in Africa - what's probably now Northeast Africa is where the first modern humans originated from. It's equatorial; it's very hot; there's huge amounts of sunlight. So the risk there is that people will end up with ultraviolet radiation penetrating their bodies.

Now, the first thing people think of when you say "ultraviolet" is skin cancer. But, actually, skin cancer isn't the sole explanation for why people have black skin. Most people don't get skin cancer until after the age at which they would have reproduced anyway and therefore it wouldn't really have any evolutionary benefit to them to have black skin.

In fact, why they black skin is to prevent ultraviolet radiation breaking down the chemical folate in the skin. Folate is really important for the production of DNA. In order to conserve their reserves of folate, people who were at first evolving in Africa actually evolved to have black skin. The common ancestor that we share going back in time with, say, chimpanzees about 6 million years ago, those animals all had pink skins but they have fur to protect them. As soon as they became hairless, our early ancestors had to evolve dark skin to protect them from the sun.

But when they migrated north, out of Africa, and they ended up in high latitudes, like in Britain, where sunlight is something we don't see very often, there was just not enough sunlight - especially coupled with dark skin - to produce enough Vitamin D which gets made in your skin, so people then became a bit Vitamin D deficient. So we lost the genes that made us have black skin in order to produce more Vitamin D. The benefit of doing that is that you have stronger bones. The down side is you're slightly more vulnerable to the ultraviolet in the sunlight, but as there's much less sunlight it was a worthwhile gamble to take.