That's a good question.

The short answer is that sunlight can actually damage our DNA and the body protects itself by making skin pigment.

When light hits the skin cells it can damage the DNA and cause cancer. Skin cells divide fast, so some forms of skin cancer are very dangerous. The more pigment there is in our skin, the more light gets stopped before it can damage our cells. So why don't we all have very dark skin? Sunlight also helps our bodies make vitamin D, which allows us to absorb calcium. A lack of vitamin D causes a disease called rickets. People who have had rickets have weak, bowed legs. We get plenty of vitamin D in milk and other vitamin-fortified foods, so rickets is an extremely rare disease in the US today but it used to be a real problem.

People who live in very sunny places and have light skin are at a disadvantage because of the risk of cancer.

Before we had good medical treatments, these people may have had a much higher death rate, leaving their darker-skinned relatives to be the parents of the next generation and pass on the genes for darker skin.

People who live in places and with very little sun or where it's cold and they are always inside or covered up used to be at risk for rickets If they had darker skin they were at a disadvantage because they would absorb even less sunlight. Before we had vitamin D supplements, these people with their weak legs may have been less able to hunt and escape predators, resulting in a higher death rate, leaving their lighter-skinned relatives to be the parents of the next generation and pass on the genes for lighter skin.

Protect yourself from over-exposure to the sun by using sunblock properly and by wearing a hat and long sleeves when you will be out in the sun for a long time.

Ultraviolet rays do two things to you:

They cause skin cancer (as you probably already know), but they also make vitamin D from other chemicals in your skin. A few minutes of direct, full-body sunshine every day will give the average light-skinned person enough vitamin D to survive. After that, any more UV rays will do more harm than good.

So, the body needs a balancing mechanism which will keep UV rays to a minimum while still letting enough through to make the vitamin D. This mechanism is the skin pigment melanin, which darkens the skin and blocks UV rays. Long, frequent exposure to the sun triggers increased production of melanin, because the body figures it is getting plenty of vitamin D, so it might as well avoid getting skin cancer too.

Cover up, and your body reduces the pigment because it figures there is a greater risk of rickets (caused by not getting enough vitamin D). This mechanism doesn't work as well as it could- it has long response times, it can't drastically change someone's skin color, and it is tied to sunlight instead of vitamin D levels. Ideally we would change color like chameleons in response to our vitamin D levels, but that mechanism hasn't evolved yet. So, to cope with varying degrees of sunlight around the world, humans have evolved different base levels of pigment.

Africa and Australia have plenty of sun- enough to give white people skin cancer. Northern Europe is often cloudy, and people must cover most of their bodies to stay warm, and the sun circles closer to the horizon so the UV rays must pass through more blocking ozone on their way through the atmosphere.

Before the days of vitamin D milk, Africans would have died of rickets if they stayed in Finland too long. So, each place has a different skin color for optimal survival- people who are too dark or too light gradually die off. If there aren't any major migrations, evolution will cause skin color to gradually converge to the locally optimal value for each place.

If Santa Barbara were isolated for thousands of years, the people would surely become naturally dark. In fact, if everyone drinks vitamin D milk for the next few thousand years, then everyone will have skin like the Africans- because nobody would get rickets, but light skin would still carry a risk of skin cancer.