Take a look around you. Look at the people in your neighborhood. Look at the kids in your school. Take notice of the people around your town, at the mall, in church, and at sporting events. What do you notice?
One thing that many Wonder Friends have noticed is that people come in all sorts of different colors. Why is that?
People noticed long ago that people from different parts of the world often had different skin colors. For example, people who lived in the tropics usually had darker skin than people who lived in colder climates.
Over time, scientists who studied the human body learned that variations in skin color appeared to be adaptive traits that were passed through genes from parents to children. These traits corresponded closely with geography and the Sun's ultraviolet (UV) radiation.
A person's skin color is determined by the amount of melanin in his or her skin. Melanin is a dark brown to black pigment produced by special cells called melanocytes. Melanin's purpose is to protect the skin from the Sun's harmful UV rays.
Ultraviolet radiation can cause skin cancer. Melanin acts as a natural sunscreen by absorbing these UV rays. The more melanin your skin has, the darker your skin will be and the more protection it will have against UV rays.
People with light skin have just a little melanin. People with darker skin have more melanin. People with yellow skin tones have more or another type of pigment called carotene.
People who live in the tropics are exposed to more of the Sun's harmful UV radiation. As a result, their skin tends to become darker as the body produces more melanin to counteract the effects of the Sun's rays. Over successive generations, genes are passed down from parents to their children, including the tendency to produce a certain amount of melanin given the area where they live.
Likewise, northern peoples tend to have lighter skin colors, because they don't receive as many of the Sun's harmful UV rays. As a result, their bodies do not need to produce as much melanin, which makes their skin color lighter.
Also, lighter skin tones in northern areas allow more UV rays to penetrate the skin to help produce the essential amounts of vitamin D that the body needs. The body must always strike a careful balance to make sure it receives just enough UV radiation to make essential vitamin D, while avoiding overexposure that can lead to skin cancer.
For example, in some northern coastal areas, such as Alaska and Canada, you'll notice that native peoples tend to have darker skin than you might otherwise expect. This is usually because they eat a diet rich in seafood that provides all the vitamin D their bodies need. As a result, their skin produces more melanin, which makes it darker.