Melanin

Melanin is the natural pigment that gives color to skin, hair and eyes. But color is almost a side effect: its main job is to protect cells from the damage done by ultraviolet radiation, the invisible part of sunlight that can reach and damage genetic material.

There is not one melanin, there are two. Eumelanin is brown or black. Pheomelanin is red or yellow. The balance between the two is what explains why one person's skin, hair and eyes are one color and another person's are a different color; it is not that some people have pigment and others do not.

And the protection is almost a matter of architecture. Melanin is packaged into melanosomes, small packets with a membrane of their own, and those packets line up over the nucleus of the keratinocyte, the most abundant cell in the skin. The nucleus holds the DNA, the cell's instruction manual, and the pigment sits right on top of it like a cap that shades it. It is a parasol set over the one thing you do not want to burn.