Researchers in Denmark have found that every person with blue eyes descends from just one "founder," an ancestor whose genes mutated 6,000 to 10,000 years ago. Before then, everyone had brown eyes.

Eiberg's team then tested 155 blue-eyed people from Scandinavia, Turkey, Jordan and India, looking to see whether they, too, had similar DNA sequences on that gene. To their amazement, they found that each individual had identical DNA sequences in that region of that gene, an indication that the original mutation happened recently enough that it hasn't had time to change.

Everyone has two genes for eye color, one from their father and one from their mother. Brown eyes are dominant, so even if someone has one blue and one brown-eye gene, he'll still have brown eyes.