Scientists have tracked down a genetic mutation which took place 6,000-10,000 years ago and leaves all blue-eyed humans to have a single, common ancestor.

Turns out that all humans originally had brown eyes, but a single mutation turned off a gene code's(OCA2) ability to produce these brown eyes. This mutation reduces the production of melanin in the iris and, in a sense, "dilutes" the brown eyes to blue.  Although, completely turning off this gene would result in albinism.

“From this we can conclude that all blue-eyed individuals are linked to the same ancestor,” says Professor Eiberg. “They have all inherited the same switch at exactly the same spot in their DNA.” Brown-eyed individuals, by contrast, have considerable individual variation in the area of their DNA that controls melanin production.

Cool.