You might not think of having blue eyes as much of an oddity, but, originally, we all had brown eyes. It's thought that the blue-eyed gene originated as recently as 10,000 years ago somewhere near the Black Sea when a genetic mutation turned the brown eyes blue. Brown eyes are caused by the levels of melanin in the iris (the same substance that gives you a suntan), the genetic mutation essentially "switches off" the gene that produces melanin in the eyes (but not the skin and hair), diluting the eyes to a blue colour. The blueness is caused by the lack of melanin, rather the presence of something else, which is also why Caucasian babies are all born with blue eyes. Due to the lack of genetic variation in blue-eyed people, compared to brown-eyed, scientists reckon that all blue-eyed people have a single common ancestor. Also, can we take a moment to appreciate how impressive it is that science can track down the time and place that a gene mutated? Anyway, the reason blue eyes caught on in such a big way is still a subject of much debate. It could be to do with the increased sensitivity to light that blue eyes afford, because the lighter iris reflects more light into the eye, which could be why blue eyes are more common in places with less intense sunlight. It could be that the mutation originally proliferated because it served as a kind of prehistoric paternity test. As it is basically impossible (short of another genetic mutation) for two blue-eyed parents to produce a brown-eyed baby, it would benefit a blue-eyed man to seek out a blue-eyed mate as a way of ensuring that the offspring he was investing his time and resources in was definitely his. A study that found that blue-eyed men tend to prefer blue-eyed women, but women and brown-eyed men didn't care either way, seem to support this. I have also just performed this experiment with a sample size of 1 (the guy opposite me), and found it to be true. Science.