Whether a person has blue eyes or brown eyes depend on whether the iris contains melanin. Scientists believe that irises lacking melanin began as a mutation in one person that passed along well enough that 10% of the world's population now has blue eyes. Did the mutation provide some kind of benefit that encouraged its propagation?
A team from Liverpool John Moores University conducted an experiment with 39 volunteers, 25 with blue eyes and 14 with brown eyes. They found that those with blue eyes could read under dimmer light than those with brown eyes. The blue-eyed subjects could read under light down to only 0.7 lux on average, as compared with an average of 0.82 lux for the brown-eyed subjects. That may seem like a small difference, but it is statistically significant. However, the sample size was small.
If being able to see under dimmer light is an advantage conferred by blue eyes, it's no wonder that the mutation propagated most successfully in Europe, particularly northern Europe, where the sun hits the earth at a fairly extreme angle in the winter. No one knows where the mutation began, but it may have held an advantage only for those working under less sunlight. -via Real Clear Science
(Image credit: Rob Unreall)