Genetic research led by scientists at the University of Copenhagen in 2008 found that all people with blue eyes likely trace their eye color to a single common ancestor.

This individual carried a genetic mutation in the OCA2 gene, which occurred approximately 6,000–10,000 years ago, most likely in the Black Sea region.

The mutation essentially "turned off" the ability to produce brown melanin in the eyes, resulting in blue instead. Before this mutation, everyone had brown eyes.

Blue eyes are now understood to be the result of this genetic bottleneck, making it a relatively recent human trait in evolutionary terms.

Sources:

- Eiberg et al., University of Copenhagen (2008)
- LiveScience
- BBC