Every blue-eyed person on the planet is descended from a single European who lived around 6,000 to 10,000 years ago, and who first developed a specific mutation that accounts for the now widespread iris coloration.
Originally, all humans had brown eyes, although genetic variation relating to a gene called OCA2
resulted in changes to the amount of pigment produced by different
individuals, resulting in the emergence of different shades of brown.
Armed with this information, scientists had for many years searched for
the source of blue eyes on the OCA2 gene, but without success.
More recently, a mutation to a separate, nearby gene called HERC2 has been identified as the cause of blue eyes. This alteration switches off OCA2,
the gene that determines the amount of the brown pigment melanin that
we make. It is thought to have first occurred when humans began to
migrate from Africa to Europe, meaning that every person with blue eyes
is a descendent of a single early European.
The fact that every blue-eyed person alive today has this same
mutation is pretty compelling evidence for this theory, although the
identity of the initial mutant remains something of a mystery. To date,
the earliest set of sapphire-colored peepers
ever found belong to a 7,000-year-old skeleton that was discovered in
northern Spain. Naturally, the eyes had long since decayed, however
genetic analysis revealed that they would have appeared blue in color.