A University of Copenhagen team has identified the gene which around 6-10,000 years ago underwent a genetic mutation in one individual who eventually gave rise to all blue-eyed people.
Professor Eiberg from the Department of Cellular and Molecular Medicine began his research in 1996, when he "first implicated the OCA2 gene as being responsible for eye colour", as ScienceDaily puts it.
Over the next decade, he and his colleagues "examined mitochondrial DNA and compared the eye colour of blue-eyed individuals" in countries including Denmark, Jordan and Turkey.
Eiberg explained: “Originally, we all had brown eyes. But a genetic mutation affecting the OCA2 gene in our chromosomes resulted in the creation of a 'switch', which literally 'turned off' the ability to produce brown eyes."
Specifically, ScienceDaily explains, the OCA2 gene "codes for the so-called P protein, which is involved in the production of melanin". The “switch”, located in the gene adjacent to OCA2, doesn't turn off the gene entirely, but "limits its action to reducing the production of melanin in the iris", thus “diluting” brown eyes to blue.
That the switch doesn't entirely disable the OCA2 gene is significant, because a complete shut-down of melanin production would result in albinism.
The proof that all blue-eyed people have a common ancestor comes from the fact that whereas eye colours ranging from brown to green are caused by relatively large differences in the amount of melanin in the iris, controlled by "considerable individual variation" in the area of the DNA responsible for melanin production, the variation in iris melanin levels across all blue-eyed individuals is very small.
Eiberg elaborated: “From this we can conclude that all blue-eyed individuals are linked to the same ancestor. They have all inherited the same switch at exactly the same spot in their DNA.”
Eiberg noted that the blue eyes mutation is neither "positive nor negative", since it doesn't affect chances of survival. He concluded: "It simply shows that nature is constantly shuffling the human genome, creating a genetic cocktail of human chromosomes and trying out different changes as it does so.” ®