As a brown-eyed person, I’ve always been jealous of people with striking blue eyes, but it turns out there’s a reason I never got that genetic gift: It comes from a select lineage. At one point in human history, everyone had brown eyes, until a single person developed the blue-eyed mutation. Every other blue eyed person is descended from that one common ancestor.
The colour of our eyes is related to the amount of melanin in them – the same pigment that dictates our skin colour. Dark brown eyes have lots of melanin, while blue eyes have relatively little.
The first blue-eyed person would have had a genetic anomaly causing them to produce less melanin in their irises, which then was passed around to future generations. How did this spread from just one person to a significant percentage of the population, though?
Some have theorised that, just like today, blue eyes were seen as attractive or desirable, allowing the trait to spread rapidly throughout the population. Around 8 per cent of the world population has blue eyes, though in localised populations in parts of Europe this can be as high as 80 per cent.
A team from the University of Copenhagen were the first to track down the genetic mutation causing blue eyes, saying it likely occurred around 6000 to 10,000 years ago.
“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,” said Professor Hans Eiberg from the Department of Cellular and Molecular Medicine.
The mutation was very specific – the OCA2 gene also is responsible for the pigmentation of our skin and hair, but in this case only the part of it that controlled melanin production in the iris was affected.
Interestingly, there are no blue pigments in blue eyes – all eye colours, from blue to green to brown and hazel are created from different levels of the same brown melanin pigment. They aren’t so much differently pigmented as a different level of the same pigment.