A team at the University of Copenhagen have tracked down a mutation which took place 6-10,000 years ago and what they think is the cause of the eye colour of all blue-eyes humans today.

“Originally we all had brown eyes,” said Professor Eiberg from the department of Cellular and Molecular Biology. “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.”

This gene codes for a protein called P protein which is involved in the production of melanin – the pigment that gives colour to our hair eyes and skin. The switch, which is located in the gene adjacent to OCA2 does not turn off the gene entirely, but limits its action to reducing the production of melanin in the iris.

If the OCA2 gene had been entirely turned off, humans would be without melanin in their hair, eyes or skin – a condition known as albinism.

Variation in the colour of the eyes from brown to green can all be explained by the amout of melanin in the iris, but blue-eyed individuals only have a small degree of variation in the amount of melanin in their eyes.

“From this we can conclude that all blue-eyed individuals re linked to the same ancestor,” said Profesor Eiberg. “They all inherited the same switch at exactly the same spot in their DNA. Brown eyed individuals by contrast have considerable individual variation in the area of their DNA that controls melanin production.”

The mutation of brown eyes to blue represents neither a positive or negative mutation. It is one of several muations such as hair colour, freckles and baldness which neither increases or reduces a human’s chance of survival.

Eiberg said: “It simple 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.”