People with Blue Eyes Share a Single Ancestor
New research published in Human Genetics shows that people with blue eyes trace their ancestry back to a single individual. Researchers tracked down a genetic mutation which took place 6–10 000 years ago and is the cause of the eye colour of all blue-eyed humans without albinism alive on the planet today.
While blue eyes evolved only once, blonde hair has evolved at least twice: in Melanesian populations, blonde hair evolved independently to European populations, involving a mutation in a different gene.
“Originally, we all had brown eyes,” said Professor Hans Eiberg from the University of Copenhagen. “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.” The OCA2 gene codes for the P protein, which is involved melanin production. This ‘switch’, located in the gene next to OCA2, does not completely shut off production but instead is limited to reducing the production of melanin in the iris, effectively ‘diluting’ brown eyes to blue. The switch’s effect on OCA2 is very specific therefore. If the OCA2 gene is completely destroyed or turned off, albinism would be the result.
Eye colours from brown to green depend on the amount 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 are linked to the same ancestor,” said Professor Eiberg. “They have 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.
Professor Eiberg and his team studied mitochondrial DNA and compared the eye colour of blue-eyed individuals in countries as diverse as Jordan, Denmark and Turkey. His research stretches back to 1996, when he first implicated the OCA2 gene as being responsible for eye colour.
The mutation of brown to blue eyes does not confer any evolutionary advantage, as with others such as hair colour.
As Professor Eiberg explained, “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.”
Source: University of Copenhagen
