This indicates that the mutation originated in just one person who became the ancestor of all subsequent people in the world with blue eyes, according to a study by Professor Hans Eiberg and colleagues at the University of Copenhagen.
The scientists are not sure when the mutation occurred but other evidence suggested it probably arose about 10,000 years ago when there was a rapid expansion of the human population in Europe as a result of the spread of agriculture from the Middle East.
“The mutations responsible for blue eye colour most likely originate from the north-west part of the Black Sea region, where the great agricultural migration of the northern part of Europe took place in the Neolithic periods about 6,000 to 10,000 years ago,” the researchers report in the journal Human Genetics.
Professor Eiberg said that brown is the “default” colour for human eyes which results from a build-up of the dark skin pigment, melanin. However, in northern Europe a mutation arose in a gene known as OCA2 that disrupted melanin production in the iris and caused the eye colour to become blue.
“Originally, we all had brown eyes,” said Professor Eiberg. “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.”
Variations in the colour of people’s eyes can be explained by 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, he said.
“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,” said Professor Eiberg.
Men and women with blue eyes have almost exactly the same genetic sequence in the part of the DNA responsible for eye colour. However, brown-eyed people, by contrast, have a considerable amount of individual variation in that area of DNA.
Professor Eiberg said he has analysed the DNA of about 800 people with blue eyes, ranging from fair-skinned, blond-haired Scandinavians to dark-skinned, blue-eyed people living in Turkey and Jordan.
“All of them, apart from possibly one exception, had exactly the same DNA sequence in the region of the OCA2 gene. This to me indicates very strongly that there must have been a single, common ancestor of all these people,” he said.
It is not known why blue eyes spread among the population of northern Europe and southern Russia. Explanations include the suggestions that the blue eye colour either offered some advantage in the long hours of daylight in the summer, or short hours of daylight in winter, or that the trait was deemed attractive and therefore advantageous in terms of sexual selection.
Source The Independent