ALEXANDER McWHINNIE

It has been a staple of countless cheesy discos and weddings for more than 30 years but now researchers have discovered that Crystal Gayle's song Don't It Make My Brown Eyes Blue was on to something after all: brown eyes did, indeed, turn blue sometime between 6000 and 10,000 years ago.

A single mutation in an individual probably living in the region north-west of the Black Sea is now thought to be the origin of blue eyes. According to scientists in Denmark, the trait then spread with the large migrations of people seeking new agricultural land at the end of the last Ice Age.

The discovery would also explain why blue eyes are restricted to the peoples of northern Europe and southern Russia. Around 8% of the planet's population is blue-eyed. "Originally, we all had brown eyes," said Professor Hans Eiberg of the University of Copenhagen, part of the research team who conducted the study. "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."

A variety of genetic sequencing is found among the brown-eyed. It is the "default" setting for humans, according to Professor Eiberg. Researchers found, though, that this was not the case among the blue-eyed. They examined individuals from across a wide area of the world - in countries as diverse as Jordan, India, Denmark and Turkey - and discovered that people with blue eyes have almost exactly the same DNA sequencing.

In fact, 99.5% of their sample had the same tiny mutation.

"They have all inherited the same switch at exactly the same spot in their DNA," said Professor Eiberg, implying, remarkably, that a single mutation is responsible.

But how could a single mutation have had such a drastic effect on eye colour? The reason lies in how the colour blue is produced in the eye. It is not the result of a blue-coloured pigment but rather the absence of the usual dark brown melanin from the front region of the iris. The mutation switched off production of the pigment in this area, allowing light to travel through the clear "stroma" of the middle of the iris and then be reflected off the back wall of the eye.

During its journey through the eye, the light encounters large collagen molecules. Light of shorter wavelength is more susceptible to being reflected and "scattered" by these molecules. This phenomenon is known as "Rayleigh scattering" and is the same effect that occurs in the atmosphere, where light travelling over longer distances is scattered by gas molecules. Short-wavelength light appears blue to us, which is why the iris and the sky appear blue.

Quite why the trait has survived and appears to have flourished is not known, however. It may have offered a survival advantage in the cloudy and darker north or it may have enhanced sexual attractiveness. In ancient societies men would have been in short supply anyway as many would be killed either in hunting expeditions or skirmishes with other groups. Anything which gave a female an advantage in mating with the few available men is thus likely to have had a real effect in spreading the gene into future generations.

Young babies often start life with blue eyes which, as the exposure to daylight stimulates melanin production, change colour to a darker shade.