By MICHAEL HANLON

Last updated at 08:54 01 February 2008

Throughout history they have been the eyes that are prized.

Frank Sinatra's were legendary, Paul Newman's melted a million hearts while Cameron Diaz's dazzle in modern Hollywood.

But how - and why - blue eyes arose has always been something of a genetic mystery. Until now.

According to a team of researchers from Copenhagen University, a single mutation which arose as recently as 6-10,000 years ago was responsible for all the blue-eyed people alive on Earth today.

The team, whose research is published in the journal Human Genetics, identified a single mutation in a gene called OCA2, which arose by chance somewhere around the northwest coasts of the Black Sea in one single individual, about 8,000 years ago.

The gene does not "make" blue in the iris; rather, it turns off the mechanism which produces brown melanin pigment. "Originally, we all had brown eyes," says Dr Hans Eiberg, who led the team.

And most people still do. The finding that a rare mutation, probably dispersed in the rapid wave of colonisation that followed the end of the last ice age, highlights one of the great mysteries of human evolution: the oddness of Europeans.

Those from Europe and the Near-East have many characteristics that set them apart from the rest of the human race.

Not only are Europeans far more likely to have blue eyes (95 per cent in some Scandinavian countries), they also have a far greater range of skin tones and hair colour than any other ethnic grouping.

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It is only in Europe that you will find large numbers of blondes and redheads, brunettes, pale skins and olive skins, blueeyed and green-eyed people living together in the same communities. Across the rest of the world people are almost uniformly darkhaired and dark-eyed.

Why this should be remains unknown, and in particular how such mutations can have arisen so quickly since Europe was colonised by Africans just a few tens of thousands of years ago.

One theory is that Europe's cold weather and dark skies played a part. Fair skin is better at making Vitamin D from the 8 per cent of the world's population have blue eyes weak sunlight found in northern latitudes.

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Another suggestion is that the strange skin, eye and hair colours seen in Europe are down to ancient interbreeding with the Neanderthals, who died out about 25,000 years ago.

Maybe the Neanderthals were blonde or red-haired and it is their genes which we have inherited. The trouble with this theory is that there is no evidence, from the scraps of Neanderthal DNA that have been recovered from bones, that there was any substantial interbreeding between them and Homo sapiens at all.

Perhaps the most plausible theory is that blonde hair and blue eyes arose because of a mechanism called sex selection.

This is where males and females choose as their mates those who have one unusual physical characteristic, not necessarily associated with "fitness" per se but simply something unusual.

The gigantic (and otherwise useless) tail of the peacock is the best example.

Sex selection comes to the fore when there is a lot of competition for mates of one sex or the other. The theory is that in Europe, where men had to spend weeks at a time out on the hunt, males were in very short supply.

In such societies, women who had flaxen locks stood a better chance of standing out and attracting the attention of the few men that would have been available for mating.

Even back then, the blue-eyed blonde was not only in demand, but also definitely would have had more fun.