(USA TODAY) - If you have blue eyes, you may be related to every other blue-eyed person in the world.
Researchers in Denmark have found that every person with blue eyes descends from just one "founder," an ancestor whose genes mutated 6,000 to 10,000 years ago. Before then, everyone had brown eyes.
Lead scientist Hans Eiberg, a geneticist at Copenhagen University, began in 1973 to study a Danish father with 17 children who carried the gene for both blue and brown eyes. Over time, researchers were able to trace the blue-eyed trait to one specific area near a gene called OCA2. The paper appears in the journalHuman Genetics.
Eiberg's team then tested 155 blue-eyed people from Scandinavia, Turkey, Jordan and India, looking to see whether they, too, had similar DNA sequences on that gene. To their amazement, they found that each individual had identical DNA sequences in that region of that gene, an indication that the original mutation happened recently enough that it hasn't had time to change.
Everyone has two genes for eye color, one from their father and one from their mother. Brown eyes are dominant, so even if someone has one blue and one brown-eye gene, he'll still have brown eyes.
That means that the recessive genes for blue eyes can be invisible for generations, with blue-eyed children popping up only when both parents carry at least one blue-eye gene, Eiberg says.
Blue eyes are actually what happens when the human default - brown eyes - is turned off. Brown eyes are caused by the pigment melanin, which also gives color to hair and skin, building up in the eyes. It's why many light-skinned babies are born with blue eyes but gradually develop brown eyes as their body produces melanin in their irises. The blue-eye mutation turned off the gene that produces melanin in the eyes.
When the original mutation occurred, the person who carried it would still have had brown eyes, says Eiberg. But in the generations that followed, a man and a woman who each had one blue-eye gene mated, producing the world's first blue-eyed baby.
Imagine what a surprise that was, Eiberg notes. "If the child was the only one to have blue eye color and everyone else has brown, it could have been very interesting."
That one mutation now exists in an estimated 300 million people, Eiberg says. Why it has become so common in some populations is unknown. It has been suggested that blue eyes and the lighter coloring that often accompanies them might have given an advantage to people in colder climates because they could absorb more vitamin D from the sun. Others have suggested it's simply a roll of the historical dice, with the original group that had the blue-eye genes coming through a genetic bottleneck.
"Maybe the population almost went extinct. If just a few individuals survived at some point and they were blue-eyed," then it could have happened much more recently, says Klaus Kjaer, a geneticist at the University of Copenhagen who also worked on the paper.
Blue eyes, though somewhat rare in the USA, are common in countries near the mutation epicenter, which previous researchers have pinpointed as probably somewhere in the Balkans or near the Black Sea.
In Estonia, 99% of people have blue eyes, Eiberg says. In Denmark 30 years ago, only 8% of the population had brown eyes, though through immigration, today that number is about 11%. In Germany, about 75% have blue eyes.
Because the research shows that all blue-eyed people are related, Kjaer notes it's "interesting" that his blue-eyed wife is therefore related to Brad Pitt.