What do Frank Sinatra, Cameron Diaz, Brad Pitt and Gwyneth Paltrow have in common? According to a new study, they all share the same ancestor.
The paper, published Thursday by Danish geneticist Hans Eiberg in the journal Human Genetics, links all baby blues to a single mutation that occurred 10,000 years ago.
Eiberg says the mutation shuts off the production of the pigment responsible for brown eye color, resulting in a pure blue iris. Because the mutation is so specific, it can only be explained one way: "There must be a common ancestor for people with blue eye color," Eiberg told SPIEGEL ONLINE.
Eiberg started his search for the elusive mutation close to home. Using the Copenhagen Family Bank, a massive genetic database with detailed information on over 6,000 Danes, Eiberg found a family with three generations of blue-eyes. Looking at DNA from their blood, Eiberg homed in on a single, tiny blip in the genetic code. "All of the family had the same mutation," he said.
Originally, Eiberg says, everyone in the world had brown eyes. But the mutation acts as a switch that shuts off the OCA2 gene, which controls the eye's production of melanin. Melanin is the pigment that gives color to eyes and hair.
The mutation limits the OCA2 gene, restricting production of melanin in the eye. The result: The eye's brown color is diluted, giving people with the mutation pure blue eyes. (Shutting melanin production down entirely would result in albinism, affecting hair and skin color as well.)
People without the off-switch, on the other hand, have eye color ranging from deep brown to blue flecked with brown. (Green-eyed people can thank an entirely different part of the genome for their pretty peepers.)
The mutation is extremely specific: All people with blue eyes have the exact same genetic variation, and anyone with brown or green eyes do not. As a result, Eiberg said, it must have been passed down from a single person. "It's not a guess," Eiberg says. "It has to be."
To make sure the Danish family wasn't a fluke, Eiberg tested hundreds more samples, including people from Turkey with dark hair, light skin and blue eyes and Jordanians with dark hair, dark skin and blue eyes. They all had the same mutation as the Danes. "I have analyzed 800 samples," Eiberg says. "Out of the 800, 799 eyes are the same."
Eiberg has long been fascinated by the genetics of eye color. In 1996, he discovered the OCA2 gene, which helps control eye color. The blue-eye mutation works directly to turn off the OCA2 gene's production of melanin in the eye.
Eye color is a good example of how research is complicating our understanding of heredity. "Eye color is a textbook example of how genes work in a simple way, and now it turns out it's a bit more complicated than that," said Zoltan Bochdanovits, a statistical geneticist at the Vrije University in Amsterdam. "They do present quite convincing evidence it's a single mutation causing this."
Where and when the mutation occurred is more speculative, but based on the number of people with pure blue eyes in the world today, Eiberg argues that the original Ol' Blue Eyes lived between 6,000 and 10,000 years ago. Existing research on prehistoric population movements suggest that the original blue-eyed babe may have lived around the Black Sea, near modern-day Ukraine or Turkey, and that their descendants migrated to Northern Europe during the Stone Age.
Like freckles, hair color or baldness, there's no real physical advantage to being blue-eyed. Says Eiberg: 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.
That doesn't mean eye color isn't important. After all, that long-distant ancestor managed to get quite a few copies of his or her mutation passed along. "I can very much imagine mate choice depends on eye color in humans," said Vrije University's Bochdanovits. "Personally, I tend to believe if you're blond and blue-eyed you have an advantage, at least in some populations."