The Tweet: “All blue eyed people can be traced back to one person who lived near the Black Sea about 10,000 years ago.”

Between 6,000 and 10,000 years ago a mutation in the eye colour gene HERC2 resulted in the blue eyes common to Northern Europeans, according to researchers form the University of Copenhagen. In a study from 2007,  Eiberg et al. analysed the genomes of 100 Danish families. They found that a single haplotype was responsible for all the blue eyes in this population.

More than 97% of blue-eyed individuals analysed carried the same haplotype, with the remaining 3% of haplotypes likely to be the result of more recent recombination events. The researchers then compared several Mediterranean blue-eyed individuals with their Danish sample and found that they too shared the same haplotype.

The researchers conclude: “The mutations responsible for the blue eye color most likely originate from the neareast area or northwest part of the Black Sea region… The high frequency of blue-eyed individuals in the Scandinavia and Baltic areas indicates a positive selection for this phenotype.” But the high frequency of blue-eyed people in this area may be misleading.

Blue-eyed Europeans were not as uncommon as they are today. Blue eyes were widespread across Europe until about 7,000 years ago when a migrating population of Middle Eastern farmers changed the balance. Research published in 2014 shows that modern Europeans are descended from a blend of three different populations in varying amounts. The indigenous Europeans were dark-skinned, blue-eyed hunter-gatherers. Interestingly, the light-skinned, brown-eyed appearance of modern Europeans came from a migrant population from the Middle East. This population was made up of farmers rather than hunter-gatherers whose diet of vegetables had a lower vitamin D content than their European counter-parts. This meant that the fair-skinned Middle Easterners had to synthesize more vitamin D through sunlight.

“Hunters and gatherers get vitamin D through their food — because animals have a lot of it. But once you’re farming, you don’t get a lot of it, and once you switch to agriculture, there’s strong natural selection to lighten your skin so that when it’s hit by sunlight you can synthesize vitamin D” lead author David Reich told the BBC. So the success of agriculture as a phenomenon lead to the spread of light-skin in the European population, carrying with it the gene for brown eyes.

Because blue-eyes were so much more common before the influx of brown-eyed Middle Easterners it’s difficult to say with certainty that they originated in the Baltic. Nevertheless, the unusual combination of dark-skin and blue-eyes that characterized indigenous European hunter-gatherers 10,000 years ago no longer exists.