Despite the apparent prevalence of the traitamong swanky crooners and British spies and creepy old creatures who just hung onto thering for too long, only about 8% of the world's human population has blue eyes. Here in theU.S. it's about twice that, but that's still a lot less than it was at the turn of thetwentieth century when nearly half of all Americans had the trait. So what happenedéAre blueeyed humans going the way of the dodoé Well, no. We cleared up that rumor about redheadsgoing extinct; the same goes for blue eyes. While the trait is becoming more rare, it'sunlikely it will disappear all together. Which
is crazy when you consider that 10,000 yearsago blue eyes didn't even exist. In fact, there's a good chance that blueeyed peoplemay all share one common ancestor. Studies over the past decade have actually tracedthe trait to a mutation that most likely arose among browneyed people in a single humanin the Black Sea region of southeastern Europe between 6,000 and 10,000 years ago. The mutation affected what's known as theOCA2 gene, which helps our bodies produce melanin, the brownish pigment that gives colorto our hair and eyes and skin. The mutation created kind of a dimmer switch for the pigmentation,but it didn't affect the entire gene. Instead,
it only affected the production of melaninin the iris, the ring structure around the eye that regulates how much light gets in. The bulk of the iris is a thick layer of melaninproducingcells called the stroma. And the OCA2 mutation turns the production of melanin in the stromaway down, but the pigmentation still shows up elsewhere, like in the hair and skin andother parts of the iris. Basically, the mutation keeps the stroma from being brown. So why blue thené Well, strictly speaking,blue eyes aren't actually blue. Instead, in people with this mutation, the stroma is fullof nearly colorless cells. And when the light
strikes them, they scatter the wavelengthsback out, in a process similar to what makes smoke or fog look blue when light passes throughthem. If there is some yellowish pigment in thestroma, then the blue light will combine with that to make green. Throw in a little bitof brown, and you have hazel. So how did we go from having no blueeyedpeople, to hundreds of millions of them in less than 10 millenniaé No one's entirelysure why the trait spread so quickly through Europe. Some scientists think the mutationcould have helped prevent certain eye disorders related to long, dark northern winters.
But another factors appears to be that, forwhatever reason, lots of blueeyed people simply mated with other blueeyed people inthe past, which kept the trait in circulation. Because for each of your genes, you have twodifferent versions, called alleles, one from your father and one from your mother. If youhave at least one dominant allele for a gene, that's the trait you have. The blue eyes come from a recessive allele,which means if you inherit one allele for blue eyes and another for brown, you're goingto have brown eyes. But you still carry the recessive blue allele, which can be passedon. Which means that that first person that
had that blueeyed mutation didn't have blueeyes. They had to pass that onto their children, and their children had to pass it onto theirchildren, until eventually they came back together to make someone with two blueeyedalleles. When both parents have blue eyes, they bothhave two recessive blueeyed genes, which means their children will also have blue eyessince there's no dominant gene to mask the recessive one. This is how you end up withScandinavian countries that are 95% blueeyed, and it also explains why the percentage ofblue eyeers is dropping in much of the western world.