Pure white people have black eyes
Apparently nature only invented blue eyes once. Sometime between 6000 and 10,000 years ago, the first person suddenly looked naively into the world because of a gene mutation.
All other people with blue eyes are descended from this one ancestor, Danish scientists working with Hans Eiberg now report in the specialist journal Human Genetics (on-line). Everyone with blue eyes seems to be related to Angelina Jolie and also to Brad Pitt.
"Originally we all had brown eyes"
"Originally we all had brown eyes," says Eiberg, who is a professor at the University of Copenhagen. Then apparently a mutation spontaneously developed in a gene in a person, so that his eyes turned blue.
From this blue-eyed Adam, the mutation must have spread across the northern hemisphere. Ten percent of all people around the world now have blue eyes. Most of them are in the Baltic Sea region: Finland is the main residence of the blue-eyed - they make up 90 percent of the population.
Mutations in individual genes occur again and again. That one is as widespread as the blue eye mutation is only possible if blue eyes have an advantage in evolution, says Thomas Meitinger, head of human genetics at the Technical University of Munich.
We can only speculate what advantage this could be. Blue eyes are usually associated with lighter skin - "and that has advantages in the production of vitamin D, which is important for teeth and bones," explains Meitinger. For the production of the vital vitamin, UV light is necessary, which especially reaches northern Europe weaker.
This could have created a selection pressure that favored people with blue eyes. Perhaps, thousands of years before Siegfried the dragon slayer, people were so fascinated by blue eyes that their wearers became attractive sexual partners.
Regardless of the attraction: if you look at it soberly, blue eyes are the result of a genetic defect. The iris only appears in blue because it lacks the pigment melanin, which usually gives it a brown color and which is also the cause of suntanned skin. Gray and green are nothing more than nuances - such eyes contain more melanin than blue, but less than brown eyes.
Diluted to blue eyes
The gene mutation of the first person with blue eyes changed the OCA2 gene, which is involved in the production of melanin, explains Eiberg. As a result, less melanin reached the iris. "Brown eyes are diluted to blue eyes, so to speak."
OCA2 is an old acquaintance for geneticists. If it is completely destroyed, a form of albinism develops. "Those affected can hardly produce any more melanin," says Meitinger, who researched this very gene twenty years ago. "At that time, however, you didn't know that it had something to do with normal eye color."
In any case, geneticists have long been struggling to finally understand the inheritance of eye color. "The mechanisms are highly complex, and many genes are involved," says Meitinger. "That is why it is difficult to predict what eye color a child will have."
It was not until the beginning of 2007 that a working group led by the Australian David Duffy from the University of Queensland shed light on iris genetics: At that time, OCA2 proved to be the main actor in the genes' game around eye color.
Hans Eiberg had also had OCA2 in mind as an important eye color gene for a long time. He has now found the special change in the genetic make-up of blue-eyed people by examining an extended Danish family that has only had blue eyes for three generations. He found exactly the same mutation in all family members and also in hundreds of other Danes.
But that's not all: blue-eyed people from Turkey and Jordan also carry the same changes in their genes. Obviously, they all inherited this mutation from the same human. "That's not a guess," says Eiberg. "It has to be like that." Thomas Meitinger also agrees: "Nothing can be shaken about the Blue-Eyes Adam."
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