If you have blue eyes, then you have one major thing in common with the next person you see with blue eyes, and the next blue eyed person you see after that, and after that and so forth.
Research has shown that everyone in the world who has blue eyes shares one single common ancestor, who lived between 6,000 and 10,000 years ago, and that the blue eyes were caused by a genetic mutation in this one ancestor.
According to a research team at the University of Copenhagen, humans all originally had brown eyes and this one ancestor who had the genetic mutation was "the cause of the eye colour of all blue-eyed humans alive on the planet today".
"A genetic mutation affecting the OCA2 gene in our chromosomes resulted in the creation of a 'switch', which literally 'turned off' the ability to produce brown eyes," explains Professor Eiberg in a statement.
The statement goes on to explain: "The OCA2 gene codes for the so-called P protein, which is involved in the production of melanin, the pigment that gives colour to our hair, eyes and skin. The 'switch', which is located in the gene adjacent to OCA2 does not, however, turn off the gene entirely, but rather limits its action to reducing the production of melanin in the iris - effectively 'diluting' brown eyes to blue."
The OCA2 gene was not totally turned off or destroyed though, otherwise we wouldn't have any melanin in our hair, eyes or skin, which is known as albinism.
The way the researchers figured out that all blue-eyed people share one common ancestor was to look at the fact that variations in eye colour from brown to green are due to the degree of melanin in the iris, but those with blue eyes only have a small variation in the amount of melanin in their eyes.
"From this we can conclude that all blue-eyed individuals are linked to the same ancestor," says Professor Eiberg. "They have all inherited the same switch at exactly the same spot in their DNA."