New research shows that people with blue eyes have a single, common ancestor. Scientists have tracked down a genetic mutation that took place 6,000-10,000 years ago and is the cause of the eye color of all blue-eyed humans alive on the planet today.
Researchers have implicated the OCA2 gene in several eye colors. The gene is involved in the production of melanin, a pigment that gives hair and skin their hues. It also codes for brown eyes and can lead to green or hazel eyes when mutated. Despite years of searching, however, scientists have not found a mutation for blue eyes on the gene. The mutation responsible for blue eye color affects a specific gene called OCA2. This gene controls the production of melanin, a pigment that gives color to our skin, hair, and eyes. While everyone has this gene, blue-eyed individuals have a specific mutation that causes a reduction in the amount of melanin produced in the iris, resulting in blue eyes.
Originally, we all had brown eyes, but 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. The OCA2 gene codes for the so-called P protein, which is involved in the production of melanin, the pigment that gives color 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 switch's effect on OCA2 is very specific, therefore, if the OCA2 gene had been totally destroyed or turned off, human beings would be without melanin in their hair, eyes, or skin color - a condition known as albinism.
According to researchers at the University of Copenhagen, variations in the color of the eyes from brown to green can all be explained by the amount of melanin in the iris, but blue-eyed individuals only have a small degree of 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. They have all inherited the same switch at exactly the same spot in their DNA. Brown-eyed individuals, by contrast, have considerable individual variation in the area of their DNA that controls melanin production.
The mutation of brown eyes to blue represents neither a positive nor a negative mutation. It is one of several mutations such as hair color, baldness, freckles, and beauty spots, which neither increase nor reduce a human's chance of survival. 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.