In 2008, a study led by Hans Eiberg of the University of Copenhagen claimed that all blue eyes are linked to a single ancestor who lived between 6,000 and 10,000 years ago. The concept has been consistently featured in the news and social media posts for the next 15 years after the paper’s launch. Simply put, we fall in love.

But where did this concept come from? This 2008 study builds on research that began in 1996, all focused on the genetic study of the OCA2 gene.

The study shows that the OCA2 gene code plays an important role in the production of melanin, the pigment that colors hair, skin and eyes. Eiberg’s theory is that between 6,000 and 10,000 years ago, a mutation occurred that turned on the gene’s ability to dilute brown eyes with blue.

“At first we all had brown eyes,” she said in 2008. “But a genetic mutation affecting the OCA2 gene on our chromosomes has created a switch that literally turns off the ability to produce brown eyes.”

Each eye color is directly related to the amount of melanin in the iris. Green eyes even less often than blue eyes have low melanin levels considering they are not as low as blue eyes. Just a small change is all it takes to go from brown to blue. “From this we can conclude that all blue-eyed individuals are related to a single ancestor,” says Eiberg. “They all inherited the same key in exactly the same part of their DNA.”

He calls this transition a specific genetic mutation event, and he believes it created the first blue-eyed human solely through a mutation in the regulatory gene HERC2. This combination is the only known way for eyes to turn blue (even red hair can appear for one of a dozen reasons). This mutation remained in effect for the next generation, meaning the reduction in melanin production in the iris allowed the brown color to continue to be diluted to blue.

Since the change thousands of years ago, the development of blue-eyed people has only progressed. Several research papers have concluded that the first mutation probably occurred somewhere in Europe, possibly during the Neolithic expansion. The blue-eyed march continued until the population dispersed.

About 10 percent of all people have blue eyes, although this number varies greatly by region, with Scandinavian countries more likely to have blue eyes. According to Eiberg, Eiberg considers a neutral mutation, the eye color sample “showing that nature is constantly shuffling the human genome, creating a genetic cocktail of human chromosomes and experimenting with different changes in the process.”