If you have blue eyes and encounter another azure-eyed person, you can confidently say, “Hey there, cousin!” Scientists have determined that, until a specific gene mutated, all humans had brown eyes.
In 2008, a team of researchers at the University of Copenhagen pinpointed a genetic mutation that occurred 6,000 to 10,000 years ago.
“Originally, we all had brown eyes,” Professor Hans Eiberg from the Department of Cellular and Molecular Medicine, who led the study, said in a report. “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 regulates the brown pigment in the eyes, and the mutation from the HERC2 gene limits the amount of melanin the eyes can produce. As a result, this genetic code causes the person’s eyes to appear blue instead of brown. When the OCA2 gene is entirely “switched off,” the result is albinism.
Green eyes are explained by having a significantly lower amount of melanin in the iris than brown eyes do. However, blue eyes only slightly vary in the amount of melanin.
The team researched the mitochondrial DNA of blue-eyed individuals from various countries, including Jordan, Denmark and Turkey. They determined that more than 97% of blue-eyed people share a single haplotype, a grouping of genomic variants that tend to be inherited.
“From this we can conclude that all blue-eyed individuals are linked to the same ancestor,” Eiberg said. “They inherited the same switch at 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.
This type of genetic mutation is neither positive nor negative. Mutations can affect features like hair color, freckles or baldness.
“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,” Eiberg said.