People with blue eyes have a single, common ancestor, according to new research.
A team of scientists has tracked down a genetic mutation that leads to blue eyes. The mutation occurred between 6,000 and 10,000 years ago, so before then, there were no blue eyes.
"Originally, we all had brown eyes," said Hans Eiberg from the Department of Cellular and Molecular Medicine at the University of Copenhagen.
The mutation affected the so-called OCA2 gene, which is involved in the production of melanin, the pigment that gives color to our hair, eyes and skin.
"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," Eiberg said.
The genetic switch is located in the gene adjacent to OCA2 and rather than completely turning off the gene, the switch limits its action, which reduces the production of melanin in the iris. In effect, the turned-down switch diluted brown eyes to blue.
If the OCA2 gene had been completely shut down, our hair, eyes and skin would be melanin-less, a condition known as albinism.
"It's exactly what I sort of expected to see from what we know about selection around this area," said John Hawks of the University of Wisconsin-Madison, referring to the study results regarding the OCA2 gene. Hawks was not involved in the current study.
Baby bluesEiberg and his team examined DNA from mitochondria, the cells' energy-making structures, of blue-eyed individuals in countries including Jordan, Denmark and Turkey. This genetic material comes from females, so it can trace maternal lineages.
They specifically looked at sequences of DNA on the OCA2 gene and the genetic mutation associated with turning down melanin production.
Over the course of several generations, segments of ancestral DNA get shuffled so that individuals have varying sequences. Some of these segments, however, that haven't been reshuffled are called haplotypes. If a group of individuals shares long haplotypes, that means the sequence arose relatively recently in our human ancestors. The DNA sequence didn't have enough time to get mixed up.
"What they were able to show is that the people who have blue eyes in Denmark, as far as Jordan, these people all have this same haplotype, they all have exactly the same gene changes that are all linked to this one mutation that makes eyes blue," Hawks said in a telephone interview.
Melanin switch The mutation is what regulates the OCA2 switch for melanin production. And depending on the amount of melanin in the iris, a person can end up with eye color ranging from brown to green. Brown-eyed individuals have considerable individual variation in the area of their DNA that controls melanin production. But they found that blue-eyed individuals only have a small degree of variation in the amount of melanin in their eyes.
"Out of 800 persons we have only found one person which didn't fit — but his eye color was blue with a single brown spot," Eiberg told LiveScience, referring to the finding that blue-eyed individuals all had the same sequence of DNA linked with melanin production.
"From this we can conclude that all blue-eyed individuals are linked to the same ancestor," Eiberg said. "They have all inherited the same switch at exactly the same spot in their DNA." Eiberg and his colleagues detailed their study in the Jan. 3 online edition of the journal Human Genetics.
That genetic switch somehow spread throughout Europe and now other parts of the world.
"The question really is, 'Why did we go from having nobody on Earth with blue eyes 10,000 years ago to having 20 or 40 percent of Europeans having blue eyes now?" Hawks said. "This gene does something good for people. It makes them have more kids."
Jeanna Bryner
As a genetics enthusiast with a deep understanding of human evolution and the molecular basis of traits, I find the recent research on the genetic origin of blue eyes particularly fascinating. The study, led by Hans Eiberg and his team at the University of Copenhagen, delves into the intricate details of a genetic mutation that resulted in the emergence of blue eyes around 6,000 to 10,000 years ago.
The key player in this genetic narrative is the OCA2 gene, responsible for melanin production—the pigment determining the color of our hair, eyes, and skin. Eiberg and his colleagues identified a specific genetic switch adjacent to the OCA2 gene that, when mutated, effectively "turned off" the ability to produce brown eyes. Rather than a complete shutdown, this switch limited the gene's action, leading to a reduction in melanin production in the iris and the development of blue eyes.
To establish the common ancestry of blue-eyed individuals, the researchers examined DNA from the mitochondria, the energy-producing structures within cells, focusing on maternal lineages. Through the analysis of DNA sequences on the OCA2 gene and the associated genetic mutation, they identified a shared haplotype among blue-eyed individuals in regions spanning from Denmark to Jordan. This finding suggests that all blue-eyed individuals inherited the same genetic switch from a common ancestor.
What sets blue-eyed individuals apart is not only the shared genetic mutation but also the limited variation in the amount of melanin in their eyes. Unlike brown-eyed individuals, who exhibit considerable diversity in the DNA controlling melanin production, blue-eyed individuals show a high degree of uniformity in this regard.
The intriguing question arises: Why did the prevalence of blue eyes increase from virtually nonexistent 10,000 years ago to 20 or 40 percent of Europeans having blue eyes today? The genetic switch associated with blue eyes appears to confer some evolutionary advantage, potentially influencing reproductive success. John Hawks of the University of Wisconsin-Madison, though not directly involved in the study, notes that the gene might play a role in increasing the number of offspring, offering an evolutionary explanation for the widespread distribution of blue eyes.
In summary, the research by Eiberg and his team sheds light on the genetic basis of blue eyes, tracing their origin to a single common ancestor and providing insights into the selective forces that may have contributed to the prevalence of this eye color in certain populations. The study, published in the January 3 online edition of Human Genetics, adds a valuable chapter to our understanding of human evolution and the intricate interplay of genes that shape our physical traits.
