What’s With Blue Eyes?
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. 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.
Eiberg 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.
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."
New research shows that people with blue eyes have a single, common ancestor. A team at the University of Copenhagen have tracked down a genetic mutation which took place 6-10,000 years ago and is the cause of the eye colour of all blue-eyed humans alive on the planet today.
What is the genetic mutation
"Originally, we all had brown eyes," said Professor Hans Eiberg from the Department of Cellular and Molecular Medicine. "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 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 switch's effect on OCA2 is very specific therefore. If the OCA2 gene had been completely destroyed or turned off, human beings would be without melanin in their hair, eyes or skin colour -- a condition known as albinism.
Limited genetic variation
Variation in the colour 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," says Professor Eiberg. "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.
Professor Eiberg and his team examined mitochondrial DNA and compared the eye colour of blue-eyed individuals in countries as diverse as Jordan, Denmark and Turkey. His findings are the latest in a decade of genetic research, which began in 1996, when Professor Eiberg first implicated the OCA2 gene as being responsible for eye colour.
Nature shuffles our genes
The mutation of brown eyes to blue represents neither a positive nor a negative mutation. It is one of several mutations such as hair colour, baldness, freckles and beauty spots, which neither increases nor reduces a human's chance of survival. As Professor Eiberg says, "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."
Blue eyes are indeed becoming less common in the world. One study showed that about 100 years ago, half of U.S. residents had blue eyes. Nowadays only 1 in 6 does.
What is happening in the U.S. will undoubtedly happen throughout the world as well. Especially as Europe opens itself up to more immigration. This is one of the reasons blue eyes are becoming less common in the U.S. -- immigration.
In the recent past, lots of brown-eyed people have moved here. Even with no other explanation you'd expect a smaller percentage of blue eyed people because of this.
But this is only part of the story. Another reason blue eyes are declining is because they were at an artificially high level before. In the past, blue-eyed people tended to have kids with other blue-eyed people. When this happens you get blue eyed kids.
Today people use ethnicity less often as a way to pick a partner. This means that blue eyed people have kids with brown eyed people more often than before. And often that means mostly brown eyed kids.
These two facts -- more brown eyed people and more mixed eye color marriages -- will undoubtedly continue into the future. Which means that blue eyed people will continue to decline in numbers.
But they won't go away completely. The version of the eye color gene that leads to blue eyes doesn't disappear from the human race when someone with blue eyes has a brown eyed child. This gene version instead goes into hiding, waiting for the right opportunity to cause blue eyes again.
What I'll do for the rest of the answer is go over the history of blue eyes according to our DNA. From that you'll get a good feel for how blue eyes work. And why they'll become even rarer.
History of Blue Eyes
Back in the day, everyone had brown eyes. Basically, like brown eyed people today, our ancestors had brown eyes because of a gene called OCA2.
This gene tells our cells to make lots of pigment in the front part of our eye. Lots of pigment there means brown eyes.
Then sometime a few thousand years ago, someone developed a small glitch in their DNA. This glitch was caused either by damage from the outside or from some mistake made by this person's own cells.
Whatever the cause, this change made the OCA2 gene stop working in the eye. When OCA2 isn't working, it doesn't make any pigment. And when no pigment gets made in the front part of the eye, you end up with blue eyes*. Except this person undoubtedly had brown eyes.
Why? Because this person still had one working OCA2 gene.
Remember, we have two copies of most of our genes -- one from mom and one from dad. Since it is very unlikely that two glitches will happen in the same gene at the same time, this person probably had one working and one non working OCA2 gene.
The working copy could still do the job of making pigment on its own. So this person had brown eyes.
But the blue version of OCA2 was now released into human DNA through this person's children. At some point two people who each carried a blue OCA2 met and had a child. This child happened to get a blue OCA2 from each parent and so had no working OCA2 genes. The end result was no pigment and so blue eyes*.
This blue-eyed person must have been special somehow because the blue OCA2 quickly swept through the European population. Soon there were places where blue eyed people outnumbered brown eyed ones.
We don't know what made these blue eyed people special. It may have been that they were irresistible to the opposite sex and so had lots more babies than brown eyed people. Or blue eyes may have been tied to some useful Northern European trait like pale skin.
Whatever the reason for their eye color, these blue eyed people established themselves and spread. And, for various reasons, they tended to have children with only each other.
So there were pockets of blue eyed people in a world of brown eyes. When populations started to move around more easily, there started to be a blending of blue and brown eyed people. And then brown eyed people started having kids with blue eyed people.
Since brown beats out blue (brown is dominant, blue is recessive), the blue eyes started to fade into the background. The same number of blue OCA2 genes were there it was just that they were now hidden more of the time.
*This is a simplified version of the story. There are other genes that make pigment too but not as well as OCA2. So some people can have two non working OCA2 genes and have green or hazel eyes and not necessarily blue.
Blue Eyes are Down but not Out
To try to make this clearer, I've created a couple of hypothetical examples. In these examples, a little b stands for the blue OCA2 and a big B stands for brown.
Since we have two copies of the OCA2 gene, there are three possibilities: BB, Bb, bb. BB and Bb will give brown eyes (since B is dominant) and bb will give blue.
The first example on the right shows what happens with no mixing. What you see is that the population increases over time but you end up with the same proportion of blue and brown eyed people.
This is because the brown eyed people can only pass down a B so all their kids will have brown eyes. Same thing with the blue eyed people except they only pass down a b. And so have only blue eyed kids.
In the second example (see below), I have shown what happens in two generations when there has been an influx of brown eyed people and completely random mixing.
The end result is more brown eyed people but with the same number of blue OCA2 gene versions. This is because many more people have a copy of each version. These folks have brown eyes but they have a hidden blue OCA2 version. If they meet someone else who carries a blue OCA2, then they have a shot at a blue eyed child.
Eventually assuming no new immigration and no differences in the number of babies each group has, the populations will settle down to some new number and stay constant. Figuring out that number isn't easy because eye color is so complicated.
If it were due to a single gene, we might be able to make some sort of guess. When I try to apply it, I get that the number of blue eyed people in the world will fall from its approximate number of about 350 million today to 49 million in some distant future.
This assumes random mixing, that there are 350 million blue eyed people in the world, that eye color is due to a single gene and that there are essentially no carriers of the blue eye gene in predominantly brown eyed populations (think China, India, most of Africa, etc.). Not sure how useful this exercise actually was but it was fun! (At the end I'll show how I got the numbers for those of you interested.)
Even if the exact numbers aren't right, what we can conclude is that blue eyes will continue to become rarer and more exotic. But that they won't disappear completely.