The Genetic History Behind Blue Eyes

M ore than one in four people in the US have blue eyes. In the UK, it’s three out of every seven; in the Netherlands, it’s three out of five, and in Iceland, it’s three out of every four. Depending on where you live, it’s a very popular ocular color to have.

But that’s kind of weird, isn’t it? Blue isn’t exactly a ubiquitous color in nature – even those creatures you’re thinking about right now, like certain insects and fish, aren’t really blue, they’re just exploiting physics to make you think they are. In mammals, the cerulean hue is even rarer: there’s the blue whale, we suppose, and a fair few monkey butts , but no animal has naturally blue fur, or hair. So why would blue eyes be a thing?

Well, here’s the secret: they’re not.

The myth of blue eyes

Seriously. Blue eyes don’t exist . “Blue eye color is determined by melanin, and melanin is actually brown by nature,” explained licensed optometrist Gary Heiting in an article for eye care website All About Vision.

“Brown melanin is the only pigment that exists in the eye; there is no pigment for hazel or green – or blue,” he wrote. “Eyes only appear to be these colors because of the way light strikes the layers of the iris and reflects back toward the viewer.”

Yeah, it turns out that when someone compares their beau’s blue eyes to the hue of the open seas or sky, they may not just be bein’ all poetical. They might be invoking the romance of physics instead.

“In the animal kingdom there are many examples in which the observed color is the result of optical phenomena such as light scattering, interference or diffraction by microscopic structures present in the tissues,” explains George Britton in The Biochemistry of Natural Pigments .

“The most familiar example of this effect is the blue of the sky,” he writes.

If you’ve ever wondered why the sky is blue – and let’s be honest, we all have at some point – here’s a crash course that’ll make your nearest physicist wince: imagine Ant-Man running an obstacle course.

No, honestly, bear with us. Say he starts off attempting the race at his insect-size: he’ll get to the first barrier – say, those little tires you have to hop in and out of – and be completely stymied. He’ll just bounce off and stay back near the start line.

So, he grows a bit. Now he’s the size of a toddler, and he can make it past the tires – but once he hits the mini hurdles, he’s still too small. He’s foiled again.

Not one to back away from a challenge, he ups his size again, until he can pass the hurdles without a problem. But then, he comes to a wall. He’s only the size of Paul Rudd, he can’t get over that – he bounces off again, stopped once more from reaching the end.

Finally, he decides screw it, and grows to his Giant-Man size. Well, this is great: he’s big enough to step over everything and go straight to the finish line. Race won.

That’s basically what’s going on when light from the sun comes down to our planet. While it’s in space, where there is, for all intents and purposes, nothing around to get in its way, the light is white – which is a misleading color for it to be, really, since it’s the result of all the different colors possible composed together. Once it enters our atmosphere, however, things get messy: there’s all kinds of microscopic particles of dust and gas and suchlike in the air, and the light starts getting scattered in all directions.

And the smaller the wavelength of light, the more likely it is to get caught up bouncing around up there. Colors at the bottom of the visible light spectrum – that is, the reds and oranges – are like Giant-Man: too big to be concerned with tiny dust motes, and so hurtle on down to get absorbed by the ground. But the smallest wavelengths – the blues and violets – get stuck bouncing around in the sky, making it appear to be the cool tone we all know.

“Very small particles, smaller in diameter than the wavelength of red or yellow light, will reflect or scatter more of the short-wave than of the long-wave components of white light,” Britton summarizes. “Colors produced in this way are known as structural colors.”

Now, if you want to be persnickety about it (and we read the comment sections, so we know that you do) the phenomenon that makes the sky appear blue and the phenomenon that makes your irises appear blue are two different effects – Rayleigh scattering and Tyndall scattering, respectively. But frankly, they’re the same dang thing; the distinction between the two is not in what’s going on, but in the size and location of the particles the light is being bounced off.

“Most non-iridescent blue colors in animals are Tyndall blues,” Britton notes. “[The] blue color of human eyes is due to the scattering of white light by minute protein particles in the iris.”

Green eyes, and hazel, are a result of the same scattering effect – but in those cases, there’s slightly more melanin in the iris, so the light is absorbed or reflected in different ways. That’s also why eye color can seem to “change” based on nearby lighting: “It’s an interaction between the amount of melanin and the architecture of the iris itself,” Heiting told CNN . “It’s a very complex architecture.”

The same effect is responsible for the blue color of many birds, Britton adds, whose feathers feature incredibly tiny air-filled membrane pockets that scatter the light away. But in all these cases, clarifies Britton, “no blue pigment can be isolated from the tissues.”

In other words: your baby blues? They’re just a trick of the light.

The ancestor of blue eyes

In humanity’s collective family tree, some branches loom larger than others. The one labeled “Genghis Khan”, for example, is thick enough to cover around one in 200 men alive today; more than a fifth of Irish fellas can claim to be descended from Niall of the Nine Hostages – even though the man may well have never existed ; and technically , every European has Charlemagne lurking somewhere in their lineage.

All of these dudes left an impressive impact on the world’s genetic makeup – but there’s one person in the history books who outbred them all. And who was it, you ask?

Well… we don’t know. Unlike old Genghis and Charlemagne, this individual lived so long ago that there’s no way we could know their name or even when and where they came from to any degree better than “ probably the Near East like 40 or 50,000 years ago.”

But we do know one thing: they had blue eyes – and nobody else did.

“Originally, we all had brown eyes,” Hans Eiberg, a professor in the Department of Cellular and Molecular Medicine at the University of Copenhagen, said back in 2008 . “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.”

Now, we know what you’re thinking – how did the team know that only one ancestor was responsible for all these different blue eyes? Well, here’s the thing: what Eiberg and his colleagues had discovered wasn’t just that the aquatic hue is the result of a mutated OCA2 gene – it was far more definitive than that.

“[Blue-eyed people] have all inherited the same switch at exactly the same spot in their DNA,” Eiberg explained.

“From this we can conclude that all blue-eyed individuals are linked to the same ancestor.”

It’s the only known way to produce blue eyes – for comparison, there’s at least eight different gene mutations that are responsible for red hair – and it’s quite specific. The “switch” that Eiberg referred to couldn’t simply switch the gene from “on” to “off” – that would result in full-on albinism. Instead, it had to be “diluted”, he explained, limiting its ability to produce the brown pigment to such an extent that the eyes looked blue.

And the weirdest part of all? Apart from looking a bit interesting, the mutation that gives blue eyes seems to be pretty useless. It’s not like the evolution of paler skin and lactose tolerance, both of which allowed residents of more northerly latitudes to absorb more vitamin D – it appears to be neutral, evolutionarily speaking. A fluke.

“It simply shows that nature is constantly shuffling the human genome,” Eiberg said, “creating a genetic cocktail of human chromosomes and trying out different changes as it does so.”

An earlier version of this article was published in January 2024 .