The Wonder of Eye Colour
Human Eye Colours - The Wonder and Science Behind Them
You've probably heard from your parents that the human eyes are the "gateway to the soul." And, if you've ever stared deeply into another's eyes, you can probably understand why so many people agree with that statement. Indeed, numerous psychologists are now revealing how the human eyes are key indicators of a person's authentic emotional state. We can tell when someone is sad, faking a smile, or angry simply by looking into their eyes.
Just behind fingerprints, perhaps there's nothing more uniquely our own than our eyes. More specifically, our eye colour. Besides giving us a sense of personal autonomy and identity, our eye colours often link us to our genetic ancestors.
Although you may choose to change your natural eye colour with contacts lenses for fun, you can never escape your "true colours." In this brief article, we'll take a look at both the most common and the rarest eye colours and explain a bit about the history and chemistry behind each pigmentation.
One interesting thing you learn once you start studying eye colours is that everyone's eyes are actually brown. That's right, even if you have the bluest of blue eyes, deep down they are actually just a different shade of an original "brownness." The reason this is so all has to do with the pigment melanin (which comes from the Greek word melas, meaning "black" or "dark"). Melanin is more commonly associated with skin or hair colour, but it also has an enormous impact on our eye pigmentation.
You see, whenever we talk about "eye colour," we're really just talking about how light strikes the topmost layer of the iris. The topmost layers of your irises have different amounts of melanin. Therefore, the more melanin you have, the more likely you will have darker eye colours. So, while "brown eyes" are the most common eye colour, anyone with brown irises can correctly tell a blue-eyed person, "Technically we all have brown eyes. My brown eyes just aren't mutated like yours are!"
Current estimates claim that a little over 55 percent of people around the world have brown eyes. Nearly 100 percent of Africans and Asians have brown eye colour. Depending on the amount of melanin in a person's iris, sometimes people could mistake brown eyes for black eyes. There's actually no such thing as a "black" eye (unless you just got beaten up), but there are varying degrees of "brownness."
Most geneticists consider hazel eyes to be the next most common eye colour around the world. Sometimes people don't break up the distinction between hazel and brown eyes because they consider hazel just a lighter version of brown. Interestingly, if you look closely enough at someone with true hazel eyes, you'll actually see a mixture of yellow and green in their irises. Most people with hazel eyes have larger amounts of melanin on the borders of the iris, but they have less melanin the closer you get to the pupil. Since there are varying degrees of melanin in these people's eyes, hazel-eyed people often identify themselves with brown or green eyes depending on the overall darkness of their eyes. Indeed, depending on the lighting, a person with hazel eyes could actually appear to have blue eyes. Percentage-wise, if you are torn between blue, green, or brown, chances are you have either hazel or brown eyes. Scientists believe around 5 to 8 percent of the global population has hazel eyes.
As mentioned above, people often confuse hazel eyes with green eyes (and vice versa). If you are unsure whether you have green or hazel eyes, chances are you have hazel eyes. Current estimates show that only around 1 to 2 percent of people around the world have green eyes. About 80 percent of that 2 percent are Caucasian and come from Europe. Also, most people with green eyes tend to be female.
Although green eyes are super rare, they have been around for quite a long time. We actually have documents going back to the Bronze Age that suggest many people in Siberia had green eyes. Today, it's most common to see green eyes in Iceland, where around 80 percent of the native born population have either green or blue eyes.
The production of green eyes is quite interesting. People with green eyes have what's called a yellow coloured stroma in their iris. When light hits the irises, a phenomenon known as Rayleigh scattering occurs. Rayleigh scattering refers to the way light waves scatter into tiny molecules when there's less melanin, which tends to make the eyes look blue (since blue tends to scatter more than any other colour). By the way, Rayleigh scattering is often used to explain why the sky appears blue. Since green-eyed people have less melanin plus a yellow stroma, the yellow mixes with the blue to produce green.
While not as rare as green eyes, blue eyes are often the most desirable eye colour on the planet. Perhaps this has to do with all the beautiful blue-eyed celebrities we've been bombarded with on TV and movies over the past few decades. Stars like Brad Pitt, Resse Witherspoon, and, of course, the late Frank Sinatra are all legendary for their beautiful blue eyes. Since the genes responsible for blue eyes are recessive, however, they are relatively rare unless you were born in an area where this trait is common. Almost everyone in the Balkans region has blue eyes, and many native northern Europeans have blue eyes as well. Today, there are around 8 percent of people around the world with blue eyes.
As mentioned before in reference to green eyes, blue eyes are caused by a lack of melanin in the iris and the Rayleigh scattering effect. Of course, since blue-eyed people don't have the yellow stroma that green-eyed people have, the colour that appears in their iris is just blue.
Research out of Copenhagen University in Denmark actually claims that everyone who has blue eyes is directly related to one ancestral "founder" that arrived on the scene some 6,000 to 10,000 years ago. Before that time, researchers believe everyone's eyes were brown. The lead geneticist on this study, Hans Eiberg, looked at the genes of people with blue eyes in Turkey, Jordan, India, and Scandinavia. He found that every single person had the exact same gene trait in their DNA: OCA2. This finding led Eiberg to believe everyone who carries the genetic mutation of blue eyes is directly related to each other.
Some geneticists consider silver eyes a rare variation on blue eyes. Basically, silver eyes are the same as blue eyes except for the fact that silver eyes have even less melanin. When Rayleigh scattering occurs in these eyes, the resultant colour is an unmistakable "stainless steel" silver. Silver eyes are really only common in Eastern Europe. Even in Eastern Europe, however, the rate of silver eyes is extremely low. There aren't any official estimates, but the percentage of people with silver eyes around the world is probably around the same levels green eyes.
One of the rarer variants of eye colour in both Asia and South America is amber. Amber coloured eyes have a medium amount of melanin, but they also have a strange yellowish tinge to them, which distinguishes them from typical brown eyes. This yellow tinge gives amber eyes a kind of golden/copper appearance. The main reason for them giving off this colour is due to a pigment known as lipochrome. Liopchrome is associated with the colours of amber, gold, and green, and can be found in the eyes of a few rare humans and some other mammal species. The current global estimate puts amber eye rates around 5 percent.
While green and silver eyes are extremely rare, it's even more rare to have what's called Heterochromia iridum. Kate Bosworth is a good example. This refers to having eyes that are two different colours. Geneticists estimate that only around 0.6 percent of the global population have two eyes that could be considered completely different colours. It would be more common to have multicoloured eyes. There's no need to worry, however, since Heterochromia iridum is completely harmless and requires no treatment.
Even if you are one of the lucky few with Heterochromia iridum, chances are the two colours you do possess are quite close to each other on the colour spectrum. That means that you might not even notice you have two different eye colours! Of course, you could always fix that by purchasing a pair of coloured contacts.
Author: John Dreyer Optometrist Bsc(Hons), MCOPTOM, DipCLP
Created: 24 May 2017, Last modified: 17 Jul 2019