Ever wondered why we call the colored part of our eye the “iris”?
The ring of pigmented tissue surrounding the pupil of the eye actually borrowed its name from the Greek goddess of the rainbow.
Like the rainbow that fills the sky with different colors, the human eye can also take on a rainbow of colors, depending on the level of a pigment called melanin.
For generations, scientists thought that eye color was controlled by a single gene for melanin, and that only a few outcomes were possible from a dominant brown gene variant crossed with a recessive blue variant.
Today, our understanding of the entire genome has broadened that picture significantly.
We now know there are at least 10 separate genes that all play a role in determining eye color.
It’s not all or nothing, but a much more nuanced story.
In fact, of these 10 genes, there are two that account for the majority of eye color outcome, and at least eight other genes that also play a smaller, more moderating role.
The two main genes are called OCA2 and HERC2, both located on chromosome 15.
In a fascinating twist, the “HERC2 gene contains a segment of DNA that controls the activity (expression) of the OCA2 gene, turning it on or off as needed,” according to the National Institute of Health.
So even different genes can, in effect, cancel each other out. The picture is complicated.
This and other surprising recent developments about eye color that contradict what we previously thought.
Here are three ways that our understanding of eye color has changed in recent years.
1. Eye Color Can Change Over A Person’s Lifetime
Unlike hair and skin, eyes don’t produce pigment continuously.
In up to 15 percent of people of European ancestry, eye color lightens or darkens over the lifetime.
For example, very brown eyes can mellow into hazel or vice versa.
These changes are normal and gradual over a long period of time, but if drastic changes occur in a short time span, you should see your doctor, as this could be a sign of a more serious eye condition, such as Horner’s syndrome, retinal glaucoma, or even melanoma of the iris.
In some cases, trauma, such as a blow to the head, can cause a change in eye color.
Often this means that the affected eye and unaffected eye will be slightly different colors. For example, pop icon David Bowie had one blue and one hazel eye, which he assumed to have come from a childhood injury to the face.
Heterochromia—having two different colored eyes—can also occur naturally, without any history of trauma.
The unique coloration can be accompanied by any of a few rare genetic disorders, but more likely it is not.
It is worth a trip to the eye doctor to see, especially if the cause has not been determined.
But, heterochromia can also be a perfectly benign variation.
Eye color can in fact change from an early age.
Babies are born with less melanin, and their eyes will gradually darken over time as melanin accumulates in the iris.
Within a few months to a year, their eye color will mature into their permanent color.
2. We All Used To Have Brown Eyes
Baby blues are actually new to the scene—a genetic mutation that showed up relatively recently in human evolution.
"Originally, we all had brown eyes," according to Hans Eiberg from the University of Copenhagen, who led a team that examined mitochondrial DNA—passed down only on the mother’s side—and found that the blue variant of the OCA2 gene appeared somewhere between 6,000 and 10,000 years ago.
In that comparatively short span of time, in terms of generations, there was a sea change: no one on earth had anything but brown eyes for most of human history, and now around 30 percent of Europeans have blue eyes—all because of one genetic mutation.
Everyone who has that gene, the blue variant of OCA2, does in fact share a common ancestor.
And the mitochondrial DNA research allows scientists to pinpoint that that common ancestor lived less than ten thousand years ago—a blink of an evolutionary eye.
3. Scientists Can Predict Eye Color
Despite the complex interaction among the 10 melanin-related genes, the field of genetics is bringing eye-color testing rapidly into focus.
Our understanding of the human genome has progressed so radically from just a decade ago that now it is possible for scientists to take a sample of DNA and predict eye color, hair color, and skin color of the subject with very high accuracy.
This cutting-edge technology has significant implications for both forensic scientists working crime scenes and for physical anthropologists tracing human history.
And best of all, researchers have made the data public and free for all to use.