Welcome to Brainstuff, a production of iHeartRadio. Hey Brainstuff, Lauren Vogelbaum here. Elizabeth Taylor was one of the most famous actors of the twentieth century, and one of her trademarks was her violet eyes. Although it's difficult to see in photographs a people who knew Taylor claim they truly did have a purplish cast. So how is that possible? And
what are the world's rarest eye colors? The eye color is controlled by two factors, the amount and distribution of pigments called melanins in your iris, and the physical structure of your iris. Let's break that down a little, okay. The iris is the ring shaped membrane that sits behind the eyes protective clear cornea. The purpose of the iris is to control the amount of light that enters your eye through the pupil, which is the hole in the
center of the iris. This happens with the motion of these layers of smooth, involuntary muscle in the iris that make it constrict or dilate, thus making your pupil smaller or bigger and letting less or more light into your eye. The iris is made up of an intricate web of muscles and connective tissues. The two main ones that contain melanins are the fibrous front layer, the stroma, and a
thin back layer, the pigment epithelium. Melanins are a group of pigments found in several places in our bodies, but relevant to today in our eyes, skin, and hair. The two main types melanin found there are eumelanin, which can be brown to black in color, and fiamelanin, which can be yellow to red. The back layer of the iris can produce mostly eumelanin, the front layer can produce both types.
Brown eyes have a lot of these pigments, Green and hazel eyes contain less, and blue eyes contain very little, but those two are the only pigments, and human eyes, like Elizabeth Taylor, didn't have purple pigment in her irises. If you have blue, green, or yiss purple eyes, it's because you have a lack of melanins in different layers of your iris. The amount and their distribution, coupled with the way that light scatters through the layers, results in
eye color. Researchers think that different eye colors might have evolved because mutations and melanin production proved useful in different environments. For the article, this episode is based on How Stuff Works. Spoke with optimologist Osuwoma Abogo, MD, whose name I hope I'm saying correctly. I did look it up. She said, dark iris color is associated with less scattering of light
in the eye. This trait may be protective under conditions of bright sunlight and high ultraviolet radiation, alike for people who live in the equatorial regions of the world. Blue eye color, on the other hand, is associated with greater light scattering in the eye a higher level of melatonin suppression, traits that may have been adaptive under highly seasonal sunshine regimes in northwestern Eurasia. How people end up with their
own unique eye color is complex. The research has discovered that at least ten genes help determine eye color, though two genes located on chromosome fifteen may influence it the most. Abuga said, A people used to believe eye color could be easily determined based on your parents' eyes, but the genetics of eye color is actually much more complicated. Research has shown that the color of your eye may actually be linked genetically to the color of your skin and
hair in some cases. Basically, the color of your eyes determined by a complex mix of genes, some of which are still being studied. Brown is the most common eye color in the world. Between seventy and eighty percent of the world's population have eyes that are some shade of brown, from tawny to nearly black. These eyes all contain a lot of melanin, but the exact shade depends on how much the two layers of the iris contain and of
what types. Mostly eumelanin will create darker brown eyes, having more feomelanin in the stroma creates lighter shades of brown. It's thought that all ancient humans living more than ten thousand years ago had brown eyes. The first light eyed person probably had a genetic mutation that caused their body to produce less melanin, and this mutation was passed on to their descendants after brown, blue eyes are relatively common, between eight and ten percent of people in the world
have them. Again, there's no blue pigment in our eyes, though, or rather in blue eyed people. There's very little melanin in either layer of the iris, and the stroma might contain no pigment at all, but because the stroma is so textured and fibrous, light scatters through it and off of it, and the stroma appears blue. This is called structural color. It's similar to what gives butterfly wings and
peacock feathers their colors. But in the human eye, the strama looks blue for essentially the same reason that the sky looks blue, Because although a full spectrum of light is hitting the strama a, blue light scatters more than other visible wavelengths. That means more blue light reaches the eye of the observer. This is also why blue eyes can seem to change color. The shade of blue that they appear changes based on how much light is available and how it scatters due to angles and other factors
like the colors around you. In dim light, for example, blue eyes can appear kind of stormy, but in direct light they can be very bright because more light is reflecting off of the iris than happens with any other color of eye. Historically, blue eyes and gray eyes have been combined into a single category, but recently researchers have
discovered that there are some differences. Around three eighty percent of the world's population have gray eyes and Like most light colored eyes, the coloration is the product of very little melanin in the iris. As with blue eyes, the stroma may have no pigment at all. However, gray eyes have more collagen in the stroma than blue eyes, affecting the way that light scatters within the iris. But okay, let's talk about green eyes. Only about two percent of
the world's population sports this eye color. Green eyes are far more common in parts of Europe than in the world at large, and women have them more often than men. For green eyed people, the back layer of the iris has a low concentration of U melanin and the stroma contains a low amount of faeomelanin, So the green color you see is a mixture of the different pigments and a bit of the light scattering that you see in
blue eyes. There's not much data on hazel eyes, which are mixed green brown, but it's thought that around five percent of the world's population has them. This coloration is probably a result of different concentrations of melanin in different sections of the iris, and a mix of few melanin and fao melanin in the stroma. But one of the rarest eye colors in the world isn't just one color, it's two. People with a condition called heterochromia have irises
of two different colors. Less than one percent of the world's population has this. In complete heterochromia, the eyes have two completely different colored irises. In partial heterochromia, just a portion of the iris is a different color from the rest. An infant can be born with heterochromia and have completely healthy eyes, but it can also be acquired later as a symptom of an injury, disease, or syndrome. About equally as rare are red or violet eyes, and they often
point to an underlying condition. Albinism, a genetic condition in which a person is born with little or no melanin in their entire body. Albinism the appearance of hair, skin, and eyes. The eyes can a pair a very pale blue, a very pale purple, or even reddish in some lights as the result of light reflecting off of blood vessels in the eye. When just a very little bit of melanin is present, these red reflections mix with the pigment
to create violet eyes. However, Elizabeth Taylor doesn't seem to have had albinism. It's more likely that she had a variant of blue eyes that appeared particularly rich due to their particular structure, and maybe she played their color up with complimentary colors in her makeup, clothing, and hair. Again, the genetics and physics of how our eyes appear are complicated and still being studied. Today's episode is based on the article do you have one of these six rarest
eye colors in the world? On how stuffworks dot com written by Jeslyn Shields. Brainstuff is production of iHeart Radio in partnership with how stuffworkst Com and is produced by Tyler Klang. Four more podcasts my heart Radio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows.