Speaker 1: Welcome to Brainstuff, a production of iHeartRadio. Hey Brainstuff, Lauren
Speaker 1: Vogelbaum here. Elizabeth Taylor was one of the most famous
Speaker 1: actors of the twentieth century, and one of her trademarks
Speaker 1: was her violet eyes. Although it's difficult to see in
Speaker 1: photographs a people who knew Taylor claim they truly did
Speaker 1: have a purplish cast. So how is that possible? And
Speaker 1: what are the world's rarest eye colors? The eye color
Speaker 1: is controlled by two factors, the amount and distribution of
Speaker 1: pigments called melanins in your iris, and the physical structure
Speaker 1: of your iris. Let's break that down a little, okay.
Speaker 1: The iris is the ring shaped membrane that sits behind
Speaker 1: the eyes protective clear cornea. The purpose of the iris
Speaker 1: is to control the amount of light that enters your
Speaker 1: eye through the pupil, which is the hole in the
Speaker 1: center of the iris. This happens with the motion of
Speaker 1: these layers of smooth, involuntary muscle in the iris that
Speaker 1: make it constrict or dilate, thus making your pupil smaller
Speaker 1: or bigger and letting less or more light into your eye.
Speaker 1: The iris is made up of an intricate web of
Speaker 1: muscles and connective tissues. The two main ones that contain
Speaker 1: melanins are the fibrous front layer, the stroma, and a
Speaker 1: thin back layer, the pigment epithelium. Melanins are a group
Speaker 1: of pigments found in several places in our bodies, but
Speaker 1: relevant to today in our eyes, skin, and hair. The
Speaker 1: two main types melanin found there are eumelanin, which can
Speaker 1: be brown to black in color, and fiamelanin, which can
Speaker 1: be yellow to red. The back layer of the iris
Speaker 1: can produce mostly eumelanin, the front layer can produce both types.
Speaker 1: Brown eyes have a lot of these pigments, Green and
Speaker 1: hazel eyes contain less, and blue eyes contain very little,
Speaker 1: but those two are the only pigments, and human eyes,
Speaker 1: like Elizabeth Taylor, didn't have purple pigment in her irises.
Speaker 1: If you have blue, green, or yiss purple eyes, it's
Speaker 1: because you have a lack of melanins in different layers
Speaker 1: of your iris. The amount and their distribution, coupled with
Speaker 1: the way that light scatters through the layers, results in
Speaker 1: eye color. Researchers think that different eye colors might have
Speaker 1: evolved because mutations and melanin production proved useful in different environments.
Speaker 1: For the article, this episode is based on How Stuff Works.
Speaker 1: Spoke with optimologist Osuwoma Abogo, MD, whose name I hope
Speaker 1: I'm saying correctly. I did look it up. She said,
Speaker 1: dark iris color is associated with less scattering of light
Speaker 1: in the eye. This trait may be protective under conditions
Speaker 1: of bright sunlight and high ultraviolet radiation, alike for people
Speaker 1: who live in the equatorial regions of the world. Blue
Speaker 1: eye color, on the other hand, is associated with greater
Speaker 1: light scattering in the eye a higher level of melatonin suppression,
Speaker 1: traits that may have been adaptive under highly seasonal sunshine
Speaker 1: regimes in northwestern Eurasia. How people end up with their
Speaker 1: own unique eye color is complex. The research has discovered
Speaker 1: that at least ten genes help determine eye color, though
Speaker 1: two genes located on chromosome fifteen may influence it the most.
Speaker 1: Abuga said, A people used to believe eye color could
Speaker 1: be easily determined based on your parents' eyes, but the
Speaker 1: genetics of eye color is actually much more complicated. Research
Speaker 1: has shown that the color of your eye may actually
Speaker 1: be linked genetically to the color of your skin and
Speaker 1: hair in some cases. Basically, the color of your eyes
Speaker 1: determined by a complex mix of genes, some of which
Speaker 1: are still being studied. Brown is the most common eye
Speaker 1: color in the world. Between seventy and eighty percent of
Speaker 1: the world's population have eyes that are some shade of brown,
Speaker 1: from tawny to nearly black. These eyes all contain a
Speaker 1: lot of melanin, but the exact shade depends on how
Speaker 1: much the two layers of the iris contain and of
Speaker 1: what types. Mostly eumelanin will create darker brown eyes, having
Speaker 1: more feomelanin in the stroma creates lighter shades of brown.
Speaker 1: It's thought that all ancient humans living more than ten
Speaker 1: thousand years ago had brown eyes. The first light eyed
Speaker 1: person probably had a genetic mutation that caused their body
Speaker 1: to produce less melanin, and this mutation was passed on
Speaker 1: to their descendants after brown, blue eyes are relatively common,
Speaker 1: between eight and ten percent of people in the world
Speaker 1: have them. Again, there's no blue pigment in our eyes, though,
Speaker 1: or rather in blue eyed people. There's very little melanin
Speaker 1: in either layer of the iris, and the stroma might
Speaker 1: contain no pigment at all, but because the stroma is
Speaker 1: so textured and fibrous, light scatters through it and off
Speaker 1: of it, and the stroma appears blue. This is called
Speaker 1: structural color. It's similar to what gives butterfly wings and
Speaker 1: peacock feathers their colors. But in the human eye, the
Speaker 1: strama looks blue for essentially the same reason that the
Speaker 1: sky looks blue, Because although a full spectrum of light
Speaker 1: is hitting the strama a, blue light scatters more than
Speaker 1: other visible wavelengths. That means more blue light reaches the
Speaker 1: eye of the observer. This is also why blue eyes
Speaker 1: can seem to change color. The shade of blue that
Speaker 1: they appear changes based on how much light is available
Speaker 1: and how it scatters due to angles and other factors
Speaker 1: like the colors around you. In dim light, for example,
Speaker 1: blue eyes can appear kind of stormy, but in direct
Speaker 1: light they can be very bright because more light is
Speaker 1: reflecting off of the iris than happens with any other
Speaker 1: color of eye. Historically, blue eyes and gray eyes have
Speaker 1: been combined into a single category, but recently researchers have
Speaker 1: discovered that there are some differences. Around three eighty percent
Speaker 1: of the world's population have gray eyes and Like most
Speaker 1: light colored eyes, the coloration is the product of very
Speaker 1: little melanin in the iris. As with blue eyes, the
Speaker 1: stroma may have no pigment at all. However, gray eyes
Speaker 1: have more collagen in the stroma than blue eyes, affecting
Speaker 1: the way that light scatters within the iris. But okay,
Speaker 1: let's talk about green eyes. Only about two percent of
Speaker 1: the world's population sports this eye color. Green eyes are
Speaker 1: far more common in parts of Europe than in the
Speaker 1: world at large, and women have them more often than men.
Speaker 1: For green eyed people, the back layer of the iris
Speaker 1: has a low concentration of U melanin and the stroma
Speaker 1: contains a low amount of faeomelanin, So the green color
Speaker 1: you see is a mixture of the different pigments and
Speaker 1: a bit of the light scattering that you see in
Speaker 1: blue eyes. There's not much data on hazel eyes, which
Speaker 1: are mixed green brown, but it's thought that around five
Speaker 1: percent of the world's population has them. This coloration is
Speaker 1: probably a result of different concentrations of melanin in different
Speaker 1: sections of the iris, and a mix of few melanin
Speaker 1: and fao melanin in the stroma. But one of the
Speaker 1: rarest eye colors in the world isn't just one color,
Speaker 1: it's two. People with a condition called heterochromia have irises
Speaker 1: of two different colors. Less than one percent of the
Speaker 1: world's population has this. In complete heterochromia, the eyes have
Speaker 1: two completely different colored irises. In partial heterochromia, just a
Speaker 1: portion of the iris is a different color from the rest.
Speaker 1: An infant can be born with heterochromia and have completely
Speaker 1: healthy eyes, but it can also be acquired later as
Speaker 1: a symptom of an injury, disease, or syndrome. About equally
Speaker 1: as rare are red or violet eyes, and they often
Speaker 1: point to an underlying condition. Albinism, a genetic condition in
Speaker 1: which a person is born with little or no melanin
Speaker 1: in their entire body. Albinism the appearance of hair, skin,
Speaker 1: and eyes. The eyes can a pair a very pale blue,
Speaker 1: a very pale purple, or even reddish in some lights
Speaker 1: as the result of light reflecting off of blood vessels
Speaker 1: in the eye. When just a very little bit of
Speaker 1: melanin is present, these red reflections mix with the pigment
Speaker 1: to create violet eyes. However, Elizabeth Taylor doesn't seem to
Speaker 1: have had albinism. It's more likely that she had a
Speaker 1: variant of blue eyes that appeared particularly rich due to
Speaker 1: their particular structure, and maybe she played their color up
Speaker 1: with complimentary colors in her makeup, clothing, and hair. Again,
Speaker 1: the genetics and physics of how our eyes appear are
Speaker 1: complicated and still being studied. Today's episode is based on
Speaker 1: the article do you have one of these six rarest
Speaker 1: eye colors in the world? On how stuffworks dot com
Speaker 1: written by Jeslyn Shields. Brainstuff is production of iHeart Radio
Speaker 1: in partnership with how stuffworkst Com and is produced by
Speaker 1: Tyler Klang. Four more podcasts my heart Radio, visit the
Speaker 1: iHeartRadio app, Apple Podcasts, or wherever you listen to your
Speaker 1: favorite shows.