1 00:00:01,840 --> 00:00:07,800 Speaker 1: Welcome to Brainstuff, a production of iHeartRadio. Hey Brainstuff, Lauren 2 00:00:07,840 --> 00:00:12,840 Speaker 1: Vogelbaum here. Elizabeth Taylor was one of the most famous 3 00:00:12,880 --> 00:00:16,000 Speaker 1: actors of the twentieth century, and one of her trademarks 4 00:00:16,160 --> 00:00:19,759 Speaker 1: was her violet eyes. Although it's difficult to see in 5 00:00:19,800 --> 00:00:23,079 Speaker 1: photographs a people who knew Taylor claim they truly did 6 00:00:23,120 --> 00:00:27,600 Speaker 1: have a purplish cast. So how is that possible? And 7 00:00:27,960 --> 00:00:32,199 Speaker 1: what are the world's rarest eye colors? The eye color 8 00:00:32,240 --> 00:00:35,519 Speaker 1: is controlled by two factors, the amount and distribution of 9 00:00:35,560 --> 00:00:39,640 Speaker 1: pigments called melanins in your iris, and the physical structure 10 00:00:39,800 --> 00:00:44,080 Speaker 1: of your iris. Let's break that down a little, okay. 11 00:00:44,520 --> 00:00:47,680 Speaker 1: The iris is the ring shaped membrane that sits behind 12 00:00:47,720 --> 00:00:51,879 Speaker 1: the eyes protective clear cornea. The purpose of the iris 13 00:00:51,920 --> 00:00:54,120 Speaker 1: is to control the amount of light that enters your 14 00:00:54,160 --> 00:00:56,680 Speaker 1: eye through the pupil, which is the hole in the 15 00:00:56,720 --> 00:01:00,080 Speaker 1: center of the iris. This happens with the motion of 16 00:01:00,200 --> 00:01:03,640 Speaker 1: these layers of smooth, involuntary muscle in the iris that 17 00:01:03,720 --> 00:01:07,600 Speaker 1: make it constrict or dilate, thus making your pupil smaller 18 00:01:07,680 --> 00:01:12,279 Speaker 1: or bigger and letting less or more light into your eye. 19 00:01:12,440 --> 00:01:14,679 Speaker 1: The iris is made up of an intricate web of 20 00:01:14,800 --> 00:01:18,919 Speaker 1: muscles and connective tissues. The two main ones that contain 21 00:01:19,000 --> 00:01:22,399 Speaker 1: melanins are the fibrous front layer, the stroma, and a 22 00:01:22,520 --> 00:01:27,720 Speaker 1: thin back layer, the pigment epithelium. Melanins are a group 23 00:01:27,760 --> 00:01:30,720 Speaker 1: of pigments found in several places in our bodies, but 24 00:01:30,920 --> 00:01:34,760 Speaker 1: relevant to today in our eyes, skin, and hair. The 25 00:01:34,800 --> 00:01:38,319 Speaker 1: two main types melanin found there are eumelanin, which can 26 00:01:38,400 --> 00:01:41,640 Speaker 1: be brown to black in color, and fiamelanin, which can 27 00:01:41,680 --> 00:01:45,040 Speaker 1: be yellow to red. The back layer of the iris 28 00:01:45,040 --> 00:01:48,840 Speaker 1: can produce mostly eumelanin, the front layer can produce both types. 29 00:01:49,680 --> 00:01:52,400 Speaker 1: Brown eyes have a lot of these pigments, Green and 30 00:01:52,440 --> 00:01:56,000 Speaker 1: hazel eyes contain less, and blue eyes contain very little, 31 00:01:58,160 --> 00:02:01,880 Speaker 1: but those two are the only pigments, and human eyes, 32 00:02:02,560 --> 00:02:05,800 Speaker 1: like Elizabeth Taylor, didn't have purple pigment in her irises. 33 00:02:06,480 --> 00:02:10,040 Speaker 1: If you have blue, green, or yiss purple eyes, it's 34 00:02:10,080 --> 00:02:13,120 Speaker 1: because you have a lack of melanins in different layers 35 00:02:13,120 --> 00:02:17,240 Speaker 1: of your iris. The amount and their distribution, coupled with 36 00:02:17,280 --> 00:02:20,400 Speaker 1: the way that light scatters through the layers, results in 37 00:02:20,440 --> 00:02:24,959 Speaker 1: eye color. Researchers think that different eye colors might have 38 00:02:25,040 --> 00:02:30,040 Speaker 1: evolved because mutations and melanin production proved useful in different environments. 39 00:02:31,320 --> 00:02:34,040 Speaker 1: For the article, this episode is based on How Stuff Works. 40 00:02:34,080 --> 00:02:37,680 Speaker 1: Spoke with optimologist Osuwoma Abogo, MD, whose name I hope 41 00:02:37,720 --> 00:02:40,640 Speaker 1: I'm saying correctly. I did look it up. She said, 42 00:02:41,200 --> 00:02:44,560 Speaker 1: dark iris color is associated with less scattering of light 43 00:02:44,680 --> 00:02:48,400 Speaker 1: in the eye. This trait may be protective under conditions 44 00:02:48,400 --> 00:02:52,200 Speaker 1: of bright sunlight and high ultraviolet radiation, alike for people 45 00:02:52,200 --> 00:02:55,640 Speaker 1: who live in the equatorial regions of the world. Blue 46 00:02:55,680 --> 00:02:58,200 Speaker 1: eye color, on the other hand, is associated with greater 47 00:02:58,320 --> 00:03:01,920 Speaker 1: light scattering in the eye a higher level of melatonin suppression, 48 00:03:02,280 --> 00:03:05,600 Speaker 1: traits that may have been adaptive under highly seasonal sunshine 49 00:03:05,639 --> 00:03:11,560 Speaker 1: regimes in northwestern Eurasia. How people end up with their 50 00:03:11,600 --> 00:03:15,240 Speaker 1: own unique eye color is complex. The research has discovered 51 00:03:15,280 --> 00:03:18,280 Speaker 1: that at least ten genes help determine eye color, though 52 00:03:18,360 --> 00:03:22,560 Speaker 1: two genes located on chromosome fifteen may influence it the most. 53 00:03:23,240 --> 00:03:26,120 Speaker 1: Abuga said, A people used to believe eye color could 54 00:03:26,160 --> 00:03:29,120 Speaker 1: be easily determined based on your parents' eyes, but the 55 00:03:29,160 --> 00:03:32,840 Speaker 1: genetics of eye color is actually much more complicated. Research 56 00:03:32,880 --> 00:03:35,080 Speaker 1: has shown that the color of your eye may actually 57 00:03:35,080 --> 00:03:37,720 Speaker 1: be linked genetically to the color of your skin and 58 00:03:37,800 --> 00:03:41,480 Speaker 1: hair in some cases. Basically, the color of your eyes 59 00:03:41,520 --> 00:03:44,440 Speaker 1: determined by a complex mix of genes, some of which 60 00:03:44,440 --> 00:03:49,360 Speaker 1: are still being studied. Brown is the most common eye 61 00:03:49,440 --> 00:03:52,520 Speaker 1: color in the world. Between seventy and eighty percent of 62 00:03:52,560 --> 00:03:55,200 Speaker 1: the world's population have eyes that are some shade of brown, 63 00:03:55,320 --> 00:03:59,720 Speaker 1: from tawny to nearly black. These eyes all contain a 64 00:03:59,800 --> 00:04:03,280 Speaker 1: lot of melanin, but the exact shade depends on how 65 00:04:03,360 --> 00:04:06,240 Speaker 1: much the two layers of the iris contain and of 66 00:04:06,280 --> 00:04:11,520 Speaker 1: what types. Mostly eumelanin will create darker brown eyes, having 67 00:04:11,560 --> 00:04:14,840 Speaker 1: more feomelanin in the stroma creates lighter shades of brown. 68 00:04:15,960 --> 00:04:18,520 Speaker 1: It's thought that all ancient humans living more than ten 69 00:04:18,560 --> 00:04:21,960 Speaker 1: thousand years ago had brown eyes. The first light eyed 70 00:04:22,000 --> 00:04:25,080 Speaker 1: person probably had a genetic mutation that caused their body 71 00:04:25,120 --> 00:04:28,120 Speaker 1: to produce less melanin, and this mutation was passed on 72 00:04:28,160 --> 00:04:33,960 Speaker 1: to their descendants after brown, blue eyes are relatively common, 73 00:04:34,360 --> 00:04:36,400 Speaker 1: between eight and ten percent of people in the world 74 00:04:36,440 --> 00:04:40,760 Speaker 1: have them. Again, there's no blue pigment in our eyes, though, 75 00:04:41,360 --> 00:04:44,279 Speaker 1: or rather in blue eyed people. There's very little melanin 76 00:04:44,440 --> 00:04:47,360 Speaker 1: in either layer of the iris, and the stroma might 77 00:04:47,400 --> 00:04:51,600 Speaker 1: contain no pigment at all, but because the stroma is 78 00:04:51,640 --> 00:04:55,640 Speaker 1: so textured and fibrous, light scatters through it and off 79 00:04:55,680 --> 00:05:00,559 Speaker 1: of it, and the stroma appears blue. This is called 80 00:05:00,680 --> 00:05:04,920 Speaker 1: structural color. It's similar to what gives butterfly wings and 81 00:05:04,960 --> 00:05:08,320 Speaker 1: peacock feathers their colors. But in the human eye, the 82 00:05:08,360 --> 00:05:11,400 Speaker 1: strama looks blue for essentially the same reason that the 83 00:05:11,440 --> 00:05:15,080 Speaker 1: sky looks blue, Because although a full spectrum of light 84 00:05:15,120 --> 00:05:18,040 Speaker 1: is hitting the strama a, blue light scatters more than 85 00:05:18,080 --> 00:05:21,400 Speaker 1: other visible wavelengths. That means more blue light reaches the 86 00:05:21,440 --> 00:05:25,640 Speaker 1: eye of the observer. This is also why blue eyes 87 00:05:25,680 --> 00:05:28,479 Speaker 1: can seem to change color. The shade of blue that 88 00:05:28,520 --> 00:05:31,520 Speaker 1: they appear changes based on how much light is available 89 00:05:31,720 --> 00:05:35,120 Speaker 1: and how it scatters due to angles and other factors 90 00:05:35,160 --> 00:05:38,480 Speaker 1: like the colors around you. In dim light, for example, 91 00:05:38,600 --> 00:05:41,480 Speaker 1: blue eyes can appear kind of stormy, but in direct 92 00:05:41,560 --> 00:05:44,440 Speaker 1: light they can be very bright because more light is 93 00:05:44,480 --> 00:05:47,279 Speaker 1: reflecting off of the iris than happens with any other 94 00:05:47,320 --> 00:05:52,599 Speaker 1: color of eye. Historically, blue eyes and gray eyes have 95 00:05:52,680 --> 00:05:56,599 Speaker 1: been combined into a single category, but recently researchers have 96 00:05:56,640 --> 00:06:00,400 Speaker 1: discovered that there are some differences. Around three eighty percent 97 00:06:00,480 --> 00:06:03,720 Speaker 1: of the world's population have gray eyes and Like most 98 00:06:03,800 --> 00:06:06,480 Speaker 1: light colored eyes, the coloration is the product of very 99 00:06:06,480 --> 00:06:09,680 Speaker 1: little melanin in the iris. As with blue eyes, the 100 00:06:09,720 --> 00:06:13,599 Speaker 1: stroma may have no pigment at all. However, gray eyes 101 00:06:13,640 --> 00:06:16,920 Speaker 1: have more collagen in the stroma than blue eyes, affecting 102 00:06:16,920 --> 00:06:21,640 Speaker 1: the way that light scatters within the iris. But okay, 103 00:06:21,800 --> 00:06:25,320 Speaker 1: let's talk about green eyes. Only about two percent of 104 00:06:25,360 --> 00:06:28,800 Speaker 1: the world's population sports this eye color. Green eyes are 105 00:06:28,800 --> 00:06:30,599 Speaker 1: far more common in parts of Europe than in the 106 00:06:30,600 --> 00:06:33,200 Speaker 1: world at large, and women have them more often than men. 107 00:06:34,920 --> 00:06:37,640 Speaker 1: For green eyed people, the back layer of the iris 108 00:06:37,760 --> 00:06:40,960 Speaker 1: has a low concentration of U melanin and the stroma 109 00:06:41,040 --> 00:06:45,640 Speaker 1: contains a low amount of faeomelanin, So the green color 110 00:06:45,680 --> 00:06:48,320 Speaker 1: you see is a mixture of the different pigments and 111 00:06:48,360 --> 00:06:50,000 Speaker 1: a bit of the light scattering that you see in 112 00:06:50,040 --> 00:06:55,040 Speaker 1: blue eyes. There's not much data on hazel eyes, which 113 00:06:55,040 --> 00:06:58,599 Speaker 1: are mixed green brown, but it's thought that around five 114 00:06:58,640 --> 00:07:02,400 Speaker 1: percent of the world's population has them. This coloration is 115 00:07:02,480 --> 00:07:05,800 Speaker 1: probably a result of different concentrations of melanin in different 116 00:07:05,839 --> 00:07:08,800 Speaker 1: sections of the iris, and a mix of few melanin 117 00:07:08,839 --> 00:07:13,000 Speaker 1: and fao melanin in the stroma. But one of the 118 00:07:13,120 --> 00:07:16,080 Speaker 1: rarest eye colors in the world isn't just one color, 119 00:07:16,440 --> 00:07:20,760 Speaker 1: it's two. People with a condition called heterochromia have irises 120 00:07:20,800 --> 00:07:23,680 Speaker 1: of two different colors. Less than one percent of the 121 00:07:23,680 --> 00:07:28,080 Speaker 1: world's population has this. In complete heterochromia, the eyes have 122 00:07:28,160 --> 00:07:32,200 Speaker 1: two completely different colored irises. In partial heterochromia, just a 123 00:07:32,240 --> 00:07:34,400 Speaker 1: portion of the iris is a different color from the rest. 124 00:07:36,000 --> 00:07:38,680 Speaker 1: An infant can be born with heterochromia and have completely 125 00:07:38,720 --> 00:07:41,520 Speaker 1: healthy eyes, but it can also be acquired later as 126 00:07:41,560 --> 00:07:47,520 Speaker 1: a symptom of an injury, disease, or syndrome. About equally 127 00:07:47,560 --> 00:07:50,720 Speaker 1: as rare are red or violet eyes, and they often 128 00:07:50,760 --> 00:07:55,280 Speaker 1: point to an underlying condition. Albinism, a genetic condition in 129 00:07:55,320 --> 00:07:57,720 Speaker 1: which a person is born with little or no melanin 130 00:07:57,760 --> 00:08:01,920 Speaker 1: in their entire body. Albinism the appearance of hair, skin, 131 00:08:02,080 --> 00:08:05,679 Speaker 1: and eyes. The eyes can a pair a very pale blue, 132 00:08:05,720 --> 00:08:08,720 Speaker 1: a very pale purple, or even reddish in some lights 133 00:08:08,840 --> 00:08:11,440 Speaker 1: as the result of light reflecting off of blood vessels 134 00:08:11,440 --> 00:08:14,760 Speaker 1: in the eye. When just a very little bit of 135 00:08:14,760 --> 00:08:18,040 Speaker 1: melanin is present, these red reflections mix with the pigment 136 00:08:18,120 --> 00:08:23,960 Speaker 1: to create violet eyes. However, Elizabeth Taylor doesn't seem to 137 00:08:24,000 --> 00:08:26,720 Speaker 1: have had albinism. It's more likely that she had a 138 00:08:26,800 --> 00:08:29,880 Speaker 1: variant of blue eyes that appeared particularly rich due to 139 00:08:29,920 --> 00:08:34,120 Speaker 1: their particular structure, and maybe she played their color up 140 00:08:34,120 --> 00:08:38,840 Speaker 1: with complimentary colors in her makeup, clothing, and hair. Again, 141 00:08:39,000 --> 00:08:42,079 Speaker 1: the genetics and physics of how our eyes appear are 142 00:08:42,240 --> 00:08:50,520 Speaker 1: complicated and still being studied. Today's episode is based on 143 00:08:50,520 --> 00:08:52,920 Speaker 1: the article do you have one of these six rarest 144 00:08:52,960 --> 00:08:55,240 Speaker 1: eye colors in the world? On how stuffworks dot com 145 00:08:55,280 --> 00:08:58,560 Speaker 1: written by Jeslyn Shields. Brainstuff is production of iHeart Radio 146 00:08:58,600 --> 00:09:00,960 Speaker 1: in partnership with how stuffworkst Com and is produced by 147 00:09:00,960 --> 00:09:04,280 Speaker 1: Tyler Klang. Four more podcasts my heart Radio, visit the 148 00:09:04,320 --> 00:09:07,400 Speaker 1: iHeartRadio app, Apple Podcasts, or wherever you listen to your 149 00:09:07,440 --> 00:09:08,160 Speaker 1: favorite shows.