Brown eyes get their color from melanin, the same pigment that colors your hair, skin and other cells. But blue eyes don’t have any blue pigment in them. Blue eyes get their color the same way water and the sky get their blue color. They scatter light so that more blue light reflects back out.
The colored part of the eye is called the iris. It’s a structure that contains muscle and other kinds of cells. You can see the iris in action when it squeezes or relaxes to let in more, or less light through the pupil. The iris is made up of two layers. For almost everyone — even people with blue eyes — the back layer (called the pigment epithelium) has brown pigment in it. The more melanin there is in the iris, the darker brown it will be.
The front layer of the iris (called the stroma) is made up of overlapping fibers and cells. For people with brown eyes, some of the cells also have brown pigment in them. If there is no pigment at all in this front layer, the fibers scatter and absorb some of the longer wavelengths of light that come in. More blue light gets back out and the eyes appear to be blue. For people with green or hazel eyes, one, or both, of the layers of the iris also has some light brown pigment in it. The light brown pigment interacts with the blue light and the eye can look green or speckled. Many people have variations in the color of their irides, often with one color near the pupil and another at the edge. This variation happens when different parts of the iris have different amounts of pigment in them.
Since blue eyes get their color from the light that’s coming in and is then reflected back out, they can appear as different colors depending on the lighting conditions. Green and hazel eyes are a mixture of pigment color and color from scattered light, so they can also look different in different lighting conditions.
Eye Color Is Not Inherited from A Single Gene
Scientists used to think that a single gene determined your eye color. More current research suggests that as many as 16 different genes could be responsible for eye color. This helps explain why two parents with the same eye color can have children with an entirely different eye color.
When babies are born, their eyes may sometimes appear blue while their melanin is still developing. Within about 12 months, cells will begin to produce melanin, and as more melanin builds up in the iris, eye color may darken.
All Blue-Eyed People May Have a Common Ancestor
Scientists trace the historic change back to a single common ancestor in Europe about 6,000-10,000 years ago. That person had a change in a gene that controls melanin production. This change, or mutation, is believed to have reduced the production of melanin in the iris.
Brown Eyes Are More Common in Hotter Climates
High levels of melanin in the eyes, hair and skin help protect people from the sun’s damaging rays. That explains why brown eyes occur more frequently in the hotter climates of Africa and Asia than in Europe. In Iceland, brown eyes are in the minority. As people move to less sunny locations, their need for protective pigment decreases. Eye color may have evolved as our early ancestors moved toward cooler parts of the world.