True fact: about 10,000 years ago, every human on Earth had brown eyes.
Now, half of all people in the U.S. have brown eyes. Worldwide, that number could be even greater.
The brown color comes from melanin, a pigment in our eyes that also gives a brown color to hair, skin and other cells.
The colored part of the eye is called the iris.
The iris is made up of two layers of muscle and other kinds of cells. In most people, the back layer has at least some brown pigment in it, even if their eyes don't look brown. In people with brown eyes, the iris’s front layer also has some brown pigment in its cells. The more melanin there is in the iris, the darker brown it will be.
Other Eye Colors
How did blue and other lighter-colored eyes come into the picture?
Scientists trace the historic change back to a single common ancestor. 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.
Some eyes appear blue, green or hazel not because they have different color pigments, but because they have less melanin. With less melanin in the eye, less light is absorbed. That means more light is scattered out from the iris. When light is scattered, it reflects differently along the light color spectrum. So eyes with the least melanin in them will appear blue. If the iris has a little more melanin, then it will appear green or hazel.
Brown Eyes Aren’t 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.
A Baby’s Eye Color May Change to Brown
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.
Brown Eyes Are More Common in Certain Places
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.
Connection Between Eye Color and Eye Health
People with brown eyes have a lower incidence of eye cancer, macular degeneration and diabetic retinopathy. Ophthalmologists are not exactly sure why, but believe that melanin pigment may give brown eyes more protection. On the flip side, a study found that people with brown eyes were just about twice as likely to develop cataracts than individuals with light eyes. The researchers did not look at why this is true, but they did rule out other things that could make cataracts more likely, like smoking and eye injury.