Melanin

When you notice someone's eye color, you're looking at a part of the eye called the iris, a ring around the pupil filled with melanin. Melanin is a pigment that gives color to our hair, eyes and skin. Depending on the amount of melanin in the iris, it can be shades of gray, green, hazel, amber, brown or even red, and everyone's eye color is unique. Eye color isn't just for aesthetics. Like most of your biology, eye color has a purpose. Thousands of years ago, the first humans had something in common, they all had brown eyes. Just like melanin in skin cells protects against the sun's damaging UV rays, melanin in our eyes keeps harmful rays from causing long-term damage like cataracts or rare types of eye cancer. As humans migrated and settled in climates with less harsh sun exposure, they required less melanin and human genetics adapted. Though the most common eye color is still brown across the world, it explains why people with European ancestors tend to have lighter hair, skin and eye colors. Interestingly, all melanin is brown. The more melanin an eye has, the darker it will appear. In eyes with less melanin, like blue or green, less light is absorbed and it scatters around the eye in a phenomenon called the Tyndall effect. It's similar to why the sky appears blue.