When you notice someone's eye colour, 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 colour to our hair, eyes and skin. Depending on the amount of melanin in the iris, it can be shades of grey, green, hazel, amber, brown or even red, and everyone's eye colour is unique.

Eye colour isn't just for aesthetics. Like most of your biology, eye colour has a purpose. Thousands of years ago, the first humans had something in common, they all had brown eyes. Just like melanin in skin cell 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 colour is still brown across the world, it explains why people with European ancestors tend to have lighter hair, skin and eye colours.

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.