Asked by: Anonymous
The function of the iris is to limit the amount of light that passes through the lens to the retina. To make the iris opaque, it is coated with the pigment melanin, in varying degrees for each of us.
Melanin makes the eyes dark brown and 10,000 years ago, everyone’s eyes were this colour. Then a mutation turned off the pigmentation on the front of the iris.
This allows light to reach the fibres of the stromal cells beneath, and they reflect light back as blue. Green, grey and olive eyes are just half-way colours.
Read more:
- Why do our eyes come in different colours?
- What are the wiggly things I see in my eyes when I look at the sky?
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.