Apparently, blue pigment is incredibly hard to produce in nature. Second best option is to trick our eyes with texture. What is called structural coloration.
Here is how it works: the iris has a couple of layers, some with pigment, some without. In between, there is a spongy layer called stroma.
In people with blue eyes, no layer has pigment except for the back layer with brown pigment. The stroma, although without pigment, has small particles suspended in it, and all the light that enters is scattered back, and as a result of the Tyndall effect, creates a blue hue.
Blue eyes have one single ancestor.
Between 6,000–10,000 years ago, a genetic mutation in one single European ancestor, caused the blue eye colour on all humans alive on the planet today. I guess that one guy was not killed by a mountain lion or something.
Green eyes are more uncommon than blue eyes.
Around 8% of the world’s population has blue eyes. (That ancestor was very successful!)
But less than 2% have green eyes. Ha! Not at all the impression I used to have. It’s like when you discover that diamonds are not that unique, but rather carefully doled to create that impression.