For most of prehistory, there was no variation in eye colour; everyone had brown eyes. And most of the world's population still does.
Around 10,000 years ago, almost certainly around the Baltic Sea, a variant in the eye colour of a single individual appeared; this was the first person to have blue eyes.
And all those who have blue eyes are descended from that person.
In only a few thousand years, the most common eye colour in Northern Europe flipped from brown to blue.