In humans, brown eyes result from a relatively high concentration of melanin in the stroma of the iris, which causes light of both shorter and longer wavelengths to be absorbed.
Dark brown eyes are dominant in humans and in many parts of the world, it is nearly the only iris color present. Dark pigment of brown eyes is common in Europe, South Europe, East Asia, Southeast Asia, Central Asia, South Asia, West Asia, Oceania, Africa, Americas, etc. as well as parts of Eastern Europe and Southern Europe. The majority of people in the world overall have brown eyes to dark brown eyes.
Light or medium-pigmented brown eyes can also be commonly found in South Europe, among the Americas, and parts of Central Asia (Middle East and South Asia).
From Wikipedia: Brown Eyes