Green eyes are a unique human trait, found in a small percentage of the global population. Their appearance results from a complex interplay of genetics, pigment composition, and how light interacts with the eye. Understanding when and how green eyes first appeared offers insights into human migration patterns and genetic evolution.
How Eye Color Works
The color of human eyes is primarily determined by the amount and type of melanin present in the iris. Two main types of melanin contribute to eye color: eumelanin (brown and black tones) and pheomelanin (yellow and red hues). The iris also contains a fibrous tissue called the stroma, which scatters light.
Green eyes result from a specific combination of these factors. They contain a low to moderate amount of eumelanin, less than brown eyes but more than blue eyes. The presence of yellowish pheomelanin, combined with light scattering by the collagen fibers in the stroma, creates the green appearance. Rayleigh scattering causes shorter blue wavelengths of light to reflect, mixing with the yellow pigment to produce the perception of green.
The Genetics of Green Eyes
Eye color inheritance is a complex polygenic trait, influenced by multiple genes. Two genes, OCA2 and HERC2, located on chromosome 15, are the most significant determinants. The OCA2 gene produces the P protein, essential for melanin production within the iris. The HERC2 gene, positioned close to OCA2, acts as a regulatory switch, influencing OCA2 expression.
Variations within these genes can lead to reduced melanin production, resulting in lighter eye colors like green or blue. Other genes, such as TYR, SLC24A4, and TYRP1, also play roles by affecting melanin production, transport, or storage. The interactions among these genes explain why eye color can vary, even within families.
The Ancient Roots of Green Eyes
Before lighter eye colors emerged, all humans had brown eyes, an adaptation providing protection from intense UV radiation near the equator. The mutation leading to lighter eye colors, including green, is estimated to have occurred between 6,000 and 10,000 years ago. Early evidence suggests green eyes were present in southern Siberia during the Bronze Age, over 4,000 years ago.
Researchers propose the earliest green-eyed individuals originated in or around the Caucasus Mountains, a region serving as a natural land bridge between Asia and Europe. The spread of green eyes is linked to early Indo-European expansions, as migrating populations carried these genetic variations across Europe, Iran, and India.
The persistence of this trait was influenced by a lack of selective disadvantage in higher latitudes with reduced ultraviolet exposure. Sexual selection also played a role, as rare or visually striking traits can be considered attractive, increasing their likelihood of being passed down.
Where Green Eyes Are Found Today
Green eyes are among the rarest eye colors globally, present in only about 2% of the world’s population. Brown eyes remain the most prevalent, accounting for 70-79% of individuals worldwide.
The highest concentrations of green eyes are found in parts of Europe, particularly in Northern, Western, and Central regions. Countries such as Iceland, Ireland, Scotland, and the Netherlands report a higher incidence of green eyes. In Iceland, a proportion of women, around 18-21%, have green eyes.
While most common in populations of European descent, green eyes can also be found in isolated groups in the Middle East, Central Asia, and specific villages in northwestern China, reflecting historical migrations and genetic mixing.