Human eye colors, from deep browns to light blues, result from genetic inheritance and biological processes. Some are widespread, but rare hues offer insights into human biology and ancestry.
The Rarity of Green Eyes
Green eyes are one of the least common eye colors globally. Only about 2% of the world’s population possesses green eyes. This contrasts significantly with brown eyes, which are the most prevalent, accounting for 70% to 80% of people worldwide. Blue eyes are the second most common, found in approximately 8% to 10% of the global population. The scarcity of green eyes stems from the specific genetic combination required for their appearance, which involves a moderate amount of melanin, a pigment also responsible for skin and hair color.
The Science Behind Green Eye Color
The green appearance of eyes does not come from a green pigment within the iris, as human eyes do not contain blue or green pigments. Instead, eye color is a structural phenomenon determined by the amount and type of melanin in the iris, along with how light scatters within it.
Green eyes have a moderate concentration of eumelanin, a brown-black pigment, in the iris’s front layer (stroma), which is more than blue eyes but less than brown eyes. They also contain a yellowish pigment called lipochrome.
The perceived green color results from a combination of these elements. Light entering the eye scatters off the low melanin and yellowish pigment, a process similar to Rayleigh scattering. The scattered blue light then mixes with the iris’s yellowish hue, creating the green coloration.
Eye color inheritance is complex, involving multiple genes rather than a simple dominant or recessive pattern. Genes like OCA2 and HERC2, located on chromosome 15, play significant roles in regulating melanin production and distribution.
Geographic Distribution of Green Eyes
Green eyes are most frequently observed in specific regions, particularly across Northern and Central Europe. Countries like Ireland, Scotland, and Iceland exhibit a higher prevalence of green-eyed individuals compared to the global average. In Ireland and Scotland, a significant portion of the population, over 75%, has either blue or green eyes. This concentrated distribution is often linked to historical migrations and genetic lineage.
While less common elsewhere, green eyes can be found outside of Europe. For example, the village of Liqian in China is noted for a higher-than-usual incidence of green eyes, which some theories attribute to potential historical Roman ancestry. The prevalence of green eyes in certain populations can also be influenced by the “founder effect,” where isolated groups carrying genes for lighter eyes establish a population, leading to a higher frequency of these traits over time due to limited genetic diversity.