The color of human eyes exhibits a remarkable spectrum of hues, from the deepest brown to the lightest blue. Hazel eyes stand out as a visually complex phenotype, often described as a multi-toned combination of green, gold, and brown. This captivating color is relatively uncommon globally, making up approximately 5% of the world’s population. Understanding the origin of hazel eyes requires examining the specific physics of color perception and the underlying biological switches that govern pigment production.
Defining Hazel Eye Color
Hazel eye color is not produced by a specific hazel pigment but instead arises from a moderate concentration of the dark brown pigment melanin within the iris. The color is positioned on the continuum between the high melanin levels found in brown eyes and the very low levels present in blue eyes. This intermediate amount of pigment is distributed unevenly across the two layers of the iris, creating the depth and variation characteristic of the hue.
The unique appearance of hazel is heavily influenced by how light interacts with the iris structure, a physical phenomenon known as Rayleigh scattering. When light enters an eye with a low to moderate amount of melanin, the shorter, blue-green wavelengths scatter more readily. This scattering, combined with the small amount of yellow-brown pigment present, produces the shifting, greenish-gold appearance that defines the color. Hazel eyes are distinguished by their variability, often appearing to change color depending on the lighting or surroundings.
The Genetic Mechanism Behind Light Eyes
The formation of intermediate eye colors like hazel is directed by a complex interplay of multiple genes, making it a polygenic trait. This complex inheritance pattern refutes the older, simplified model that suggested eye color was determined by a single dominant or recessive gene. At least 16 different genes contribute to eye color variation, but the most significant control center is the locus on chromosome 15 involving the $OCA2$ and $HERC2$ genes.
The $OCA2$ gene provides instructions for creating the P protein, which plays a role in the maturation of melanosomes, the cellular structures that produce and store melanin. The amount of functional P protein directly influences the concentration of melanin deposited in the iris, with less protein resulting in lighter eye colors. A nearby gene, $HERC2$, acts as a regulator for $OCA2$.
A specific sequence variation within $HERC2$ functions like an on/off switch, significantly reducing the expression of the $OCA2$ gene. This reduction in $OCA2$ activity is responsible for the overall decrease in melanin that leads to light-colored eyes, including blue and green. Hazel color often results from a different, less severe variation in this same genetic region, such as a specific single nucleotide polymorphism (rs1800407) found within the $OCA2$ gene itself. This variation is associated with the moderate melanin levels that produce green and hazel eyes.
The Geographical Genesis of the Light Eye Mutation
The story of hazel eyes is intertwined with the emergence of all light eye colors from the original, universally present brown eyes of early humans. Scientific evidence suggests that the mutation responsible for the entire spectrum of light eyes is relatively recent, occurring between 6,000 and 10,000 years ago. This initial, single mutation occurred in the regulatory region of the $HERC2$ gene, significantly reducing $OCA2$ expression and effectively “diluting” the melanin to produce the first blue eyes.
This foundational light-eye mutation is theorized to have originated in a single common ancestor in the northwestern Black Sea region of Europe. The subsequent spread of this trait is attributed to a founder effect, where the trait rapidly became common in an isolated population. As this blue-eye gene spread and mixed with populations retaining brown-eye genes, the partial expression of the light-eye trait gave rise to intermediate colors. Hazel eyes require a moderate amount of melanin and are a product of this genetic admixture, developing later as the initial blue-eye mutation was introduced into a broader population base.
Current Prevalence and Global Distribution
Today, hazel eyes maintain their status as a less common eye color, representing roughly 5% of the global population. The highest concentrations of people with hazel eyes are found in regions where populations with the light-eye genetic heritage have historically blended with populations possessing a heritage of darker eyes. This includes parts of Europe, particularly Western and Southern Europe, and the Middle East and North Africa.
The prevalence of hazel eyes in the United States, estimated to be around 18%, is notably higher than the global average. This figure reflects the high degree of genetic diversity and significant historical immigration from European, Mediterranean, and Middle Eastern populations. The geographical distribution of hazel eyes serves as a map of human migration, marking the zones where light-eye genes spread and intermingled with established, darker-eyed populations.