The human eye comes in a variety of colors, from brown, hazel, green, gray, blue and amber. This range of eye colors is unusual compared to most mammals which typically only have brown or dark colored eyes. So why did humans evolve to have such diversity in eye pigmentation?
The genetics behind eye color
Eye color is determined by the amount of melanin pigment in the iris of the eye. The melanin is produced by melanocytes, specialized cells in the iris. The more melanin present, the darker the eye color. Brown is the most common eye color worldwide likely because having a large amount of melanin pigment confers some advantages such as UV light protection.
The main gene responsible for eye color is the OCA2 gene located on chromosome 15. Different versions (alleles) of this gene result in varying amounts of melanin production and correspond to different eye colors:
- The allele that produces the most melanin results in brown eyes.
- Decreased melanin production results in lighter colors like green and hazel.
- Mutations that significantly limit melanin result in shades of blue.
- A total loss of melanin pigment results in gray or very light blue colors.
So in summary, eye color is determined by different alleles coding for varying levels of melanin in the iris.
The evolution of eye color diversity
So when did the different eye colors besides brown evolve in humans? And how did eye color diversity come about?
The current consensus is that brown eye color was the original eye color of early humans. This is because most humans indigenous to Africa, Asia, and the Americas have brown eyes. Over time as some populations migrated out of Africa into Europe, mutations in the OCA2 gene reduced the melanin content, giving rise to lighter eye colors like green, hazel, and blue.
One theory for how this happened is that lighter eye colors evolved due to sexual selection. As scarcer traits, rare light eye colors were considered attractive and people with those traits were preferred mates. So over time, alleles for light eyes became more common in European and Western Eurasian populations. However, the actual evolutionary mechanism is still debated.
Regardless of how it evolved, today the prevalence of lighter eye colors correlates strongly with latitude and UV light exposure. Populations around the equator have predominantly brown eyes, while light eyes become more common at higher latitudes. This gradient is thought to be due to sexual and natural selection balancing the benefits and risks of decreased melanin and lighter eye color.
Advantages of eye color variation
While the origins are unclear, today we know that eye color variation does confer some advantages to humans:
- UV protection – Brown eyes with more melanin protect against UV damage but reduce visibility in low light conditions.
- Light absorption – Lighter eyes with less melanin absorb more light to increase visibility in low light European environments.
- Social bonding – Unique eye colors may have facilitated social bonding as a way to identify kinship and family relations.
So in summary, the diversity in eye colors helped adapt early human populations to new environments in Europe while also serving possible social purposes.
Geographic distribution of eye colors
Today, eye color distribution varies significantly around the world as shown in this table:
| Continent/Region | Most common eye color |
|---|---|
| Africa | Brown |
| Asia | Brown |
| Europe | Blue/Green |
| Americas | Brown |
In general, brown eyes predominate in indigenous populations close to the equator while light eyes become more common at higher latitudes. However, even in Northern Europe, brown eyes still account for over half of the population. Blue eyes in particular are a rare trait globally found in only about 8% of people.
Changes in eye color with age
While eye color is mostly determined at birth, there can be subtle changes as people age. Two main things happen:
- Infants – Melanin production continues in babies for 1-2 years so dark eyes can get darker.
- Elderly – Iris pigment breaks down so light eyes can darken slightly.
However, the basic eye color doesn’t drastically change after childhood except under certain conditions like disease, trauma, or medications.
Genetic diseases linked to eye color
Certain genetic conditions are also associated with typical eye colors:
- Albinism – Lack of melanin results in very light blue/gray eyes.
- Heterochromia – Different eye colors within an individual due to uneven melanin.
- Ocular albinism – Low melanin in eyes only, not skin and hair.
So while eye color diversity is considered normal in humans, in some cases it can indicate an underlying medical condition.
Conclusion
In summary, eye color diversity in humans evolved relatively recently in our history, after early humans migrated out of Africa into Europe. Mutations lowering melanin levels gave rise to lighter eye colors that were advantageous in northern latitude environments. Today, while brown eyes are still the global majority, light eye colors persist in European and Western Eurasian populations at higher frequencies. The variation in eye color continues to enhance human adaptability while also serving social purposes.