Of all the organs of the human body, the eye is especially attractive. Seeming to be alive as it moves back and forth in the eye socket, the eye allows the sighted to interact with the physical world and allows us – in a superficial but meaningful way – to peer into the minds of others.
A distinctive feature of the human eye is color mixing. The iris surrounding the pupil can be blue, green, gray, hazel, brown, and even red.
Differences in melanin pigment levels primarily explain the different shades – more melanin makes the eyes darker, and less leaves eyes that reflect blue.
How much melanin is in the iris depends on the expression of about a dozen different genes, and possibly more. The two most important are definitely OCA2 and HERC2. OCA2 produce a protein that controls the maturation of melanosomes that produce melanin. HERC2 controls the expression of OCA2.
When humans originated in the Horn of Africa at least a quarter of a million years ago, human eyes were dark brown or nearly black.
This is because OCA2 was expressed at a high level, which in turn resulted in the production of more melanin, which stained the skin dark brown and, as a side effect, darkened the iris.
Brown skin is less likely to get sunburn or skin cancer, which is good for people in the sunny equatorial climate of Central Africa.
But when humans started migrating out of Africa between 50,000 and 70,000 years ago, the selection pressure that led to increased melanin production disappeared.
In Northern Europe, where sunlight can be a scarce commodity in winter, lighter skin tones have become preferred as they allow more vitamin D to be absorbed from sunlight.
This meant that there was less melanin in the body, which allowed for diversification of eye color, as other genes that more subtly influenced eye color mutated and their influence became more evident.
Blue eyes, for example, are extremely common in northern Europe after they gained prominence about 7,000 years ago.
Blue eyes may help regulate circadian rhythms, Rick Sturm, associate professor of molecular genetics at the University of Queensland, told ABC Science. This makes them especially useful in high latitudes, where the length of sunlight is highly dependent on the time of year.
Now that humans have multiplied and spread across the globe, new eye colors could possibly evolve as genes mutate and people of different backgrounds mix their genes through reproduction.
A rich rainbow of human eye color can be even more varied!