Ancient Humans' Eyes Were Nearly Black. Future Humans Could Have All New Colors
Of all human bodily organs, the eye is especially alluring. Seemingly alive as it twitches to and fro within the socket, the eye permits sighted interaction with the physical world and allows us – in a superficial, yet meaningful way – to gaze into the minds of others.
The human eye's hallmark trait is its medley of colors. The iris, which surrounds the pupil, can appear blue, green, gray, hazel, brown, and even red. Differences in levels of the pigment melanin primarily account for the varying hues – more melanin renders the eyes darker, while less leaves eyes reflecting light blue. How much melanin dwells within the iris depends on the expression of around a dozen different genes, and perhaps more. The two most important by far are OCA2 and HERC2. OCA2 produces a protein that controls the maturation of melanin-producing melanosomes. HERC2 controls the expression of OCA2.
When humans arose in the horn of Africa at least a quarter of a million years ago, human eyes were extremely dark brown or nearly black. That's because OCA2 was expressed at high levels, in turn leading to the production of more melanin, which colored skin dark brown and, as a side effect, darkened irises. Brown skin is less likely to be sunburned or to develop skin cancer, benefits which served humans well in Central Africa's sunny, equatorial climate.
But when humans started migrating out of Africa between 50,000 and 70,000 years ago, the selective pressures that drove heightened melanin production disappeared. In Northern Europe, where sunlight can be a scarce commodity in winter, lighter skin tones became advantageous, as they allow for more vitamin D absorption from sunlight. This meant less melanin in the body, which permitted eye color to diversify as other genes that more subtly affected eye color mutated, their influence becoming more apparent.
Blue eyes, for example, are extremely common in northern Europe after rising to prominence roughly 7,000 years ago. Besides evoking chilling thoughts of white walkers from Game of Thrones, blue eyes may actually help regulate circadian rhythms, molecular geneticist Associate Professor Rick Sturm of the University of Queensland told ABC Science. This makes them particularly useful in higher latitudes, where hours of sunlight differ drastically with the seasons.
Now that humans have multiplied and spread across the globe, new eye colors could possibly evolve as genes mutate and humans of diverse backgrounds blend their genes via reproduction. The rich rainbow of human eye color could grow even more diverse!