Eyes are said to be the windows to the soul, but they’re also a glimpse at humanity’s genetic past. Scientists estimate that between 6,000 and 10,000 years ago, the eye color of all Homo sapiens was brown — likely an evolutionary advantage, as the melanin pigment offers some protection from UV radiation. But then, something changed. Sometime during the Neolithic expansion in Europe, an individual was born with a mutation to the OCA2 gene. This gene code controls melanin production in the iris, and the mutation caused this person’s eyes to turn blue rather than the usual brown. Because blue eyes can only form as a result of this mutation, scientists theorize that all blue-eyed people — about 10% of the world population — are a relative of this original lone blue-eyed ancestor.
Strangely, this mutation doesn’t actually turn your eyes blue — in fact, blue eyes are technically not blue at all. The eye’s iris is predominantly made up of two layers: the stroma and the epithelium. Brown eyes have a brown-black melanin pigment in both these layers (though the stroma absorbs the most light), which produces the color brown. Blue eyes, on the other hand, have no melanin pigment in the stroma; in fact, blue eyes have no pigment at all. Instead, they are a reflection of white light in a process called the Tyndall effect. Because blue wavelengths of light are the shortest, they are reflected the most by the fibers in the eyes, which absorb the longer red-orange wavelengths. This bit of complicated optics is similar to how the atmosphere reflects sunlight, turning the sky (and the ocean) a dazzling blue. So while the overall effect is that people have “blue” eyes, from a pigment perspective, the truth is that they really don’t have any color at all.
Humans are the least genetically diverse among the great apes. This means that we’re a relatively young species, as enough time hasn’t passed for mutations to accumulate (200,000 years is a geologic blink of an eye). It also means that Homo sapiens likely sprung from a surprisingly small population — around just 10,000 breeding pairs or so. This is likely because early humans appear to have survived two genetic bottlenecks while exiting Africa, both of which caused the population to plummet. One theory suggests humans almost went extinct 74,000 years ago due to a massive volcanic eruption, but other studies question if that “eruption” was actually an epidemic. Usually, this low diversity can make it tougher for animals to adapt to climatic changes. Fortunately, what humans lack in genetic brawn, we make up for with our incredibly complex brains.