When you’re next staring deep into the eyes of your partner, the moment may soon be ruined by the knowledge that, regardless of whether these windows to their soul appear piercingly blue or a shimmering green, the reality is that they are brown. That’s right. All human eyes are brown. As the owner of a sparkling set of deep brown eyes, I see no disappointment in the knowledge that all human eyes are in fact a wonderful shade of brown, but for anyone feeling misled or confused, a mix of biology and physics should help explain this reality. It all comes down to the presence of the pigment melanin, also found in skin and hair, within your eye’s iris — the colored part that surrounds the pupil. “Everyone has melanin in the iris of their eye, and the amount that they have determines their eye color,” said Dr. Gary Heiting, a licensed optometrist and senior editor of the eye care website All About Vision. “There’s really only (this) one type of pigment.”

Seeing blue through the brown

Melanin — made up of melanocyte cells — is naturally dark brown in color but has the ability to absorb different amounts of light, depending on how much of it there is. The more melanin inside the iris, the more light is absorbed, meaning less light is reflected out, leaving the iris appearing brown.

But when someone has blue eyes, they have less melanin in their iris, resulting in less light being absorbed and more light reflecting, or scattering, back out. When this light is scattered, it reflects at shorter wavelengths along the blue end of the light color spectrum — leaving you seeing blue.

Green and hazel eyes are somewhere in the middle, with differing quantities of melanin resulting in different levels of light absorption and therefore different colors reflecting out. Hazel is considered a mixture of eye colors, according to Heiting.

Different light settings can also make some eyes appear to change color depending on where the person is standing.

A baby’s eyes can appear blue while their melanin is still forming.

“It’s an interaction between the amount of melanin and the architecture of the iris itself,” added Heiting. “It’s a very complex architecture.” This part of the eye is therefore unique to most individuals and can act as something like a fingerprint, due to the existence of various textures and patterns.

Blue eyes have the least amount of pigment of all eye colors. When babies are born, their eyes may sometimes appear blue early on, while their melanin is still forming. Their eye color may then darken as they develop.

“As a baby develops, more melanin accumulates in the iris,” said Heiting.

Evolution in play

Like skin color, one theory behind the evolution of eye color is the migration of our early ancestors toward cooler parts of the world. While high levels of melanin — in eyes, hair, and skin — help protect people in hotter climates, like Africa, from UV radiation, the need for the protective pigment decreases as people move to locations with less sun. “There was less need for all that melanin,” Heiting said.

Another theory conceived by professor Hans Eiberg at the University of Copenhagen was that a mutation once switched off the ability of someone’s eye to produce melanin. This would lead to light eyes in the affected individual; their rarity may have made them more attractive and aided their natural selection within the population. In one study, he analyzed genes for eye color and identified what he believed to be a common mutation causing blue eye color.

“It’s believed that’s how blue eyes came about, but it may just be the de-emphasis on the need for all the melanin,” Heiting said.

Determining color

It’s long been believed that if someone has brown eyes — or what appear to be brown eyes — their chances of having a child with lighter eyes are slim. Following suit is the theory that two people with blue eyes will automatically have a child with blue eyes due to the gene being recessive, rather than dominant.

But this is also not quite true. “It’s pretty well accepted now that eye color is a polygenic trait,” Heiting said, meaning multiple genes are involved. In fact, up to 16 genes are thought to play a role in the amount of melanin in someone’s eye, implying that the eye color trait does not follow the traditional rules of inheritance.

So if your deep, dark eyes are pining for a child with light, sparkling eyes, hope may not be lost. “Several genes have an influence on eye color,” Heitling said. “It’s not something you can predict with ease.”

Other options are colored contacts or laser surgery to change how light is reflected from your eye, but while you mull over the realization that eyes are not what you once thought they were, one thing is for sure: You’ll never look someone in the eye the same way again.