If you were to make eye contact with a chimpanzee, you’d likely notice something uncanny about them very quickly: you cannot easily tell where they’re looking. The tissue that surrounds their iris, known as the sclera, is dark brown or nearly black, which makes their gaze almost impossible to track.
Contrastingly, if you were to make eye contact with a friend or family member, you’d have no trouble instantly discerning the direction of their attention. Because human sclerae are bright white, our gaze is involuntarily legible; others can always tell, within a fraction of a second, exactly what we are focused on.
This, evolutionary biologists argue, is the product of hundreds of thousands of years of natural selection, which has sculpted the human eye into a highly precise social signaling instrument. Because of this selection, we are the only primates on Earth with uniformly white sclera. The larger question scientists have spent decades wrestling with is why.
The Cooperative Eye Hypothesis
The leading explanation for this unique exception comes from a theory known as the Cooperative Eye Hypothesis.
The hypothesis was initially formalized by Michael Tomasello and colleagues at the Max Planck Institute for Evolutionary Anthropology. Broadly, it proposes that human white sclera evolved specifically to make gaze direction visible to other humans. In turn, this would have enabled the kind of tight, wordless coordination that underpins almost all of our important social interactions, from raising children communally to building cities.
The original research, published in 2007 in the Journal of Human Evolution, tested this by presenting human infants and great apes with an experimenter who looked toward the ceiling using only his eyes, only his head, or both. Consistently, the researchers found that great apes followed the head. Human infants, on the other hand, followed the eyes almost exclusively.
From this finding, the authors concluded that humans had likely evolved a special sensitivity to eye direction — and, more importantly, that the white sclera was the anatomical feature that made this sensitivity possible.
The logic behind the hypothesis has ample intuitive appeal: if you want to be able to follow someone’s gaze, then you need contrast of some kind. A dark iris on a white background broadcasts direction like a compass needle; a dark iris on a dark background tells you almost nothing.
The Advantage Of Having Gaze-Signaling Eyes
The primary question that stems from this hypothesis centers on whether or not scleral color is associated with cooperation across species. A 2022 study published in Scientific Reports provided the first rigorous quantitative test of this question. To do so, researchers analyzed the scleral pigmentation of 108 primate species against measures of prosociality, social tolerance and lethal aggression.
The researchers found, unambiguously, that primates with brighter sclerae (such as some chimpanzees and bonobos) displayed increased cooperative behaviors. Species with dark sclerae, on the other hand, exhibited both reduced cooperation and higher rates of lethal violence between members of the same species.
In other words, the whites of your eyes seem to function, on a deep evolutionary timescale, as a peace signal. They advertise that we live in a world where making our attention known to others is safe — because those others are our allies, not enemies.
Some might still wonder how exactly, in a mechanical sense, the whites of our eyes help signal prosociality. In a 2022 study published in eLife, researchers showed images of human and chimpanzee eyes to both humans and chimpanzees under varying conditions — different brightness levels, different distances — in order to simulate real-world “visual noise.”
Both species were able to discriminate human gaze direction more accurately than chimpanzee gaze direction, particularly when viewing conditions were poor. However, when chimpanzee eyes were digitally altered to have white sclera, gaze discrimination immediately improved for both species.
Overall, the researchers concluded that, beyond visibility alone, uniformly white sclera makes gaze far more robust; it’s resistant to degradation by shadows, distance and distractions. These findings suggest that our eyes are engineered to be readable even in a busy, dimly lit savanna.
The Complication: Chimps Have White Eyes Too (Sort Of)
Science very often resists clean stories; the story of human white sclera is one of them. More specifically, the idea that humans are the only primates that possess white sclera has recently become considerably more complicated.
As explained in a 2025 study published in Biological Reviews, white sclerae occur far more commonly in chimpanzees and other mammals than we previously believed. The researchers arguing that primates actually display a wide, gradual spectrum of scleral pigmentation. Notably, it posits that human sclerae don’t stand out as sharply as the cooperative eye hypothesis assumes.
On top of this, the review also raises the possibility that sexual selection played a role in scleral brightness, as this is something that noticeably declines with age across primates. From this perspective, sclera whiteness, or even just brightness, may potentially function as a health and fertility cue.
Meanwhile, the self-domestication hypothesis offers a complementary angle. It proposes that white sclera emerged as a by-product of selection against aggression in human evolution. That is, as we became less reactive and more socially tolerant, our neural crest cell changes resulted in reduced pigment throughout the body, including, but not limited to, the sclera.
This is something seen in other mammals, too. When you breed for tameness, you consistently get a cluster of correlated traits:
These are all changes that can be traced back to alterations in neural crest cell development: the embryonic cells that give rise to both pigment-producing melanocytes and stress-reactive adrenal tissue. Reduce the neural crest cells, reduce the aggression response — but also, as a side effect, reduce melanin production in the sclera.
From this perspective, the whites of our eyes likely were not the primary target of selection at all. Instead, this brightening would have been a collateral consequence of humans becoming kinder to each other.
A Window Into Our Minds’ Eyes
What makes the white sclera story so compelling is that, despite the surface-level appearance of these arguments and hypotheses, it actually isn’t about our eyes at all. Rather, it’s about what eyes can do in a social species and, by extension, what happens when evolution finds a way to make the mind’s attention visible and legible to others.
Babies learn how to follow eye gaze from birth. We can detect direct gaze in milliseconds. We use eye contact to coordinate language learning, synchronize joint tasks and signal trust. For a species that managed to build civilizations through cooperation, the ability to cheaply and accurately broadcast your attentional state to those around you was probably one of the most consequential evolutionary innovations of all.
So, the next time someone says “look me in the eye,” remember: they’re invoking an evolutionary covenant. You are part of a species that evolved to read and be read.
The story of the human eye is a story about connection. See how much of that connection extends beyond other people. Take the science-backed Connectedness to Nature Scale to uncover your relationship with the natural world around you.