Scientists challenge theory that human eye evolution was driven by cooperation

A widely popular idea in psychology and evolutionary science suggests that the distinctive look of the human eye—particularly the white area surrounding the iris—evolved to help us communicate cooperatively. But a new scientific paper published in Biological Reviews challenges that theory, arguing that the idea lacks strong evidence and may have misrepresented both human and non-human primate eye variation. The authors propose that other evolutionary pressures, like light protection and sexual selection, may better explain the diversity in primate eye pigmentation, including our own.

The idea under scrutiny is known as the Cooperative Eye Hypothesis. This hypothesis suggests that humans evolved conspicuously white eyes to make it easier for others to follow our gaze. According to the theory, this trait supports uniquely human social behaviors such as collaboration, joint attention, and non-verbal communication. In contrast, other primates are thought to have darker, less visible eyes that obscure their gaze to avoid giving away intentions in competitive situations.

This idea has gained considerable popularity both in academia and in popular science. It’s been cited in thousands of scholarly works and widely shared in books and online media. But according to the authors of the new review, the hypothesis rests on flawed assumptions and has not held up under closer empirical scrutiny. They argue that the supposed uniqueness and uniformity of the human eye appearance is exaggerated and that the available evidence does not convincingly show that our bright eyes significantly improve communication compared to other primates.

“We have been trying to understand what drives variation in the appearance of eyes in primates for a few years,” said the lead author of the new paper, Juan Olvido Perea-García of Nicolaus Copernicus University and Universidad Las Palmas de Gran Canaria.

“A key idea when we started working on the topic was that human eyes were unique because they were the only ones with ‘white’ of the eyes. Supposedly, this evolved uniquely in humans to support also supposedly uniquely human communicative behaviors. But more and more evidence has emerged that other primates do actually have white of the eyes, it’s just that it hadn’t been looked into in enough detail.”

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“In response to this, other teams tried to ‘save’ the uniqueness of the human eye by proposing that what really distinguished human eyes from those of other primates was the degree and how homogeneously depigmented human eyes were. But this is simply and visibly not true if we look at more pigmented individuals. We wanted to bring attention to the existing variation in human external eye appearance to set the record straight.”

First proposed in the late 1990s and popularized by researchers like Michael Tomasello, the Cooperative Eye Hypothesis suggests that human eyes evolved their distinctive appearance under pressures for improved social cooperation. The key feature is the bright white area around the iris, created by the sclera and conjunctiva, which is thought to make it easier to detect eye direction. This, the theory goes, allowed humans to communicate silently and efficiently through subtle shifts in gaze, laying the groundwork for complex group behaviors and even language development.

Early support for this idea came from studies comparing eye morphology across primate species. These studies claimed that only humans show such strong contrast between the iris and surrounding eye tissue, making our gaze uniquely visible. Based on these findings, it was proposed that our ancestors experienced evolutionary pressure to evolve highly visible eyes that promote cooperative behavior, while other primates retained darker, less expressive eyes to hide their intentions.

Despite its influence, the Cooperative Eye Hypothesis has several major weaknesses, according to the new review. The authors argue that the four central claims of the hypothesis—uniqueness, uniformity, communicative advantage, and empirical support—are either false or unproven.

First, human eyes are not as unique as previously claimed. Many non-human primates, such as golden langurs and pig-tailed macaques, have similarly bright peri-iridal regions, the area surrounding the iris. In fact, some species show comparable levels of pigmentation brightness, suggesting that bright eyes have evolved multiple times across the primate family tree. Even chimpanzees, often cited as having dark, “hidden” eyes, can exhibit scleral brightness similar to humans.

Second, the idea that human eye appearance is uniform across the species is incorrect. In reality, there is substantial variation in the brightness of the sclera among different human populations. Many people, especially those with darker skin tones, show visibly pigmented conjunctiva, contradicting the idea that all humans have fully white eyes. This variation has been observed for decades in medical literature but has largely been ignored in psychological studies supporting the Cooperative Eye Hypothesis.

Third, there is little evidence that bright eyes offer a substantial communicative advantage. Experiments suggest that even with darker eyes, humans and other primates can still follow gaze accurately, especially in familiar or well-lit contexts. Some researchers have shown that the time it takes for humans to interpret gaze direction is only slightly longer when the sclera is darkened artificially—and the accuracy remains unchanged. Studies also show that chimpanzees, under the right conditions, can follow gaze just as effectively.

Finally, the experimental foundation of the hypothesis is shaky. A key study by Tomasello and colleagues in 2007 claimed that human infants follow gaze more reliably than great apes, supposedly due to our distinct eye morphology. But the new review highlights serious methodological concerns with that study, including differences in testing environments and unfamiliarity of subjects with the experimental setup. The authors argue that these differences could explain the observed results, not inherent cognitive or anatomical differences.

“Human eyes are varied in their coloration and shape, and we do not know why this is the case yet,” Perea-García told PsyPost. “We know they are used in communication, but we do not know whether we got ‘eye whites’ for that. This is counter-intuitive, because we know we follow gaze. But just because we do something with a part of our bodies, it doesn’t mean it evolved for that function. For example, we use fingers to type on the keyboard, but our fingers obviously did not evolve for that purpose.”

“Science is always moving and we should remain skeptical of what appears obvious. We don’t doubt that the scientists who say all humans have homogeneously depigmented ‘whites’ of the eye believe this to be the case. But, due to their origins in Northern Europe or Northeast Asia, they probably have not been exposed to deeply pigmented individuals. Scientists are humans, and humans are not omniscient.”

Given the limitations of the Cooperative Eye Hypothesis, the authors propose that researchers explore alternative explanations for the evolution of eye appearance in humans and other primates. Three promising ideas are photoregulation, sexual selection, and non-referential communication.

The photoregulation hypothesis suggests that darker eye pigmentation protects the eye from harmful ultraviolet light, which may explain why some primates in equatorial regions have more heavily pigmented eyes. In this view, humans may have reduced eye pigmentation in part due to lower UV exposure in some ancestral environments—or due to other anatomical adaptations that made dark eyes less necessary.

Another possibility is that depigmented sclera evolved under sexual selection. In humans, bright eyes are often associated with youth and health, traits that are commonly preferred in mate selection. This could have led to a preference for lighter sclera over time, especially if they signaled good health or reproductive potential. Interestingly, human faces also display other juvenile features compared to other apes, possibly supporting this idea.

A third option is that visible eyes may enhance facial expressions or social signals more generally, without necessarily improving gaze detection. In many primates, the eye region is marked with bright skin or fur, drawing attention to the eyes as a part of emotional or affiliative communication. In this context, bright sclera might help amplify social cues like eye contact or expressions of emotion, rather than helping others follow gaze direction specifically.

Other explanations have also been considered, including random genetic drift, self-domestication, and even species recognition. While some of these ideas remain speculative, they illustrate that there are many ways to think about how eye appearance might have evolved—beyond cooperation alone.

Despite the progress made in understanding eye evolution, much remains unknown. One major limitation is the lack of detailed measurements of conjunctival and scleral pigmentation in both humans and other primates. Surprisingly, there is still no comprehensive dataset that maps how pigmentation varies across human populations or across different primate species.

“We are still lacking detailed measurements of pigmentation in the human eye, believe it or not,” Perea-García said. “Until then, it is all educated guesses.”

The authors suggest that new technologies and genetic methods could help answer key questions—such as when in evolutionary history humans developed lighter sclera and whether these changes happened gradually or suddenly. Investigating the genetics of pigmentation in the eye, and comparing it to other traits like skin or hair color, could offer deeper insights into the evolutionary forces at play.

They also encourage researchers to consider ecological and behavioral factors, such as habitat type, social structures, and mating preferences, when studying ocular traits. Better data across cultures and species will be needed to test whether any single hypothesis—or combination of factors—can account for the wide variety of eye appearances found in primates today.

“If anyone has the time and energy, I really recommend them to watch EDGE Science’s Youtube video on the topic. It does a fairly good job at explaining how this topic has progressed over the years, and how new data keeps forcing us to draw new conclusions: https://www.youtube.com/watch?v=_Fuo7axCWJ4.”

The paper, “Look past the cooperative eye hypothesis: reconsidering the evolution of human eye appearance,” was authored by Juan Olvido Perea-García, Aurora Teuben, and Kai R. Caspar.