The Whites of Our Eyes Are Unique
The human white and highly exposed sclera
plays an important role in communicating
gaze direction and facilitating social interaction
Human Whites of the Eyes Are Unique
Another Difference Between Humans and Apes
by Jerry Bergman, PhD
The hearing ear and the seeing eye, the Lord has made them both. Proverbs 20:12
Among the many characteristics that distinguish humans from other primates is the conspicuous white sclera—the visible “white” of the eye. For decades, popular writers and evolutionary advocates have emphasized the similarities between humans and apes. One of the most influential examples was The Naked Ape (1967) by zoologist and ethologist Desmond Morris.[1] The book portrayed humans as another animal species closely related to the great apes. The international bestseller was updated and reprinted in 1999. It has 11,929 reviews on Goodreads.[2]
Terms such as “naked ape” and “hairless ape” have often been used to emphasize the genetic similarities between humans and chimpanzees. In contrast, new research is revealing important differences. One striking distinction is the human eye.
Unlike other apes, humans possess a highly visible white sclera that makes the direction of a person’s gaze easy to detect. Far from being a trivial anatomical detail, the human white sclera appears to be specially suited for the complex interactions and communications that characterizes human beings and sets them apart from other primates.
Books and articles continue, however, to claim that we are 98% similar to chimpanzees. This claim has been shown to be false. The following are examples of some of the many claims that have been made about human and ape genetic similarity.
1. The Third Chimpanzee: The Evolution and Future of the Human Animal, by Jared Diamond (2006. New York, NY: Harper Perennial).
The cover of Marks’ book illustrates the contrast between 98 percent ape and 2 percent human, the latter shown as a coat sleeve.
Jared Diamond observes at the outset of the book that humans share approximately 98 percent of their genetic material with two species of chimpanzees, leading him to characterize humans as a third species of chimp. Building on this premise, he argues that many aspects of human behavior are best understood as extensions of our ape heritage. The book’s central thesis is that the biological similarities between humans and apes are so great that our evolutionary past provides a key explanation for why humans are so very similar to apes.
2. Chimpanzee Politics: Power and Sex Among Apes, by Frans de Waal (2007. Baltimore, MD: Johns Hopkins University Press).
This classic book explores the “striking similarities between chimpanzees and human social structures.” Drawing on years of observations of chimpanzee communities, de Waal documents how chimps form political alliances, employ deception, compete for status, and negotiate reconciliation after conflicts. He argues that these behaviors reveal important continuities between apes and humans and help explain aspects of human social maneuvering, coalition-building, and diplomacy.
3. Our Inner Ape: A Leading Primatologist Explains Why We Are Who We Are, by Frans de Waal (2006. New York, NY: Penguin Publishing Group).
De Waal, a leading expert in primatology, in this book specifically compares humans to what evolutionists believe are our two closest ape cousins: the aggressive chimpanzee and the peaceful, matriarchal bonobo. Using these comparisons, de Waal seeks to explain a wide range of human physical and behavioral traits, including social relationships, body design, love, power, sex, cooperation, aggression, and violence. The book argues that many aspects of human nature can be better understood by examining our similarities to these two ape species.
4. The Gap: The Science of What Separates Us from Other Animals, by Thomas Suddendorf (2013. New York, NY: Basic Books).
In this book, Suddendorf examines both the similarities and differences between humans and other animals. While detailing cognitive abilities that make humans unique—such as advanced foresight, mental time travel, and abstract reasoning—he highlights evolutionary continuities shared with apes and other animals, including empathy, grief, and social cooperation. Surveying traits often regarded as uniquely human, such as language, intelligence, morality, and culture,
5. What It Means to Be 98% Chimpanzee, by Jonathan Marks (2002. Oakland, CA: University of California Press).
In this book, biological anthropologist Jonathan Marks examines both the similarities and differences between humans and chimpanzees. Marks notes that the claim that humans are “98 percent chimpanzee” is impressive only if we understand what the figure actually measures and what it does not.
Summary of the books: Though most of the titles reviewed here focus on the similarity of humans and apes, all of them admit major differences. Furthermore, all of the books cited depend on the 98% difference in arriving at their conclusions of similarity. Comparisons between human and chimpanzee DNA are considerably more complex than a single percentage figure implies. Depending on the methods used, some researchers have reported differences as large as 15 percent, producing an 85 percent similarity, or 450 million DNA differences (8 Sept 2025, 9 June 2025, 21 May 2025). Recent genomic studies have suggested that comparisons between human and chimpanzee DNA are considerably more complex than a single percentage figure implies.
The growing recognition of both genetic and anatomical differences between humans and apes points to a larger gap than was commonly portrayed in earlier popular literature. The unique human sclera examined in this review is likely only one example of many distinctions yet to be fully appreciated.
The New Research on Human Eye Sclera
One of the first researchers to investigate the significance of the white sclera was Shiro Kohshima[3] of Kyoto University. Kohshima compared the eyes of nearly half of all living primate species and concluded that the conspicuous white sclera is unique to humans. Because the white sclera contrasts sharply with the darker iris, human eyes make it much easier to determine the direction of another person’s gaze. As a result, human eyes are significantly more conspicuous than those of most other primates.[4] Building on this work, Hiroki Kobayashi and colleagues concluded:
Human eyes have a widely exposed white sclera surrounding the darker colored iris, making it easy to discern the direction in which they are looking. We compared the external morphology of primate eyes in nearly half of all primate species, and show that this feature is uniquely human. Humans have the largest ratio of exposed sclera in the eye outline, which itself is elongated horizontally. We suggest that these are adaptations to extend the visual field by allowing greater eye movement, especially in the horizontal direction, and to enhance the ease of detecting the gaze direction of another individual.[5]
Don’t Fire Until You See the Whites of Their Eyes
In brief, the human eye appears uniquely designed to communicate gaze direction, which, as will be explained, is a very important trait . Unlike the eyes of other primates, the highly visible white sclera enables others to quickly determine where a person is looking, thereby facilitating communication, cooperation, teaching, and social interaction.[6]
The results revealed a striking difference between humans and apes. Human infants primarily followed the direction of the experimenter’s eyes, looking toward the ceiling nearly three times more often when he shifted only his gaze than when he merely raised his head with his eyes closed. In contrast, gorillas, chimpanzees, and bonobos relied mainly on head movement rather than eye direction. The apes looked toward the ceiling approximately 2.5 times more often when the researcher raised his head with his eyes closed than when he shifted only his gaze.
Human infants primarily followed the eye direction of the scientist. They looked up nearly three times more often when he glanced towards the ceiling using only his eyes than when he just raised his head with his eyes shut. These findings indicate that humans and apes attend to different visual cues when determining where another individual is directing attention. Human infants are especially sensitive to eye gaze, whereas apes depend primarily on head orientation.
This Trait Was Inborn
Humans are remarkably sensitive to eye contact and gaze direction. Studies of newborn infants have shown that, within the first five days of life, babies look longer at faces whose gaze is directed toward them. The ability to follow another person’s gaze emerges between two and four months of age and becomes a consistent behavior by about eight months. Following another person’s gaze can reveal what interests them, where their attention is focused, whether they are being truthful, and can even help establish social bonds. Language is often complex and ambiguous, but eye contact provides an additional channel of communication. This reality is reflected in common expressions such as, “Look at me and tell me where you were last Friday.”
In light of the fact that the human eye, with its highly visible white sclera, greatly enhances our ability to determine where others are looking, and what they are attending to. The important role that gaze detection plays in communication and social interaction rases an interesting question: if this trait provides such significant advantages, why is the conspicuous white sclera found only in humans and not in our closest ape relatives?
Research has found some Sumatran orangutans and bonobos exhibit light brown sclera but the contrast produced is significantly less than existing in the human whit sclera. Furthermore, humans show considerably more of the whites of our eyes than these primates. Experimental studies have demonstrated that this difference significantly improves the ability to detect another individual’s gaze direction.[7] Although wild chimpanzees have occasionally been observed with partially white sclera the trait is evidently due to pathology or a mutation.[8],[9]
As might be expected, evolutionary researchers have proposed explanations for the origin of this distinctive human trait. One such proposal argues,
“humans are especially reliant on eyes in gaze-following situations, and thus suggest that eyes evolved a new social function in human evolution, most likely to support cooperative (mutualistic) social interactions.”[10]
While this proposal explains the potential advantage of the trait, it does not by itself explain how such a specialized feature arose or why a comparable adaptation is absent in our closest ape relatives. The importance of this ability has been emphasized by anthropologists Peter J. Richerson and Robert Boyd, who observed:
Indeed, the uniquely visible human eye appears well suited to promote the cooperation, communication, teaching, and social coordination that distinguish human societies from those of other primates.
The research leading to this conclusion compared the responses of human infants, gorillas, chimpanzees, and bonobos to different gaze cues. In the experiment, a researcher looked toward the ceiling using one of three methods: (1) moving only his eyes, (2) moving only his head while keeping his eyes closed, or (3) moving both his eyes and head.
Summary
New research has documented that we are even more different from the apes than what was assumed only a few years ago. As we have just seen, one example is the unique morphology of the human eye. The latest research has documented that humans alone normally possess a conspicuously white and highly exposed sclera, a feature that plays an important role in communicating gaze direction and facilitating social interaction. The importance of these abilities is reflected in the observation of Michael Tomasello:
References
[1] Morris, D., The Naked Ape. McGraw-Hill, New York, NY, 1967
[2] Morris, D., The Naked Ape: A Zoologist’s Study of the Human Animal. Delta Publishing Co., New York, NY, 1999.
[3] Kobayashi, H., and S. Kohshima, “Unique morphology of the human eye,” Nature 387:767-768, https://doi.org/10.1038/42842, 19 June 1997.
[4] Tomasello, Michael, “Reliance on head versus eyes in the gaze following of great apes and human infants: The cooperative eye hypothesis,” Journal of Human Evolution 52(3):314-320, March 2007.
[5] Kobayashi and Kohshima, 1997.
[6] Gredebäck, Gustaf, et al., “The development of joint visual attention: A longitudinal study of gaze following during interactions with mothers and strangers,” Developmental Science 13(6):839-848, https://onlinelibrary.wiley.com/doi/10.1111/j.1467-7687.2009.00945.x, November 2010.
[7] Kano, Fumihiro, et al., “Experimental evidence that uniformly white sclera enhances the visibility of eye-gaze direction in humans and chimpanzees,” eLife 11:e74086, https://doi.org/10.7554/eLife.7408, 2022.
[8] Goodall, J., The Chimpanzees of Gombe: Patterns of Behavior. Harvard University Press, Cambridge, MA, 1986.
[9] Boesch, Christophe, The Chimpanzees of the Taï Forest. Oxford University Press, New York, NY, 316 p., 2000.
[10] Tomasello, 2007
[11] Boyd, Robert, and Peter Richerson, “Culture and the evolution of human cooperation,” Philosophy Transactions of the Royal Society of London, Section B, Biological Science 364(1533):3281–3288, 12 November 2009.
[12] Tomasello, 2007.
Dr. Jerry Bergman has taught biology, genetics, chemistry, biochemistry, anthropology, geology, and microbiology for over 40 years at several colleges and universities including Bowling Green State University, Medical College of Ohio where he was a research associate in experimental pathology, and The University of Toledo. He is a graduate of the Medical College of Ohio, Wayne State University in Detroit, the University of Toledo, and Bowling Green State University. He has over 1,900 publications in 14 languages and 40 books and monographs. His books and textbooks that include chapters that he authored are in over 1,800 college libraries in 27 countries. So far over 80,000 copies of the 60 books and monographs that he has authored or co-authored are in print. For more articles by Dr Bergman, see his Author Profile.