Do the Eyes Have It?
Dog domestication may have helped humans thrive while Neandertals declined
A study by Hiromi Kobayashi and Shiro Kohshima of the Tokyo Institute of Technology showed that modern humans are unique among extant primates in having highly visible white sclerae surrounding the colored irises of their eyes, as well as eyelids that expose much of the sclerae. In other primates, the dark sclerae, similarly colored skin and concealing eyelids tend to mask the direction in which the animal is looking, according to the Japanese team. In humans, the white sclerae and open eyelids make the direction of a person’s gaze visible from a distance, particularly if that glance is directed in a more or less horizontal direction. The changes in the human eye may be adaptations to enhance the effectiveness of the gaze signal.
Michael Tomasello and colleagues at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, developed this idea as the “cooperative eye hypothesis.” They suggested that cooperation among humans was facilitated by the ability to recognize where others were looking. Apes will follow gaze less often than human infants, they found. If the direction of gaze and the direction of the head conflicted, apes tended to follow head direction. In a humorous aside, the researchers noted that they tried their experiment with 14 chimpanzees, 4 gorillas, 4 bonobos and 5 orangutans—but dropped the results of tests on three chimps and all five orangutans because they “did not pay attention to the gaze cues sufficiently for their skills to be reliably assessed.” Following gaze was apparently not a high priority to the apes.
The mutation causing white sclerae is universal in humans, but it turns up occasionally in apes, too. In decades of observations at Gombe National Park in Tanzania, Jane Goodall observed two chimps, probably brothers, who had white sclerae. A third, female chimp developed white sclerae as an adult. But the trait has not spread or reappeared in that population. The advantage of the white sclerae must be related to something that ancient humans did commonly and chimps don’t do or do rarely. Although chimps hunt small prey, often cooperatively, meat makes up less than 2 percent of their diet, whereas Paleolithic humans hunted much larger game that apparently provided a significant part of their diet. Obviously, silent communication among humans would be advantageous for hunting in groups. But there is another skilled gaze-reader: the domestic dog.
A dog will follow the gaze of a videotaped human if the human first attracts the dog’s attention by speaking to it and looking at it, according to results published by Ernõ Téglás, of the Central European University in Budapest, Hungary, and his colleagues. Indeed, dogs perform as well as human infants at following the gaze of a speaker in tests in which the speaker’s head is held still.
Ádám Miklósi of Eötvös Loránd University in Budapest, Hungary, and his team tested dogs and wolves, and found that dogs were far more attentive to human faces than were wolves, even socialized wolves. Although wolves excel at some gaze-following tasks, perhaps suggesting a preadaptation for communicating with humans, dogs tend to look at human faces for cues and wolves do not. Miklósi’s team believes this major behavioral difference is the result of selective breeding during domestication.
Another way of looking at this phenomenon is that the white sclerae became universal among humans because it enabled them to communicate better not only with each other but also with dogs. Once dogs could read a human gaze signal, they would have been even more useful as hunting partners. No genetic study has yet confirmed the prevalence or absence of white sclerae in Paleolithic modern humans or in Neandertals. But if the white sclera mutation occurred more often among the former—perhaps by chance—this feature could have enhanced human-dog communication and promoted domestication. Although some genetic analyses have suggested that modern humans and Neandertals interbred, even the highest estimates of cross-breeding involve very low levels of genetic exchange that might have been inadequate to spread the white sclera trait among Neandertals.
Humans love to look into their dogs’ eyes to “read” their emotions. Dogs apparently feel the same. Maybe—just maybe—this reciprocal communication was instrumental in the survival of our species.
- Germonpré, M., M. Lázničková-Galetová and M. Sablin. 2012. Palaeolithic dog skulls at the Gravettian Předmostí site, the Czech Republic. Journal of Archaeological Science 39:84–202.
- Goodall, J. 1986. The Chimpanzees of Gombe: Patterns of Behavior. Cambridge, MA: Belknap Press.
- Kobayashi, H., and S. Kohshima. 2001. Unique morphology of the human eye and its adaptive meaning: Comparative studies on external morphology of the primate eye. Journal of Human Evolution 40:419–435.
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- Téglás, E., et al. 2012. Dogs’ gaze following is tuned to human communicative signals. Current Biology 22:1–4.
- Tomasello, M., B. Hare and J. C. Lehmann. 2007. Reliance on head versus eyes in the gaze following of great apes and human infants: The cooperative eye hypothesis. Journal of Human Evolution 52:314–320.