Nature
PUBLISHED
Side-eye. It’s become one of the most powerful tools in a modern human’s optical arsenal. A quick skip of the eyes that can say so much, and something we have learned to read a lot into.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.It’s a micromovement you can spot across a room thanks to a notable trait of human eyes: our bright white sclerae. While not totally unique to humans, they are the norm for our species and quite rare among other primates.
Some of our closest relatives – chimps and bonobos – have brown sclerae. For chimps, this is much darker than their comparatively pale irises, whereas bonobos have paler sclerae compared to their irises. So, why did we buck the brown?
For a long time, scientists figured it hinged on how we’ve evolved to use our eyes as a means of communication. They argued that the contrast of a white sclera against a pigmented iris made it easy to spot where a fellow human was looking, and gave us a whole new way to share critical information.
Give a talk to an audience of humans and the eye rolls will be painfully apparent. Give that same lacklustre performance to a crowd of chimps, and you’ll be blissfully unaware of their ennui. Or at least, that was how the theory went. But in recent years, new research has rejected the idea that other primates like chimps and bonobos can’t follow the gaze of their conspecifics.
In 2019, an international team of scientists looked at the eyes of over 150 humans, bonobos, and chimpanzees. They discovered that though primate sclerae were darker, the contrast of light and dark meant their gaze was still easy to follow.
So, does this mean our white sclerae aren't the result of millions of years of socialization? Not necessarily, but it does indicate that primates are more capable of communicating with their eyes than previously recognized.
“Humans are unique in many ways, as no other animal can communicate with similar intricate language or build tools of such complexity,” said Juan O. Perea-García at the National University of Singapore in a statement accompanying the release of the research. “Gaze following is an important component of many behaviours that are thought to be characteristically human, so our findings suggest that apes might also engage in these behaviours.”
It's true that side eye is likely more apparent as a flash of white than nuanced shades of brown. However, research in 2023 revealed that white sclerae among primates isn't as rare as we once thought.
“White sclera is often considered a uniquely human trait in the scholarly literature and popular media,” wrote the authors. “Our study confirms that this is not the case. Almost 1 in 6 chimpanzees at Ngogo had full or partial white sclera in at least one eye, which tended to be more visible when gaze was averted rather than direct. Even a small amount of white or light sclera may make gaze direction more apparent.”
So, by all means go forth and use those shiny white sclerae to communicate with your mates. It works great. Just know that you side-eye a chimp to your peril.
