Humans have weirdly white eyes. Here’s why.
Popular Science...
Blue, green, amber: Someone’s eye color immediately attracts our attention. But there’s something unusual about human eyes: We have a large visible area of white that surrounds the iris. Most other mammals have entirely dark eyes with almost indistinguishable pupils. So why are we different? What is the white part of our eyes actually for?
The whites of our eyes help us connect
Scientists paid little attention to that question until 1997, when Shiro Kohshima, a Japanese biologist at Kyoto University, decided to take a closer look. He compared the eyes of nearly half of existing primates and found that only humans had white in their eyes.
His theory was that the white part of the eye (the sclera) helps us communicate because it makes it easier to tell where someone is looking. The contrast between the white sclera and dark pupil makes the outline of the eye more visible. We also have more elongated eyes than other animals, which makes it even easier to tell where someone may be looking.
Following someone’s gaze is surprisingly powerful. It can indicate if they’re telling the truth, draw attention to something, and even help us bond. Language, after all, can be complicated and ambiguous. “It’s important to build up a fast communicative step,” says Fumihiro Kano, a cognitive scientist at Kyushu University in Japan. “White sclera help towards that.”
The cooperative eye hypothesis
In 2007, Michael Tomasello, a psychologist at Duke University, expanded on Kohshima’s earlier ideas to develop the cooperative eye hypothesis. He argued that the white sclera are particularly useful for human collaboration.
For instance, the whites of our eyes help us figure out what someone is focused on. It may even have helped our ancestors hunt together and share resources. Central to his idea was the theory that humans are unusually sensitive to where others are looking.
To test this, he conducted an experiment involving human infants and gorillas, chimpanzees and bonobos. A scientist looked at the ceiling with only his eyes, only his head, or both.
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.
Apes did the opposite, relying primarily on head movement rather than eye gaze. They looked towards the ceiling roughly 2.5 times more often when the researcher lifted his head but closed his eyes.
Why eye contact is so important for babies
From an early age, humans are particularly sensitive to eye contact. In a study of newborns, within the first five days of their lives, researchers found that babies looked longer at faces whose gaze was directed at them. The ability to actively follow where others look emerges between two and four months, and by eight months it becomes consistent behavior.
“Eye gaze is a natural pointer which makes it easier to understand each other,” says Kano. “If you look at a human infant, then that infant becomes interested in you.”
Eye contact also helps develop necessary language skills. Having white sclera means that infants can more easily follow an adult’s eyes towards a certain object, hear the name of the object, and develop their vocabulary. Studies suggest that infants who follow eye gaze more frequently at ten months have a greater vocabulary.
Related 'Ask Us Anything' Stories
Is the white of the eye the real secret to human connection or is it something else?
However, recently, Juan Perea-García, an evolutionary biologist at the University of Las Palmas de Gran Canaria, questioned how important the white of the eye actually is in communication.
“The cooperative eye hypothesis taps into the bias of human exceptionalism,” says Perea-García. “That’s why it’s so compelling.” Since Tomasello’s 2007 study that proposed the theory, research has shown there are other primates with white sclera.
Perea-García also points out that, for some people from South Asia, Africa, and Australia, their sclera is not uniformly white but more pigmented. So he argues that it’s not the whiteness of the eyes that’s important for communication, but the contrast between the sclera and the iris. Chimpanzees also have dark sclera with bright irises which could serve a similar purpose.
But this may not be the whole story. While human sclera are not always uniformly white, we tend to show considerably more of the whites of our eyes than most primates and experiments suggest that difference matters.
Kano and his team compared how humans and chimpanzees interpreted images of human and chimp eyes. They found that both species were better able to discriminate gaze direction from humans. They then made both images smaller and darker. Chimp eyes became even harder to read than humans.
The team even digitally altered chimpanzee eyes to have white sclera and found that gaze discrimination immediately improved.
“Our work suggests that gaze visibility depends not only on iris-sclera contrast, but also on the visibility of the overall eye outline,” Kano says. In other words, it’s not just about how well the iris stands out. The white sclera makes the whole shape of the eye more visible against the face, something that’s difficult to discern in the dark eyes of chimpanzees. It’s these features working together that seems to make it easier to follow our gaze direction in poor visibility conditions.
The whites of our eyes also indicate health and age
White eyes may also have another purpose: They make it easier to notice changes in eye color which can indicate significant information about health or age.
As we get older, the whites of the eyes gradually become more yellow or red because of fatty deposits and more blood vessels around our eyes. This shift can occur more rapidly with poor health or diet.
However, if the sclera suddenly changes color, it can signal more serious health problems. Severe yellowing is closely related with jaundice, a failure of the liver to filter blood properly, while acute reddening may indicate an eye infection. A yellow or red sclera also affects how healthy others think you are.
Researchers tested this by digitally manipulating pictures of eyes to be more red or yellow. Individuals with yellow or red eyes were seen as less healthy, older, and less attractive. It’s an immediate frame of reference that shows how much information we get from our eyes.
So, next time you catch the eye of someone across the room and smile, take a second to appreciate the importance of the white in their eyes. Without it, that connection might never have happened.
In Ask Us Anything, Popular Science answers your most outlandish, mind-burning questions, from the everyday things you’ve always wondered to the bizarre things you never thought to ask. Have something you’ve always wanted to know? Ask us.
The post Humans have weirdly white eyes. Here’s why. appeared first on Popular Science.