The white of the eye – or the ‘sclera’ to give it its technical name – is the opaque, protective layer that encloses the entire eyeball, apart from the transparent cornea at the front of the eye. In humans, the whole sclera is white, but most mammals have a sclera that is dark at the front, or coloured to match the iris, with just a narrow band of white sclera occasionally visible.
The dark sclera in most mammals might have evolved to ensure a flash of white from their eyes wouldn’t draw attention to them as they lurked in the shadows. But in humans, our eyes may have evolved a white sclera since it made it easier to see which direction we’re looking in.
This ‘cooperative eye hypothesis’, originally proposed in 2001 by researchers at the Tokyo Institute of Technology, suggests that being able to quickly tell the direction of someone’s gaze is an important part of non-verbal communication. This may have been useful to our ancestors when hunting in groups – a quick flick of the eyes from one hunter would be enough to alert the others that they had spotted something.
Surprisingly, though, eyes with a white sclera are rare among the other great apes. This may be because their societies are less cooperative, and it’s more important to hide the direction of gaze from each other, so as not to give away the location of food, for instance.
Other studies have found that, among canines, those species who hunt in groups are much more likely to have eyes with a white sclera than those who hunt alone. But gaze signalling is not the only possible reason for the white sclera in humans and some animals. It may also have evolved as a way to signal physical health – diseases such as hepatitis can discolour the sclera yellow.