Our eyes are designed to see and be seen
If you’re facing someone, but they’re distracted, how do you know? One of the first clues is likely their eyes, which can tell you where the person’s attention is. This kind of passive communication is greatly enhanced by the coloring and shape of our eyes, which have evolved to nearly turn out irises and pupils into bullseyes, a target we can’t avoiding looking at. We’re not completely alone in these traits, but they’re much more pronounced in humans than in closely related species like gorillas.
Colored for contrast
The coloring in question is really a lack of color in our sclera. The sclera is generally an opaque mass of tissue, intended to house and protect your iris and optic nerve. Most animals have pigmentation in their sclera matching their irises, and so the two structures aren’t differentiated to casual observers. Humans’ white sclera instead creates intense contrast with our irises, which is theorized to have evolved to allow us to better track each other’s gaze and attention, communicating and responding more easily and quickly to each other.
To further optimize ocular communication, our eyes are also unusually elongated. A more perfect circle would hide the sclera, exposing just the iris and pupil. The wider openings on human eyes expose more of the sclera though, making our point of focus that much easier to track, even from a distance.
While watching each other’s gaze is highly developed in humans, we’re not the only species to make use of these concepts. Western lowland gorillas (Gorilla gorilla gorilla) (really!) were found to usually have some amount of white in their scleras. Out of the 70% with lighter eyes, 7% had eyes as white as a human, and are just as easy to “read” as a human face. This isn’t a one-way mirror though, as gorillas are known to communicate with eye contact as well.
On a basic level, this is why it’s recommended that you avoid eye-contact if you’re ever confronted by an agitated gorilla. It’s also the reason the Rotterdam Zoo began issuing glasses with fake eyes for viewing gorillas. Visitors wanted to make eye-contact with the gorillas, which the gorillas saw and recognized as threatening behavior. So the cardboard glasses have pinhole openings to see out, but are printed with photos of slightly oversized eyes, clearly looking to the side thanks to the white sclera.
Source: Melissa Hogenboom by There is something weird about this gorilla's eyes, BBC Earth