Why are humans the only species that have visible eye whites?

Humans aren't the only species that has visible eye whites, but ours are more visible than most creatues. In fact, many domesticated animals have partially visible eye whites. But most wild primates, other than humans, sclera (the part we call a "white") and iris that are about the same color or shade as their skin, to hide their eyes.

It is thought that this is a chance evolutionary advantage, allowing us to communicate with each other visually. We can see where each other is looking. Apes must follow each other's head direction, while humans can see where each other's attention is, even if their head does not turn.

Also, the amount of visible white can communicate changes in emotion...if someone is afraid, their eyes widen, showing more white. If they are annoyed, they may roll their eyes. If they are sleepy, their eyes partially close. If they are flirting, they might wink. Eyes are an unusual social communication tool, used by humans.

It's interesting that wolves lack visible whites, while dogs have partially visible whites. This may be because humans are more sympathetic to pets that have visible whites like their own, so they've been selectively bread (unconsciously) for that trait, over the past hundred thousand or so years.