Why do only humans have whites of the eyes?

  Poker players wear sunglasses to keep their eyes from revealing the cards. But if it’s a gorilla or a dog, they probably don’t need sunglasses, and it’s hard to read from the animal’s eyes because you can’t see the whites of their eyes.
  Humans are the only animals with clearly visible whites of the eyes. The whites of our eyes make our irises more prominent and therefore readable. We can easily know what someone is looking at, and often what someone is thinking. But in many animals, the white of the eye, called the sclera, is not white at all, but is pigmented, making the iris difficult to distinguish. In those animals with white sclera, it was barely visible.
  The human sclera has no pigmentation, suggesting that a contrasting and prominent iris to the rest of the face may be important. One possible reason is that the human eye is not only for seeing, but also for communication.
  One feature of the colorless sclera is that it can reveal a person’s health. So unwittingly, we can measure someone’s age and attractiveness from the color of their sclera. Red or pink sclera can be associated with poor health, while yellow-stained sclera can be a sign of jaundice or old age.
  A prominent iris can also show what someone is looking at. This is useful because other people are looking at things that may be of interest to us, and we can also infer that someone may have noticed something we didn’t notice, and that they may have a different point of view than ours.
  The ability to understand another point of view is uncommon in animals, but not absent. Some species, like chimpanzees, are better at this than others, but still very limited compared to humans. Professor Nick Haslam, head of the Department of Psychological Science at the University of Melbourne, believes most people can do this without much difficulty.
  Professor Haslam said children with autism with visible whites were less interested in white plush toys than other children and adults.
  ”Perhaps humans evolved visible sclera because our theory of mind is more developed, making it more important for our social intelligence to communicate and read the gaze direction and emotional expression of others.”

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