Draw a picture of an eye. Go ahead, I'm not looking.
You probably drew something like this:
Actually, no, you probably ignored me and kept reading, but I'll let that go. The point is, when we think of eyes, we generally think of the kind of thing above: a round or diamond-shaped opening, a pupil in the middle, surrounded by iris, and big chunks of white on either side. However, that's actually not what most eyes on this planet look like.
Take this picture of my dog (actually, take any picture of my dog, because every picture of my dog is perfect).
Her gorgeous eyeballs fill up almost the entire opening between her eyelids. You can barely see white on either side. Or take this photo of a goat's eye:
Whoa, what do we have here? A rectangular pupil, unlike our beady little circular ones, and an iris that takes up the entire opening. Again, virtually no eye whites! Is the entire animal kingdom conspiring against us to make us feel like eye freaks? Surely, we can turn to our nearest cousin, the chimpanzee — but no!
Look at this mofo.
Besides stealing all the mango, he's got eyes that are almost human-like ... but not quite. Small pupil, check. Round shape, check. Colored iris, check. Eye whites? Nope.
In fact, among the 633 species of primates, only one has visible eye whites: humans. That white part is actually called sclera, and while other primates have it, theirs are colored, typically about the same color as the iris, so we don't see them unless we get up really close. Humans have no pigment in their sclera, giving us what we commonly refer to as "the whites of our eyes."
So, why should we have white sclera while everyone else's is colored?
The answer comes down to evolution. Again, as with all good things, you can understand it best if you consider my dog.
Have you ever tried to tell your dog where her favorite toy is by pointing to it? Unless she has known you a long time and picked up on humans' weird habits, she probably looked at your finger, not where your finger is going. However, if she knows you long enough, she may learn that, in your weird little human mind, there's an invisible line between your finger and something off in the distance, and if she follows your delusional pattern of thinking and traces the invisible line, she can figure out what you're looking at.
However, try not even pointing at the toy. Try just looking at it. If your dog is like my best friend, Ella, she will stare back at you, not knowing you're trying to tell her anything at all. Even if you and your dog have known each other a long time, she may never pick up on your subtle indicator that her toy is over there by the bookcase, darn it.
That's likely because your dog doesn't have eyes like yours. If she looks at another dog, she will just see a pupil inside a colored sclera. Unless she's up close, she won't see where that pupil is actually pointing. As a result, it's not intuitive to her to follow your line of sight, because she literally doesn't know that's what you're looking at. Turn your head, and she might have a better idea. It turns out, apes are the same way. In studies, they won't follow your gaze unless you literally turn your head to face the thing you're looking at.
However, humans know to follow each other's eye movements intuitively. Why? We don't know for sure, but our best theory is called the "Cooperative Eye Hypothesis."
The Cooperative Eye Hypothesis says that one of the things that has made humans so successful is cooperation: we can work in large communities, all benefiting others in our tribe. While a herd of elephants maxes out at about 70 individuals, wolf packs don't usually have more than 30 and chimps can form communities of up to 120, humans interact with a nearly limitless number of other humans, and need to cooperate with as many as possible, to survive, especially as we began to develop a transaction-based economy.
So, subtle eye movements help us detect emotion, read one another's intentions and interact in delicate ways that promote kinship with the people around us, all of which is evolutionarily advantageous. As our ancestors developed these traits, those who could cooperate the best passed them on to their descendants.
All this cooperation meant that we needed to be able to communicate as well as possible with other members of our species. We also needed to be able to sniff out deception in the traitors among us. When we know we're being watched, we're more likely to be good. That promotes altruism, which is ultimately good for a community. You scratch my back, I scratch yours — which is why lying is something we can pick up so easily in eye movements. How can you best tell if someone's eyes are moving suspiciously? If they have big sclera that tell you exactly where their eyes are moving, to a hair's width. That's how.
Your mom said, "Look in my eyes and tell me you didn't eat the cookies," because she knew she could sniff out a lie — or more accurately, stare out a lie — if she looked right at you. She can thank millions of years of evolution for that ... and also your sclera.