"Don't shoot until you see the whites of their eyes." That's the context we're used to hearing the human sclera mentioned in. But it turns out the evolution of our sclera may have more to do with cooperation and communication than combat. According to this week's Economist, German researchers have posited that our sclera evolved to that end. In other words, if your iris is surrounded by white tissue, it's easier for others to establish where you're looking and for you to focus on the same thing. The Economist article uses the phrase "the mutual direction of gaze" and suggests a common example: a group of men standing around a car focusing on the efforts a single male.
Other primates have darker sclera, making it markedly more difficult to determine their focus. As a result, studies revealed that non-human apes paid more attention to where where a human's head was turned, while human children followed the eyes, a more accurate way of determining where the facilitator was actually looking.