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Well, it wouldn't be any fun if I just gave it away now would it? 😉
This was my reasoning. I said everything out loud, pretty much in this exact manner:
If humans are the only species that expose the whites of their eyes, that means exposing the whites of your eyes is a favorable trait (otherwise it wouldn't have survived for so long). The only effect I can think of that exposing whites would have is a negative one -- it makes camoflage less effective for hunting or hiding, especially in the dark. So, if it can't be a positive trait based on appearance, it has to be a positive trait based on something else. If not structure, then most certainly function.
From the statement, humans are the only species that expose whites (which I was not aware of), so that means that there has to be some function that human eyes have that other species' eyes do not. What is unique about human eyes?
Each eye's field of vision overlaps with the other. That in itself isn't special. What's special about this is that humans have this ability AND peripheral vision. From an evolutionary standpoint, that's big. Most species either evolved eyes that are located on the sides of the head, allowing for a greater range of vision (better for defensive purposes moreso than hunting), or eyes in the front of the head (better for hunting but definitely not for self-defense).
Now that that's established, what's so special about the whites? Well, this again goes back to function. What does having exposed whites enable us to do? Or rather, what would we not be able to do if we did not have exposed whites? The answer also has to do with the unique structure of the human eye (or rather, the eye socket).
That's as far as I'll go with that... I don't want to spoil all the fun 🙂... but the important thing to remember is that it doesn't matter if you get the question wrong, all that matters is that you have a coherent and rational line of reasoning to get to whatever your conclusion is, even if that conclusion is that you have no idea.
There's also a social cooperation argument, based on the idea that exposed whites (which contrast with the iris and pupil) allow us to determine where people are looking regardless of the direction their head is pointed. I don't recall the whole line of reasoning, but it was pretty interesting. Basically the point was that we are very socially complex species, and that eyes are important in communication.
But you're right, the answer is not what's important, it's the line of reasoning in an interview setting.