If we humans weren't so bare, we would probably not wear robes. And then there would be no reason to disrobe. If there were no bare skin, there would be no Hefner as we know it.
And, according to Mark Changizi from the Department of Cognitive Science at Rensselaer Polytechnic Institute, the reason we're bare is because we can see in color.
More talk of nudity and other dirty things after the cut...
Changizi's theory, which he details in a post over at ScientificBlogging, is based on both research and speculation. But I kind of love these evolutionary "Just So Stories" like this, not necessarily as hard science, but for their ability to inspire imagination and curiosity about who we are and where we came from. The fact is, without basically re-doing human evolutionary history in a lab, we probably won't ever know for sure why certain features evolved. Or why we have some features that other animals don't. But I do find the speculation fun.
As I have argued in my research, our color vision is a distinctive kind of color vision, one that is specialized for detecting the color changes that happen in skin due to the physiological changes in blood (e.g., oxygenation). Most varieties of color vision - like that in birds, reptiles and bees - do not have this extraordinary capability. Our color vision is for seeing blushes, blanches, red rage, sexual engorgement and the many other skin color changes that occur as one's emotion, mood, or physiology alters. Color is for seeing embarrassment, fear, anger, sexual excitement, and so on.
Our primate ancestors once had furry faces, and one was born with our style of color vision, able to detect the peculiar changes in our underlying blood physiology. Although the faces this ancestor looked at were furry, some skin would have been visible, such as around the eyes, nostrils, lips and any lighter patches of fur. This ancestor would have been born an "empath," able to see the moods of others. Color vision of this kind would thus spread over time.
And once it spread, animals could then have evolved to "purposely" signal colors indicating their mood, and then bare skin would have evolved to have more canvas for signaling. Many of our skin color changes are indeed "purposeful," i.e., not simply inevitable consequences of our underlying physiological state. For example, Peter D. Drummond has shown that peoples' faces blush more on the side which people can see.