What Do Aliens Look Like?

Science fiction writers have always had their little green men. But these humanoid aliens were based soundly on Earth-based life, not any extra terrestrial evidence. Today, we've discovered hundreds of planets around other stars. As we learn what some of these alternative Earths might look like, science and imagination have allowed us to use real science to imagine the biology of their inhabitants. Will they have two eyes? Two legs? What color will their skin be? Which species on Earth can give us clues about likely biology of aliens? And what can we learn from how life on Earth developed to help us understand what ET really looks like?