The most common theory for human evolution is that we are descended from apes. Some of these apes moved out of the forests in which they lived and onto the grasslands. The human features are supposed to be adaptations to a savannah environment.
In that case we have to consider other savannah mammals. Most walk on four legs and are covered in hair, but you don't see any of these adaptations in humans.
However, the Aquatic Ape Theory suggests that when our ancestors moved onto the savannah they were already differant from apes. It suggests that hairlessness, bipedalism, and other characteristics evolved some time earlier, when the human and ape lines first diverged.
Most of the features of human physiology are rare or unique among land mammals, but are common among aquatic ones. Perhaps some of our earliest ancestors lived in a semi-aquatic habitat for a prolonged period of time.
There is geological evidence to support this hypothesis, and nothing in the fossil record that is inconsistant with it.