Humans have many traits that separate us from our closest relative, the chimpanzee. Thinking about the differences between chimps and ourselves can give us insight into human evolution. Some differences:
We have much less hair.
We have a shorter gut.
We are better at throwing objects accurately.
We have hooded nares.
We are weaker and slower than chimps.
We don't have fangs like chimps.
We walk on two legs primarily.
We have emotional tears.
We have more subcutaneous fat than other primates, that tend to store more fat around their gut.
We have more complex language and tool use, including fire.
How do we explain these differences? Was it a single niche that brought about all these changes, or a series of different environmental pressures over millions of years?
Everything below is pure speculation:
Our nearest living ancestors live in trees, so the first change in niche for protohumans must have been the transition to living on the ground, potentially in savannas. Chimpanzees are already pretty intelligent, and are known to use a variety of tools, though with their long fingers and short thumbs they have some difficulty with precision tool use. On the other hand, baboons, who live mostly in savannas, have hands that are better able to grasp objects, though do not use tools as often as chimpanzees. Why not? Since chimpanzees are more intelligent than baboons, the most reasonable conclusion is that baboons aren't intelligent enough to significantly use tools. When protohumans made the transition to living on the ground full time, this allowed the hand to focus on adapting fully to tool use rather than tree-climbing. I think one reasonable conclusion from all these facts is that the major story of protohuman evolution is: There was an intelligent tree-dwelling primate, more intelligent than other primates, that made the transition to living on the ground, developed dexterous hands, and started using more and more tools, thus driving increasing intelligence. It seems likely that this increase in tool use created the pressure for bipedalism, allowing a greater variety of tool use (such as walking with a spear).
Once the protohumans started using more and more tools, many traits likely followed as a result. For instance, the use of fire for cooking is the likely explanation for the shorter gut as it takes less gut to digest cooked food (with cooking we basically externalized a portion of digestion, like a spider who injects digestive juices into his trapped prey). Perhaps our shorter hair is a result of our ability to create clothing out of the hides of other animals and warm ourselves by fire, thus making it unnecessary to spend the calories growing our own pelts. Our ability to throw objects accurately is likely related to hunting, perhaps with spears. We are weaker and slower since we can protect ourselves and hunt using tools rather than spend calories maintaining large muscles.
There's an interesting theory call the aquatic ape hypothesis (http://en.wikipedia.org/wiki/Aquatic_ape_hypothesis). It states that at some stage, protohumans spent enough time in the water collecting food that it affected our evolution in a variety of ways. I think the most clear example of the explanatory power of this theory is our hooded nares, which keeps water out of the nose unless you turn upside down. Other examples include reduced hair and increased subcutaneous fat, characteristics found in many mammals that spend lots of time in the water (subcutaneous fat is a better insulator than hair when wet). Some proponents of the theory push it further, saying bipedalism developed to help protohumans wade in water while keeping their head above the waterline (when apes wade across rivers they do so on two legs).
Incidentally, early, anatomically modern humans did often live on beaches. For instance, the first wave of human migration out of Africa around 70,000 years ago or so, moved primarily via the coast of the Indian Ocean, arriving in Australia around 40,000 years ago. The fact that this first migration moved along the coast suggests they were beach-combers. Similarly, some scientists believe that once Asian peoples arrived in the Americas around 15,000 years ago, they rapidly moved down the Pacific coast, then later migrated inland, again suggesting a beach-combing society.
What was the driving force behind language? Perhaps for teaching tool use or planning for complex hunting and trapping maneuvers? And why music? And why does it seem our intelligence is so complex and adaptable, beyond the needs of a hunting/gathering society (able to develop quantum mechanics or play a Rachmaninoff piano concerto, for example)? Why do we laugh at humor?