There are a number of theories as to why our ancestors evolved to walk on two legs. Some of them are patently ludicrous (see: aquatic ape hypothesis), but among the more sensible, the idea that we did it for cooling purposes is beautiful in its simplicity. This theory claims that we evolved to stand upright, in order to regulate our body heat in the hot conditions of ancient Africa.
Top image: Masashi Mochida on Flickr.
Unfortunately, this theory may have taken a major blow thanks to a new piece of research. In contrast to previous attempts to model heat loss by posture, the researchers looked at how much heat would have to be discarded while walking rather than standing still. What they found was that bipedality didn't help with shedding heat in high-temperature environments — but it didn't hurt either.
What did make a big difference? Hairlessness. So while we may not have gone upright in order to cool off, losing the dense fur that marks other primates certainly helped.