Here's a cool idea: some researchers have suggested that human bipedalism (walking on two legs rather than four) may have evolved from apes walking on slim branches. This is based on observations of orangutans doing limb walking to get to hard-to-reach fruits.
Here's the link:
http://www.nature.com/news/2007/070528/full/070528-8.html
It's apparently been a bit of a puzzle to account for where and when bipedalism evolved in our ancestors, and the prevailing theories had it coming about after they started living on the ground. The idea here is that bipedalism had advantages even before our hairy forebears left the treetops. The theory is still being debated, but it looks plausible.
Yet more evidence, if any was needed, that tree climbing is an important part of being human.