Bipedalism has very ancient roots in the human family tree. Some traits of bipedalism show up in hominid ancestors from as much as 7 million years ago. But how and why did walking upright on two legs become such a human trait? This article from Erin Wayman for the Smithsonian.com does a great job of breaking down the most recent hypotheses and the history of the study of human bipedalism.
Smithsonian has an online article series called “Becoming Human.” It’s a great overview of how the traits we consider “human” came to be.