What, in evolutionary terms, propelled us to become human? The answer lies not in our forebears' big brains or their facility with language but in their ability to walk on two feet. That remarkable fact -- standing and walking seem so mundane -- only starts the drama that Craig Stanford, codirector of the Jane Goodall Research Center, tells of our origins. Today scientists are finding far more evidence than ever before about our beginnings. The discoveries are prompting dramatic reappraisals of common beliefs about our past. Throw out the simple idea that millions of years ago some apes moved to the African savanna, where they evolved into runty hominids who eventually metamorphosed into us. Dump that textbook image of an ape transforming into a human in five stages. Newly found remnants of two-legged "proto-humans" show that our ancestry is much richer and more convoluted. In no way can we still think of ourselves as standing on the top rung of an evolutionary ladder of excellence. But what about our tremendous thinking powers? Our brains could have started to grow because, as our ancestors adapted to standing and walking upright, they became more successful at hunting ever larger animals. The meatier diet could have fueled the increase in brain size. And the switch to standing and walking tall may have allowed our forebears to develop language, let alone take over the entire world as their home. Describing his - and others' - latest research and interpretations, Stanford offers a fresh, galvanizing take on what made us human.