Science How did humans evolve to walk upright? Fossil discovery complicates the picture. (+video)

Foot bones unearthed recently in Ethiopia belonged to a contemporary of 'Lucy,' the 3.2-million-year-old early human discovered in 1974. But these bones seem to belong to a different species, one thought to have split its time between walking upright and climbing trees. 

Lucy Nicholson/Reuters

Learning to walk indicates a child's progression from infant to toddler. But walking on two legs is uncommon among mammals. In fact, standing and walking on our own two feet helps to set us apart from other mammals.