From at least 6 to 3 million years ago, early humans combined apelike and humanlike ways of moving around. Fossil bones like the ones you see here record a gradual transition from climbing trees to walking upright on a regular basis. Sahelanthropus may have walked on two legs.

When did these hominins evolve to walking upright?

While the new research suggests hominins were walking upright at least 3.6 million years ago, when exactly this distinct locomotion style first diverged from other apes remains unclear.

When did bipedalism evolve?

Evidence for bipedalism extends as far back as 4.2 million years ago, perhaps even six million years ago, but stone tools do not appear in the archeological record until 2.6 million years ago—so we can rule out tool-making as an explanation.