How did humans evolve to walk upright? Fossil discovery complicates the picture.

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.

New findings reported earlier this week in the journal Nature show that early hominins held onto their ability to climb trees longer than scientists previously thought.

Discovered in Burtele, Ethiopia, recently unearthed right foot bones are about 3.4 million years old and is thought to belong to a new species. The stiff joints of foot and toe bones suggest that the early hominin was capable of bending its foot upward to enable walking. The thumb-like big toe, however, would have helped it to also climb trees, like African great apes.

“It’s a lovely little foot to have,” paleoanthropologist Donald C. Johanson told The New York Times. Working in Ethiopia in 1974, Johanson discovered "Lucy," the first specimen of Australopithecus afarensis, a species thought to be either a human ancestor or a close relative of one. Johanson's discovery indicated that bipedalism preceded humanity's increase in brain size. 

This new finding suggests that the evolution of bipedalism was more nuanced than we knew. The dual-purpose bones are about the same age as Lucy's, but scientists think that Lucy's species mostly moved by walking upright. Since two species of hominin existed at the same time, with different shaped feet, it appears that hominins evolved to walk more than once.

There are still many unknowns surrounding the evolution of humans' bipedalism.

Hominin bones of this age are uncommon. Their small size makes them susceptible to scavengers and deterioration. So there just isn't much evidence for scientists to use in their interpretations.

Without the rest of the Burtele skeleton, it is difficult to determine precisely how this hominin moved. As this NPR story points out, bipedalism requires evolutionary changes from head to toe.

"We need more fossils to determine what sorts of bodies went with these feet," wrote Daniel Lieberman, an evolutionary biologist at Harvard, in a commentary in this week's Nature.

Scientists will continue to hunt for more fossil evidence to expand their understanding of human bipedalism. This work, according to Lieberman, "will require researchers to continue getting their feet dirty in the field and the lab."