Footprints found in Tanzania in 1978 and dated to 3.66 million years ago were largely regarded as the oldest undisputed evidence of upright walking in the human family tree.

They were discovered at Laetoli site G and are thought to belong to Australopithecus afarensis, the species that gave rise to the renowned "Lucy," maybe the world's best-known fossil.

However, the Laetoli site G footprints were not the only ancient trackways discovered at the time. A mile distant, at Laetoli site A, a set of footprints were assigned to a young bear walking upright on its hind legs since they differed so much from the tracks left by Australopithecus afarensis.

Researchers now believe the Laetoli site A footprints belonged to a different early human ancestor who also walked on two legs, a discovery that could rewrite the human history.

"These footprints demonstrate that the evolution of upright walking was more complicated and more interesting than we previously thought," Jeremy DeSilva, an associate professor in the department of anthropology at Dartmouth College and coauthor of the research, said.

"There were at least two hominins, walking in different ways, on differently shaped feet, at this time in our evolutionary history, showing that the acquisition of human-like walking was less linear than many imagine."

Striding bipedalism, the human version of walking on two legs, is unique among mammals, and it was previously thought to have a single evolutionary origin.

The research team found that the fossils' features seemed more humanlike after examining the bear paw prints and comparing them to the mystery fossil footprints, as well as human and chimpanzee footprints. For example, it appears that the feet had a comparatively large "big toe."

In addition, the walking pattern recorded in the tracks resembles a cross-step, with the foot crossing in front of the body's midline. When walking across uneven ground or regaining their equilibrium, people may cross-step.

The researchers also examined over 50 hours of footage of wild black bears and discovered that they rarely walk on their hind legs.

And that suggests that bipedalism didn't have to evolve all at once and in a straight line to the way we walk now, but that other versions may have existed at the same time in different branches of the family tree.

DeSilva stated that we would need to find fossils to learn more about this hominin's appearance. However, he said that the individual's foot measurement indicated that he was only a few inches taller than 3 feet (0.9 meter).

The research analysis is published in the journal Nature.