Here’s what you’ll learn when you read this story:

• Lucy might be one of the most iconic human ancestors, but she had a neighboring species that has gone unidentified—until now.
• The foot from this species indicated that the mystery hominin climbed trees like Lucy did, but spent more time in the branches than on foot.
• Researchers who analyzed the morphology of the foot bones and tooth specimens found that they most likely belonged to the more primitive species Australopithecus deyiremeda.

One of the most iconic skeletons of a human ancestor emerged from Ethiopia’s dry and desolate Afar rift. Lucy, who last walked 3.4 million years ago, was identified as Australopithecus afarensis. She had been buried in sediment that turned to sandstone above a bed of volcanic rock that formed out of packed ash.

And she wasn’t alone. Found not far from Lucy’s final resting place were the bones of another more mysterious hominin species.

Lucy and this other hominin had evidently coexisted in the same region. Their remains were found within about 3.1 miles (5 km) of each other, but there were obvious evolutionary differences. The individual’s foot, known as the Burtele (BRT) foot, ignited controversy amongst scientists—some insisted that it was a good representation of the diversity in foot morphology of australopithecines, while others argued that it must have belonged to a species of Ardipithecus (known for transitioning from climbing trees to walking upright).

Yohannes Haile-Selassie, a paleoanthropologist from Arizona State University, sought answers. He now thinks the BRT foot (discovered in 2009) belonged to an older and less evolved hominin species known as *Australopithecus deyiremeda. *This species did have some anatomical overlap with Lucy, but while A. afraensis alternated between climbing trees and walking on foot, A. deyiremeda was even more focused on life in the trees. A. deyiremeda was an extremely adept climber, with big toes that were opposable much in the way our thumbs are (a feature that gave it a grasping advantage). Its feet were also capable of hyperflexing, meaning that despite the climbing optimization, it was able to push off solid ground and walk.

“One of the reasons why the BRT foot [was] not assigned to A. deyiremeda, despite their provenience from the same horizon as one of the paratypes of the species, was the lack of diagnosable dentognathic specimens that could be directly compared with the holotype and paratype specimens of the species,” Haile-Selassie said in a study recently published in the journal Nature.

Both the BRT foot and specimens of teeth from the Burtele sites are now thought to have belonged to A. deyiremeda, which was also found to have more primitive dental and jawbone features than A. afarensis. Early australopithecines tended to have delayed incisor formation (much like great apes). And besides having slow-growing teeth and opposable big toes like extant great apes, the other toes of *A. deyiremeda *are more curved than those of *A. afarensis, *suggesting that they had a stronger grasp on branches.

The diets of Lucy and her neighbor species were also compared, by analyzing tooth enamel for carbon fixation in photosynthesis (the process through which carbon dioxide from the atmosphere is fixed, or converted, into organic compounds such as sugars). Through the C3 carbon fixation pathway, the most common type, plants fix CO2 into a three-carbon compound known as phosphoglycerate. Through the C4 pathway, on the other hand, plants convert CO2 into a four-carbon compound. C3 plants include grass crops such as wheat and barley (which survive in cooler climates with more rainfall), while C4 plants include crops like maize, millet and sugarcane (which thrive in dry heat).

*A. afarensis *tooth enamel analysis shows that it ate both C3 and C4 plants, while *A. deyiremeda *relied more heavily on C4 plants that tolerated the shift towards a hot, dry climate during the Pliocene and early Pleistocene. This makes *A. deyiremeda *something of a mashup. Though it was a more arboreal species than the partly arboreal (but more terrestrial) Lucy, both shared larger post-canine teeth that indicated a step forward in evolution. It probably means that these teeth evolved as adaptations to changing diets that were not necessarily aligned with the emergence of a bipedality.

“Although mid-Pliocene hominin paleobiology and locomotion remain incompletely understood the BRT foot confirms the presence of multiple bipedal forms during the Pliocene,” Haile-Selassie said. “Further postcranial fossil discoveries, particularly of early species […] will be crucial to fully understanding the origins of obligate human-like bipedality.”

Elizabeth Rayne is a creature who writes. Her work has appeared in Popular Mechanics, Ars Technica, SYFY WIRE, Space.com, Live Science, Den of Geek, Forbidden Futures and Collective Tales. She lurks right outside New York City with her parrot, Lestat. When not writing, she can be found drawing, playing the piano or shapeshifting.