Sifting through the seemingly endless sands of Ethiopia’s Afar Rift, researchers have uncovered telling remnants of a long-lost human ancestor. As reported in Nature, tooth, jaw, and foot fossils discovered at the site can now be linked to Australopithecus deyiremeda, an ancient relative that had a taste for fruit and a penchant for climbing trees. The hominin lived some 3.4 million years ago, a pivotal point in the evolutionary branch that would eventually lead to Homo sapiens.

Australopithecus deyiremeda shared the landscape with Australopithecus afarensis, the hominin species made famous by the Lucy fossil. Even though Lucy has far more acclaim, it’s likely that Australopithecus deyiremeda is a closer human relative, said Thomas “Cody” Prang, an assistant professor of anthropology who analyzed the foot structure of the species. “This species is another reminder that human evolution is more diverse than once thought,” he said.

The lead author of the study is Yohannes Haile-Selassie of Arizona State University. Funding for this project was provided by the National Science Foundation and the W.M. Keck Foundation.

Haile-Selassie and his team first uncovered the foot fragments in 2009. The shape of the toe immediately suggested that the foot — known as the Burtele foot — didn’t come from a member of A. afarensis, but the researchers couldn’t confidently assign a species based on the foot alone. Jaw bones and teeth are usually necessary to recognize a species, Haile-Selassie explained.

Further digging and sifting in the area over the years uncovered 25 teeth as well as several jaw fragments, the fossils needed for a positive ID. The teeth and jaw fragments didn’t come from the same individual as the Burtele foot, but they were close enough in place and time to strongly suggest they belonged to the same species: A. deyiremeda.

Close investigations by Prang and others offered new insights into the lifestyle of this long-ago relative. Prang traveled to Ethiopia to see the Burtele foot for himself. The big toe looked opposable, suggesting that A. deyiremeda could likely grasp and climb tree branches with relative ease. But Prang’s analysis suggested that the toe wasn’t quite as flexible as those of more ape-like hominins such as members of the genus Ardipithecus. “This toe was slightly more human-like, an important clue to A. deyiremeda’s place in our family tree,” Prang said.

            _{Fossil fragments of Australopithecus deyiremeda (Credit: Yohannes Haile-Selassie)}

Although A. deyiremeda was likely comfortable living in trees and swinging from branch to branch, it could also walk on two feet, Prang said. When it did so, it likely pushed off on its second digit rather than its big toe like modern humans do today.

Further analysis of the tooth fragments reinforced the idea that A. deyiremeda likely spent significant time in trees. Isotope readings showed that the diet was mostly plants with the C3 signature, meaning fruits and tree leaves as opposed to grasses, which have a C4 signature. “Lucy and other members of A. afarensis had a varied diet, including more C4 foods, which might suggest they spent more time eating grasses and sedges on the ground,” Prang said.

A. deyiremeda and A. afarensis had different lifestyles and body types, but they both hold an important place in our ancestry, Prang said. “Thinking about who we are and where we came from is one of the fundamental questions for humanity,” he said. 

Header image: Workers search for fossils in Ethiopia's Afar Rift. (Credit: Stephanie Melillo)