The origins of a 3.4-million-year-old foot bone uncovered in Ethiopia may finally be elucidated, prompting a reevaluation of how various ancient human ancestors cohabited.

In 2009, Johannes Haile-Selassie and his team at Arizona State University unearthed eight hominin bones that previously constituted a right foot at a site known as Burtele in northeastern Ethiopia’s Afar region.

This discovery, dubbed Bartele’s foot, features opposable big toes akin to those of gorillas, indicating that any species could have had arboreal capabilities.

Another ancient human species, Australopithecus afarensis, was known to inhabit the vicinity, with the well-known fossil of Lucy—also discovered in the Afar region—but Bartele’s foot appeared to belong to a different species. “From the outset, we realized it was not part of Lucy’s lineage,” Haile Selassie states.

There were two primary hypotheses that intrigued Haile Selassie: whether the foot was associated with another species within the genus Australopithecus or perhaps an older, more primitive group known as Ardipithecus, which existed in Ethiopia more than a million years ago and also possessed opposable thumbs.

Meanwhile, in 2015, scientists announced the identification of a previously unknown hominid species, named Australopithecus deiremeda, after jaw and tooth remains were found in the same region. Initially, there was uncertainty about whether the enigmatic leg bone was part of A. deiremeda, as its age differed from that of the jaw and tooth remains.

However, in the subsequent year, researchers made a crucial discovery. The lower jaw of A. deiremeda was located within 300 meters of Bartele’s foot, and both sets of remains were dated to the same geological era. This led the research team to conclude that Bartele’s foot belonged to A. deiremeda.

In a separate part of the study, researchers analyzed Earth’s carbon isotopes. They found that A. deiremeda primarily consumed materials from trees and shrubs, while human teeth were more adapted for a diet rich in grasses than those of afarensis.

Haile Selassie noted that this finding suggests that both hominin species occupied the same ecological niche without competing for resources. He believes these groups could have coexisted harmoniously, engaging in separate activities. “They must have crossed paths and interacted within the same habitat, each doing their own thing,” he remarked. “While members of Australopithecus deiremeda may have spent time in trees, afarensis was likely wandering the adjacent grasslands.”

This revelation enhances our understanding of human evolution. “Historically, some have argued that only a single hominid species existed at any given time, with newer forms emerging eventually,” Haile Selassie explained. “We are now realizing that our evolutionary path was not straightforward. Multiple closely related hominid species coexisted at the same time, indicating that coexistence was a fundamental aspect of our ancestors’ lives.”

Carrie Mongul, a professor at Stony Brook University in New York, expressed enthusiasm about these developments. “Understanding more about the diversity of Pliocene hominins is truly exciting,” she stated. “This period, around 3 million years ago, was rich in evolutionary significance.”