Mystery foot fossil belonged to a little-known species that lived alongside Lucy

(CNN) — Scientists say they have solved the mystery of the Burtele foot, a set of 3.4 million-year-old bones found in Ethiopia in 2009. The fossils, along with others unearthed more recently, have now been linked to a little-known species that was a contemporary of the celebrated Australopithecus afarensis skeleton Lucy.

The foot bones and a jawbone with teeth still attached belonged to an ancient human relative called Australopithecus deyiremeda, a more primitive species than Lucy, according to a study published November 26 in the journal Nature.

Should they hold up to further scrutiny, the findings could knock Lucy, one of the most recognizable names in human evolution, from her important spot in the family tree.

Coexisting species

In 2009, a team led by paleoanthropologist Yohannes Haile-Selassie, a professor at Arizona State University and the study’s lead author, found the eight foot bones within 3.4 million-year-old sediments at the Burtele locality at Woranso-Mille in Ethiopia’s Afar region. The site is close to where Lucy’s partial skeleton was discovered in 1974.

The team knew the foot came from a different species than Lucy’s because it had an opposable toe, suggesting it had a greater ability to grasp and would have easily climbed trees. However, there wasn’t enough information to name a new species based on the foot fossils alone.

Not long after, Haile-Selassie, director of the Institute of Human Origins at Arizona State, found teeth and other fossil fragments dating from 3.33 million to 3.59 million years ago and in 2015 reported that they belonged to a new species researchers dubbed Australopithecus deyiremeda. This designation was met with skepticism from some experts in human evolution given the small number of fossils.

The latest study reports the discovery of further Australopithecus deyiremeda fossils and includes the puzzling foot bones in the designation.

Lucy’s species was long thought to be the only hominin to live between 3.8 million and 3 million years ago. The finds, however, provide clear evidence that two related hominin species coexisted at the same time around what is now the Woranso-Mille site, raising questions about how they managed to live together.

The study reported that A. deyiremeda would have walked on two legs, and most likely pushed off on its second digit instead of its big toe like modern humans do today.

“What that means is that bipedality — walking on two legs — in these early human ancestors came in various forms. The whole idea of finding specimens like the Burtele foot tells you that there were many ways of walking on two legs when on the ground,” Haile-Selassie said in a statement, adding that “there was not just one way until later.”

The team was also able get a sense of what A. deyiremeda likely ate by using a technique known as isotope analysis that looks at chemical signatures of different carbon forms preserved in eight of the teeth. It found that the species consumed mostly trees and shrubs, whereas Lucy ate a wider variety of food, including grass-based plants.

“This is the first time that we’re showing that Australopithecus deyiremeda and Australopithecus afarensis were able to coexist because one, they were different in terms of their locomotive adaptation, and two, they were consuming different dietary resources,” Haile-Selassie said in a video shared by Arizona State. “So, there was no competition,” he added.

The new fossils and attribution of the foot bones should result in a broader acceptance of A. deyiremeda as a genuine species, said Fred Spoor, a research leader at the Centre for Human Evolution Research at London’s Natural History Museum. Spoor was not involved in the research but wrote a commentary published alongside the new study.

Fitting A. deyiremeda into the evolutionary tree, he added, may result in some new “twists” in the human story that could see Lucy lose her “iconic status” as the ancestor of all later hominins, including our own species, Homo sapiens.

A finding that ‘will cause quite a stir’

Lucy was shorter than an average human, reaching about 3.3 feet (1 meter) in height. She had an apelike face and a brain about one-third the size of a human brain. Her fossil showcased a mixture of humanlike and apelike traits and provided the first definitive proof that ancient human relatives walked upright 3.2 million years ago.

For decades after the skeleton’s discovery, Lucy’s species was thought to be the sole common ancestor group of all later hominins, Spoor noted. But more recent discoveries of older hominins that likely walked upright have suggested that Lucy was not the earliest human ancestor, although most still think her species was ancestral to our own lineage. The new research, Spoor said, challenged that line of scientific thought; our own genus, Homo, might not have descended directly from Lucy’s species.

When researchers compared A. deyiremeda with other Australopithecus species, they found that some of its features, particularly its foot and jaw, resembled a species that Lucy was thought to have given rise to — a hominin known as Australopithecus africanus, which lived between 2 million and 3 million years ago. Other features, including A. deyiremeda’s diet, closely resembled a more primitive Australopithecus species known as Australopithecus anamensis, which lived 4.2 million to 3.8 million years ago.

It seems likely that A. deyiremeda descended from A. anamensis, and if that’s the case, then A. afarensis might not be the ancestor of all later human species, Spoor said in a statement. It’s an unexpected finding that “will cause quite a stir” among scientists, he added.

Put simply, the study suggests that A. anamensis sits at the base of this family tree, giving rise to at least three sister species (A. afarensis, A.
deyiremeda and A. africanus). Previously, scientists drew a straight line of descent from A. anamensis to Lucy’s species to A. africanus and ultimately to Homo sapiens. The presence of multiple species makes it unclear which gave rise to later hominins.

“For decades, we’ve been inundated with textbooks and documentaries that say that Lucy and her relatives are our ancestors,” Spoor said.

“The new research suggests that A. anamensis wasn’t just the ancestor of Lucy, but that many other human species could descend from it as well, including our own.”

Ryan McRae, a paleoanthropologist at the Smithsonian National Museum of Natural History in Washington, DC, said this study was the first paper to suggest the possibility that Lucy’s species could have been an evolutionary dead end — meaning it wasn’t a direct ancestor of modern humans. Such an explanation is, of course, still just a hypothesis.

“I don’t think there is enough conclusive evidence to say which of the two options, deyiremeda or afarensis, is an ancestor for sure. It’s possible neither are and there are still more species to find,” he said via email.

“Regardless of who was ancestral to whom, though, our family tree keeps getting bushier and bushier.”

Haile-Selassie plans to return to Ethiopia and the surrounding region soon to hunt for more Australopithecus fossils, which will help expand what scientists know about these species and how they relate to each other.

“We need more fossils of A. deyiremeda and A. anamensis to answer the questions we still have about them,” he said in the statement.

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