Scientists Finally Solve Mystery of Ancient Fossil Foot

Mysterious Fossil Foot Belonged to Ancient Human that Lived Alongside ‘Lucy’

By Humberto Basilio edited by Claire Cameron

Sixteen years ago a group of anthropologists discovered 3.4-million-year-old fossilized foot bones in Ethiopia. While they suspected the foot belonged to an ancient human that likely lived alongside the species we know as “Lucy,” Australopithecus afarensis, without a skull or teeth to analyze, they couldn’t be sure.

What they did know is that unlike Lucy, which walked upright on arched feet like our own, the mystery foot had a grasping toe that was adapted for climbing trees.

Now the same team that discovered the strange foot have finally solved the mystery. In a paper published Wednesday in Nature, the researchers describe other hominin fossils found in the same area as the appendage, which they nicknamed the Burtele foot. The findings confirm that Lucy lived alongside another hominin species called Australopithecus deyiremeda, which behaved rather differently from its A. afarensis peers.

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“It’s a really exciting discovery long-awaited for all of us who have been wondering what that crazy foot was,” says University of Missouri anthropologist Carol Ward, who was not involved in the new study.

“Not only do we have different species living at pretty similar times in a similar area but they are navigating the world in a different way from one another,” she says.

Anthropologists had suspected that the Burtele foot belonged to A. deyiremeda for years: In 2015 they reported the species’ existence in the region based on jawbones that were 3.5 million to 3.3 million years old. But to conclusively link A. deyiremeda to the Burtele foot, the team needed to return to its discovery site to find more fossils.

“We have been going to the site every year for 20 years now, and the Burtele locality is revisited every year like every locality at the site,” says Arizona State University paleobiologist and study co-author Yohannes Haile-Selassie.

During the most recent visit to Ethiopia’s paleoanthropological site Woranso-Mille, the team made several pivotal discoveries: fragments of pelvic bones and, crucially, a skull and a jawbone with 12 teeth. Identified as belonging to A. deyiremeda based on the shape of the canines and molars, the jaw showed more primitive features than its A. afarensis cousins.

After analyzing the teeth, the team found that their owner ate a different diet to Lucy, preferring to eat trees, shrubs, fruits and leaves—a diet more similar to more ancient hominins, according to the team. By contrast, Lucy’s species typically ate vegetation from mixed woodland areas and grassland plants.

The Burtele foot gives clues to how A. deyiremeda managed to deftly climb trees for sustenance: its long, curved toes and flexible bones suggest a foot well adapted for scaling and holding on to trees. Even the bones of the big toe are slender and curved, suggesting it could wrap around branches.

By combining their finds—the teeth, the dietary analysis and the foot—and taking into account the absence of other hominin fossils at the site, the scientists have concluded that the mysterious Burtele foot belonged to A. deyiremeda.

The finding gives researchers more opportunity to learn about how ancient humans adapted to walk upright, Haile-Selassie says. And, he says, it shows that not all human ancestors walked on two feet.

“It is a unique mode of locomotion that underwent various experiments throughout human evolution until the emergence of Homo,” he says.

It could also help settle another debate once and for all: the 2015 discovery of A. deyiremeda was contested, with some scientists arguing the specimens actually belonged to A. afarensis, says paleoanthropologist Donald Carl Johanson, who discovered Lucy in 1974.

The new study instead suggests that A. deyiremeda inherited its foot traits from an ancestral species different from that which gave rise to Lucy’s kind, Johanson says. “Acceptance of a new hominin species always attracts criticism,” he says. “Whether the new evidence will convince a wider audience that A. deyiremeda is a valid species remains to be seen.”

Knowing that another hominin lived alongside Lucy’s species also challenges the idea that human evolution was relatively linear, Ward says. The new findings also pose questions about how ancient hominins walked.

Haile-Selassie’s team will continue returning each year to the Burtele site to learn more about the biology and geographic distribution of A. deyiremeda. “There are many questions that we can ask about this species,” Haile-Selassie says.

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