A strange ancient foot reveals a hidden human cousin

In 2009, a research team led by Arizona State University paleoanthropologist Yohannes Haile-Selassie uncovered eight bones from the foot of an ancient human ancestor in 3.4-million-year-old sediments in the Afar Rift of Ethiopia. The fossil, known as the Burtele Nature Foot, was recovered at the Woranso-Mille paleontological site and was formally introduced in a 2012 publication.

“When we found the foot in 2009 and announced it in 2012, we knew that it was different from Lucy’s species, Australopithecus afarensis, which is widely known from that time,” said Haile-Selassie, director of the Institute of Human Origins (IHO) and a professor in the ASU School of Human Evolution and Social Change.

“However, it is not common practice in our field to name a species based on postcranial elements -elements below the neck — so we were hoping that we would find something above the neck in clear association with the foot. Crania, jaws and teeth are usually the elements used in species recognition.”

Connecting the Burtele Foot to Australopithecus deyiremeda

When the Burtele foot was first described, some teeth had already been recovered from the same general area. However, scientists were unsure whether those teeth came from exactly the same sediment layer as the foot. In 2015, the team announced a new species from the region, Australopithecus deyiremeda, but did not yet assign the Burtele foot to this species, even though some of the fossils were found very close to the foot, explained Haile-Selassie.

Over the next decade, repeated field seasons and additional fossil discoveries allowed the team to build a stronger picture. Haile-Selassie said they now have enough material to confidently link the Burtele foot with the species A. deyiremeda.

Two Hominin Species Sharing the Same Landscape

The decision to place the Burtele foot in a specific species is only one part of a larger story. The Woranso-Mille site is especially important because it provides clear evidence that two closely related hominin species were living in the same area at the same time.

The Burtele foot, now associated with A. deyiremeda, is considered more primitive than the feet of Lucy’s species, A. afarensis. Unlike Lucy, the Burtele foot kept an opposable big toe, which would have been useful for climbing. On the ground, however, A. deyiremeda still walked on two legs and appears to have pushed off primarily from the second toe rather than the big toe, which is how modern humans typically walk.

“The presence of an abducted big toe in Ardipithecus ramidus was a big surprise because at 4.4 million-years-ago there was still an early hominin ancestor which retained an opposable big toe, which was totally unexpected,” said Haile-Selassie.

“Then 1-million-years later, at 3.4-million-years ago, we find the Burtele foot, which is even more surprising. This is a time when we see species like A. afarensis whose members were fully bipedal with an adducted big toe. 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, there was not just one way until later.”

Isotope Evidence Highlights Different Hominin Diets

To better understand what A. deyiremeda ate, Naomi Levin, a professor at the University of Michigan, analyzed eight of the 25 teeth recovered from the Burtele area using isotope techniques. The method begins with cleaning the tooth surface and then carefully removing only the enamel for testing.

“I sample the tooth with a dental drill and a very tiny (< 1mm) bit — this equipment is the same kind that dentists use to work on your teeth,” said Levin. “With this drill I carefully remove small amounts of powder. I store that powder in a plastic vial and transport it back to our lab at the University of Michigan for isotopic analysis.”

The findings were unexpected.

While Lucy’s species appears to have had a mixed diet, using both C3 (resources from trees and shrubs) and C4 plants (tropical grasses and sedges), A. deyiremeda relied more heavily on C3 resources.

“I was surprised that the carbon isotope signal was so clear and so similar to the carbon isotope data from the older hominins A. ramidus and Au. anamensis,” said Levin. “I thought the distinctions between the diet of A. deyiremeda and A. afarensis would be harder to identify but the isotope data show clearly that A. deyiremeda wasn’t accessing the same range of resources as A. afarensis, which is the earliest hominin shown to make use of C4 grass-based food resources.”

Dating Fossils and Reconstructing Ancient Environments

Another crucial part of the research involved pinning down the age of the fossils and reconstructing the ancient environments in which these hominins lived. Establishing how the fossil layers line up over space and time helps scientists understand when, and under what conditions, each species existed.

“We have done a tremendous amount of careful field work at Woranso-Mille to establish how different fossil layers relate, which is crucial to understanding when and in what settings the different species lived,” said Beverly Saylor, professor of earth, environmental and planetary sciences at Case Western Reserve University. Saylor led the geological work that established the stratigraphic association between the foot and Au. deyiermeda.

Juvenile Jaw Offers Clues to Growth and Development

Alongside the 25 teeth recovered from Burtele, Haile-Selassie’s team also discovered the jaw of a juvenile individual that, based on tooth anatomy, clearly belonged to A. deyiremeda. According to Gary Schwartz, IHO research scientist and professor at the School of Human Evolution and Social Change, this jaw contained a complete set of baby teeth already in place, as well as many adult teeth still developing deep inside the lower jawbone.

The researchers used CT scanning technology to visualize all of the developing teeth. Because tooth development is closely linked to overall growth patterns, this information helped the team estimate that the youngster was about 4.5 years old at the time of death.

“For a juvenile hominin of this age, we were able to see clear traces of a disconnect in growth between the front teeth (incisors) and the back chewing teeth (molars), much like is seen in living apes and in other early australopiths, like Lucy’s species,” said Schwartz.

“I think the biggest surprise was despite our growing awareness of how diverse these early australopith (i.e., early hominin) species were — in their size, in their diet, in their locomotor repertoires and in their anatomy — these early australopiths seem to be remarkably similar in the manner in which they grew up.”

How Ancient Hominins Lived Together

By combining information about movement (locomotion), diet and environment, scientists are gaining new insight into how different hominin species could live in the same region without one driving the other to extinction. Differences in how they walked, climbed and fed may have allowed them to share the landscape by using it in distinct ways.

“All of our research to understand past ecosystems from millions of years ago is not just about curiosity or figuring out where we came from, said Haile-Selassie. “It is our eagerness to learn about our present and the future as well.”

“If we don’t understand our past, we can’t fully understand the present or our future. What happened in the past, we see it happening today,” he said. “In a lot of ways, the climate change that we see today has happened so many times during the times of Lucy and A. deyiremeda. What we learn from that time could actually help us mitigate some of the worst outcomes of climate change today.”

Publication, Research Team and Funding

The paper, “New finds shed light on diet and locomotion in Australopithecus deyiremeda,” appears in the journal Nature. The international research team included scientists from Arizona State University, Washington University, St. Louis, Case Western Reserve University, Berkeley Geochronology Center, Universitat de Barcelona, University of Tampa and University of Michigan. The full list of authors are: Yohannes Haile-Selassie, Gary T. Schwartz, Thomas C. Prang, Beverly Z. Saylor, Alan Deino,Luis Gibert, Anna Ragni and Naomi E. Levin.

Funding for this work came from the National Science Foundation and the W.M. Keck Foundation. Field and laboratory research in Ethiopia was made possible through the support of the Ethiopian Heritage Authority.