In 2009, scientists found eight bones from the foot of a human ancestor in layers of ancient sediment at the Woranso-Mille site in the central Afar region of Ethiopia. The fossil remains, known as the Burtele Foot, were discovered by a team led by paleoanthropologist Yohannes Haile-Selassie, from Arizona State University (United States), but were not assigned to any fossil species of a human ancestor from the African continent.
A study now published in the journal Nature and led by Haile-Selassie solves the mystery and reveals that Burtele Foot belongs to the species Australopithecus deyiremeda, a new hominid fossil discovered years ago by the researcher's team at the Woranso-Mille site (Nature, 2015). Thus, the study of this fossil foot - dated to about 3.4 million years ago - reveals that A. deyiremeda was an Australopithecus that walked on two limbs (bipedalism) and also lived in trees, as indicated by the presence of a prehensile big toe like that of chimpanzees.
The international team of experts includes Professor Lluís Gibert, from the Faculty of Earth Sciences of the University of Barcelona, who is the only researcher from a European institution to sign the study. Geological analyses were decisive for dating and linking this foot to the remains of A. deyiremeda.
A fossilized foot that could not have belonged to Lucy
A. deyiremeda is a human ancestor. This scientific breakthrough put an end to the debate over whether Australopithecus afarensis - the famous Lucy discovered in Ethiopia by Donald Johanson and Yves Coppens in 1974 - coexisted with other hominid species in the Middle Pliocene.
"The site of Woranso-Mille is significant because it is the only site where scientists have clear evidence showing two related hominin species co-existed at the same time in the same area. 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 Yohannes Haile-Selassie, director of the Institute of Human Origins and professor at the School of Human Evolution and Social Change (United States).
The different faces of bipedalism in human evolution
Burtele Foot is more primitive than the feet of Lucy's species, which was fully bipedal. It retained an opposable big toe - important for climbing - and the toes were longer and more flexible, also suitable for grasping. But when A. deyiremeda walked on two limbs, it probably pushed off with the second toe and not the big toe, as we modern humans do.
"However, it is not common practice in our field to name a species based on postcranial elements - meaning elements below the neck - so we were hoping that we would find something above the neck in clear association with the foot. Jaws and teeth are usually the elements used in species recognition."
The discovery of Burtele Foot is even more surprising than that of the Ardipithecus ramidus fossil, an early hominid ancestor that still had an opposable big toe 4.4 million years ago. "This is a time when we see species like A. afarensis whose members were fully bipedal with an adducted big toe. So 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."
