Fossil Foot Shows Lucy Shared Space With Another Hominin Who Might Be Our True Ancestor

Hominin foot bones found in 3.4-million-year-old sediments in Ethiopia have been assigned to a recently described species. This provides further evidence of different elements of the human family tree living side-by-side in the cradle of humanity and further obscures the question of whether the best known early human fossil was a direct ancestor of ours.

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When the fossil known as Lucy was found, and eventually given the name Australopithecus afarensis, it was thought the human family tree was a straight line, and this must be our direct ancestor. However, we know the branching was as complex as the trees Lucy and her kind still sometimes climbed. For example, four years ago, evidence emerged that two sets of footprints Mary Leakey found in the same layer of volcanic ash were from different hominin species.

Those prints were made very close in time – probably the same day – so there could be no question of one species having displaced the other, even locally. Now we have another round of evidence of co-existence, this time written in bones.

When eight foot bones were found at Woranso-Mille in the Afar region, near where a volcano erupted this week, the finders could use ancient eruptions to show they dated from the same era as Lucy. 

“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 Professor Yohannes Haile-Selassie of Arizona State University in a statement. Yet the same site also included an A. afarensis, nicknamed Kadanuumuu, from the same stratum layer.

The pedal bones became known as the Burtele foot, and proved that two species more closely related to us than chimpanzees overlapped.

“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, Haile-Selassie continued. “Crania, jaws and teeth are usually the elements used in species recognition.” 

The closest Haile-Selassie and colleagues had at the time was some teeth found nearby. However, in 2015 they announced a new species, Australopithecus deyiremeda, based on other fossil specimens. They suspected the foot might have belonged to a member of this new species, but considered the data insufficient.

The discovery of several more fossils in the area that connect the foot to the fossils used to describe A. deyiremeda has given Haile-Selassie and colleagues confidence to publish.

The Burtele foot is more like those of earlier apes, still more connected to the trees, than Lucy’s, with an opposable big toe suited for climbing. That same toe would have been a hindrance when walking upright, which the authors think A. deyiremeda managed by pushing off with the second toe.

Haile-Selassie noted that when Ardipithecus ramidus was found to have retained an opposable big toe 4.4 million years ago, it was considered a surprise, given the extent of human migration to the ground. 

“Then 1-million-years later, at 3.4-million-years ago, we find the Burtele foot, which is even more surprising,” Haile-Selassie said. “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.”

When two species share space they usually need to create their own niches, for example in their primary food resources. Previous evidence has shown A. afarensis maintained a mixed diet, with both trees and tropical grasses having a significant part in their diet. Using tooth enamel from eight A. deyiremeda teeth, co-author Professor Naomi Levin of the University of Michigan concluded they lived mostly on C3 plants, which would have included fruits and nuts, but not grass seeds.

Although that is consistent with feet suited to climbing, Levin still expected more of a mixture, given the timing. “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.”

Living together would have been much easier if A. deyiremeda were only competing for half Lucy’s diet, but the findings do raise the question of why a species so much better suited to the rainforests was living outside it. 

However, a much bigger question lurks in the background. Some of the features of A. deyiremeda have more in common with A. africanus than its contemporary. A. africanus lived in southern Africa for 1.2 million years, starting not long after the Woranso-Mille fossils, and is considered likely to be our direct ancestor. 

Perhaps then A. deyiremeda was not a throwback, less adapted to a bipedal lifestyle than A. afarensis, and destined to die out earlier, having been far rarer based on fossil numbers. Perhaps instead it was A. deyiremeda that migrated south and eventually gave rise to us. For a palaeontologist in Afar, “Who’s your [great, great grand-] daddy?” is a very pertinent question.

The study is published in Nature.