Early hominin Australopithecus afarensis may not be our human ancestor after all

One of the most recognisable names in human evolution might not be as related to us as we thought.

When it was discovered in 1974, the Australopithecus afarensis fossil that came to be called ‘Lucy’ was heralded as one of the first humans. For more than 50 years, this skeleton has graced the halls of museums around the world as a landmark in our journey from apes to becoming Homo sapiens.

More recent discoveries have slightly altered this picture. These suggested that while Lucy was not one of the earliest humans, her species was still thought to be an ancestor to our own lineage.

However, newly studied remains from another species of ancient hominin, known as Australopithecus deyiremeda, have called even this into question. It now seems more likely that this species is instead descended from an earlier member of the family, Australopithecus anamensis.

If that’s the case, then A. afarensis might not be the ancestor of all later human species. Dr Fred Spoor, one of our Research Leaders in human evolution, says that this idea “will cause quite a stir” among scientists.

“For decades, we’ve been inundated with textbooks and documentaries that say that Lucy and her relatives are our ancestors,” Fred explains. “Even when A. anamensis was named, it was seen as the ideal ancestor for A. afarensis so was slotted into the evolutionary tree behind it.”

“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.”

The findings of the study were published in the journal Nature.

New fossils from Australopithecus deyiremeda

The roots of this new arrangement of the family tree lie in Burtele and Waytaleyta, two adjacent dig sites in northern Ethiopia. It was here, in 2015, that researchers found a handful of fossils that they described as a new species A. deyiremeda.

As there were so few fossils, some scientists questioned whether this species should have been named at all. Digs in the surrounding area had only turned up hominin fossils from Lucy’s species, leading them to suggest that these bones might have been misidentified.

New fossils of A. deyiremeda, including jaw and tooth fossils, have now been unearthed that confirm its status as a species. These have then been used to confirm that the earlier discovery of a fossil foot from the same rocks was also likely from the same species.

While it’s hard to be completely certain without a full skeleton containing both jaw and foot bones, Fred believes that it’s justified to connect these fossils because they’re the only Australopithecus fossils found at Burtele and Waytaleyta.

As the researchers examined the remains of A. deyiremeda more closely, they realised something surprising. It appears this hominin is more closely related to Lucy’s ancestor, A. anamensis, than Lucy’s species.

The simplest way to explain this unexpected finding is that A. anamensis was at the base of this family tree, giving rise to both Lucy’s species and A. deyiremeda.

This unexpected result suggests that, contrary to decades of scientific thought, Lucy’s species might not have been the ancestor of all later Australopithecus and our own genus, Homo. More fossils will be needed to confirm exactly how this finding affects the wider hominin family tree.

Which ancient human species lived in eastern Africa?

These new fossils also show conclusively that multiple species of ancient hominin were living in eastern Africa between 3.3 and 3.5 million years ago.

While A. afarensis and A. deyiremeda lived in what’s now Ethiopia, another species called Kenyanthropus lived much further south on the western side of Lake Turkana in Kenya.

The co-existence of the two Australopithecus species suggests that they weren’t directly competing with each other. Chemical analysis of their teeth shows this might have been because they had different diets, with A. deyiremeda feeding on leaves, fruits and nuts and A. afarensis eating a mixed diet, including grains and grasses.

The structure of their feet also suggests that they lived different lifestyles. While A. afarensis mostly walked around on two feet, A. deyiremeda would have spent a lot of time climbing trees.

“While these species had different diets and behaviours, it’s still likely that they crossed paths,” Fred adds. “To what degree, however, we don’t know.”

“Interbreeding between human species did occur, but we’re not able to extract DNA from fossils this old so we can’t say if it happened here. It’s equally possible that they largely avoided each other, similar to chimpanzees and gorillas where they overlap today.”

The lead author of this new study, Professor Yohannes Haile-Selassie, is set to return to Ethiopia and the surrounding region in the coming months on the hunt for more Australopithecus fossils, which will help to reveal more about the lives of these ancient humans.

“We need more fossils of A. deyiremeda and A. anamensis to answer the questions we still have about them,” Yohannes says. “We still don't know what the foot of A. anamensis looked like, for example, which could tell us if it had an opposable big toe like A. deyiremeda and so how related they are.”

“Finding out is very important to improve what we know about these species, and how they relate to each other.”