For a half century, the iconic “Lucy” fossil species, Australopithecus afarensis, has held the title of being the most likely direct ancestor of all humans.
But as the list of ancient human relatives has grown and more fossils have been discovered, Lucy’s position has increasingly been called into question. Now, a key paper published last month in the journal Nature could overturn that theory entirely, some scientists say.
The proposal has revealed intense disagreements in the field. Some say A. anamensis is our direct ancestor, others argue that we don’t know which Australopithecus species we descended from, and still others say the new analysis doesn’t shake up the human family tree at all.
The new discovery is “not altering our picture of human evolution in any way, in my opinion,” Zeray Alemseged, a paleoanthropologist and professor of organismal biology and anatomy at the University of Chicago who was not involved in the new study, told Live Science.
Either way, a resolution might not come until more fossils are found.
An iconic species
Understanding the roots of the debate requires going back a century. In 1925, Raymond Dart announced the discovery of the first known Australopithecus — a skull dubbed the Taung Child unearthed in what is now South Africa that dates to around 2.6 million years ago. For the next 50 years, researchers thought that humans descended directly from the Taung Child’s species, Australopithecus africanus.
But Lucy’s discovery in 1974 at the Hadar site in Ethiopia rewrote that picture. The 3.2 million-year-old fossil became the oldest known australopithecine specimen at the time.
And researchers found her species, A. afarensis, walked upright on two legs similarly to how humans do today, yet it had a smaller brain — about the size of a modern-day chimp’s. This suggested Lucy’s kind could represent a “halfway” point in human evolution between the last common ancestor with chimps and us, making her species a good candidate for our direct ancestor among the many known hominins, the lineage that encompasses humans and our closest relatives.
Then, in 1979, her status as our direct ancestor was cemented: an assessment of the evolutionary relationships among hominin fossils uncovered until that point suggested Lucy’s species gave rise to the genus Homo. In that family tree, A. africanus was demoted from our ancestor to a more distant cousin.
As more australopithecines have been unearthed, the Australopithecus family tree has become bushier and more tangled, complicating the picture of who we may have descended from. But for many anthropologists Lucy’s species still reigns, eventually giving rise to the lineage from which modern humans evolved.
Then the new Nature paper was published. Researchers had unearthed new fossil fragments and tied them to a previously discovered, enigmatic 3.4 million-year-old fossil known as the “Burtele foot”.
The new tooth and jaw fragments allowed anthropologists to ascribe the foot, for the first time, to a little described and controversial species — Australopithecus deyiremeda, a tree-climbing ancient human relative that walked upright on two legs and lived alongside Lucy’s species 3.5 million to 3.3 million years ago at the Woranso-Mille site in Ethiopia.
For Fred Spoor, a professor of evolutionary anatomy at University College London who was not involved in the recent study of the Burtele foot, the new discovery was the nail in the coffin for the theory Lucy’s species was our direct ancestor.
That’s because the paper suggested that the species tied to the Burtele foot and the South African A. africanus were more closely related to each other than either was to Lucy’s species. By that logic, then, A. africanus may not have descended from Lucy’s species, but was rather her cousin.
So, it’s possible that both A. deyiremeda and A. africanus descended from the more ancient A. anamensis, who lived in East Africa from around 4.2 million to 3.8 million years ago.
This would also make A. anamensis the direct ancestor to humans, Spoor told Live Science in an email.
For Spoor, this finding would have huge implications. “If this is correct, A. afarensis will lose its iconic status as the ancestor of all later hominins,” probably including us, Spoor wrote in an accompanying commentary about the recent research.
A fierce debate
But other anthropologists are hotly divided on the implications of the new paper.
Some Live Science spoke to thought Spoor’s conclusions were plausible, while other experts said they were “far-fetched” and “a stretch, to put it mildly.”
Because the existing fossil record in East Africa goes much further back in time than the current South African record, many believe the Homo genus arose in East Africa.
Currently the oldest known Homo fossil is a 2.8 million-year-old jawbone from Ethiopia, but models estimate the genus would have actually emerged around 0.5 million to 1.5 million years earlier.
This is older than many of the earliest South African hominin fossils, which were found thousands of miles away. That “would make it unlikely that any of those are the direct ancestor,” Carol Ward, the Curators’ Distinguished Professor of pathology and anatomical sciences at the University of Missouri, told Live Science.
Lucy’s species is still a candidate, but no longer the candidate.
Lauren Schroeder, University of Toronto Mississauga
For many, the most likely candidate for an East African ancestor is still Lucy’s species, A. afarensis, which lived in modern-day Ethiopia, Tanzania and Kenya from around 3.9 million to 3 million years ago. This wide geographic distribution and persistence for almost a million years means it had many opportunities to give rise to other species across Africa, Alemseged said.
Scientists in the “Lucy” camp argue that A. afarensis‘ fully upright mode of walking, broad diet, use of early stone tools and wide geographic range constitute strong evidence for Lucy’s ancestral position in the human family tree.
This makes Spoor’s claim that Lucy’s species wasn’t our direct ancestor a big one. But he isn’t alone in this view.
Thomas Cody Prang, an assistant professor of biological anthropology at Washington University in St. Louis and a co-author of the Nature study, said it’s possible A. afarensis evolved human-like features completely independently of modern humans, like how bats and birds independently evolved wings. Such convergent evolution has been proposed before in our family tree: For example, Prang’s team previously found that A. afarensis and modern humans independently evolved certain body proportions.
If this is true, other species living at roughly the same time as Lucy’s kind are likely ancestors to later hominins, Prang told Live Science in an email.
Prang, for his part, thinks A. deyiremeda‘s anatomy makes the species a better candidate for our direct ancestor than Lucy. That’s because the species has a combination of ancient and new traits. What’s more, a 2015 analysis flagged A. deyiremeda as being more closely related to Homo than Lucy’s species.
Others think the Nature paper resurrects A. africanus as a plausible ancestor to Homo.
Lauren Schroeder, a paleoanthropologist at the University of Toronto Mississauga and who was not involved in the new study, said that either way, many different hominin species were evolving and intermingling across Africa during this 3.5 million to 2 million period of time. That means our evolutionary history is more like a braided stream, with species separating and then recombining, and less like a straight evolutionary line.
“Early Homo could have emerged from a broader, pan-African pool of australopith diversity. So yes, Lucy’s species is still a candidate, but no longer the candidate,” for a direct human ancestor, Schroeder told Live Science in an email.
Even the authors of the new paper disagree on its implications. While Prang supports the dethroning of Lucy’s species as our direct ancestor, the study’s lead author Yohannes Haile-Selassie, a paleoanthropologist and director of Arizona State University’s Institute of Human Origins, insists that Lucy’s species is still the best candidate for the direct ancestor to Homo.
He told Live Science in an email that the more ancient traits found in A. deyiremeda and A. africanus, like having feet adapted for climbing trees, contradict the idea that they are our direct ancestors. On the other hand, Lucy’s species had more human-like feet, which Haile-Selassie said makes A. afarensis the “more likely ancestor of those which came later.”
Of course, it’s possible the smoking gun evidence that settles the debate will never come.
“We will almost certainly never know who our direct ancestor is — and the more we learn about human evolution and how diverse our past was, the more elusive that ancestor becomes,” said Ward.
But that doesn’t mean we’ll ultimately understand less of our evolutionary past, Ward said. “Even though we may never know which one was our ancestor, we can still piece together much of what that ancestor may have been like.”
