‘Lucy’ may not be our direct ancestor after all, scientists claim, sparking heated debate

For half a century, the iconic “Lucy” fossil species, Australopithecus afarensis, has held the title of being the most likely direct ancestor of all humans.

However, as the list of ancient human relatives grew and more fossils were discovered, Lucy’s position became increasingly questionable. Now, a major paper published last month in the journal Nature could completely overturn that theory, some scientists say.

They argue that, given new evidence, an older species, Australopithecus anamensis, rather than Lucy, is our direct ancestor.

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The proposal revealed sharp disagreements on the ground. Some say that A. anamensis is our direct ancestor, while others argue that we do not know which australopithecine species we descend from. Others argue that the new analysis does not shake up the human family tree at all.

The new findings “in my opinion do not change our picture of human evolution in any way,” Zelai Alemseged, a paleoanthropologist and professor of organismal biology and anatomy at the University of Chicago who was not involved in the study, told Live Science.

In any case, a solution may not be reached until more fossils are discovered.

iconic species

To understand the roots of this debate, we need to go back a century. In 1925, Raymond Dart announced the discovery of the first known australopithecus, a skull called Taun Child, dating from about 2.6 million years ago, excavated in what is now South Africa. For the next 50 years, researchers believed that humans were direct descendants of the Town Child species, Australopithecus africanus.

But that all changed in 1974 with Lucy’s discovery at the Hadar site in Ethiopia. The 3.2 million-year-old fossil was the oldest known Australopithecus specimen at the time.

And researchers discovered that her species, A. afarensis, walked upright on two legs like humans today, but its brain was smaller, about the same size as a modern chimpanzee. This suggests that Lucy’s species may represent a “midpoint” in human evolution between our last common ancestor with chimpanzees and ourselves, making her species a strong candidate for our direct ancestor among the many known hominin lineages that include humans and our closest relatives.

And in 1979, her status as our direct ancestor was cemented. Evaluation of the evolutionary relationships among previously discovered human fossils suggested that 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.

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As more australopithecines are unearthed, the australopithecus family tree becomes bushier and more complex, complicating the picture of who we descend from. But for many anthropologists, Lucy’s species still reigns supreme, producing the lineage from which modern humans eventually evolved.

Then a new Nature paper was published. Researchers unearthed new fossil fragments and linked them to an earlier discovery of a mysterious 3.4-million-year-old fossil known as the “vertefoot.”

The new teeth and jaw fragments allow anthropologists, for the first time, to attribute the foot to Australopithecus deiremeda, a controversial species that has rarely been described. Australopithecus deiremeda, a relative of ancient tree-climbing humans, walked upright on two legs and lived with Lucy’s species at the Woranso Mir site in Ethiopia between 3.5 and 3.3 million years ago.

For Fred Spoor, a professor of evolutionary anatomy at University College London, who was not involved in the recent research on Bartele’s feet, the new findings are a nail in the coffin for the theory that Lucy’s species is our direct ancestor.

Because that paper suggested that species related to Bartele foot and South African A. africanus were more closely related to each other than either to Lucy’s species. According to that logic, A. africanus may not have been a descendant of Lucy’s species, but rather her cousin.

Therefore, both A. dilemeda and A. africanus may be descendants of the older A. anamensis, which lived in East Africa approximately 4.2 to 3.8 million years ago.

This would also make A. anamensis a direct ancestor of humans, Spoor told Live Science via email.

This discovery has great significance for Spool. “If this is correct, A. afarensis would lose its iconic status as the ancestor of all subsequent humans, perhaps including us,” Spoor wrote in an accompanying commentary on the recent study.

intense discussion

But other anthropologists are sharply divided over the meaning of the new paper.

While some at Live Science thought Spoor’s conclusions were plausible, other experts said the conclusions were “far-fetched” and “overstated to say the least.”

Many believe that the genus Homo originated in East Africa because the extant fossil record of East Africa is much older than that of modern South Africa.

The oldest known hominin fossil is a 2.8-million-year-old jawbone from Ethiopia, but models estimate that the genus actually appeared about 500,000 to 1.5 million years ago.

This is older than many of South Africa’s earliest hominin fossils, which were discovered thousands of miles away. Carol Ward, Curator’s Distinguished Professor of Pathology and Anatomy at the University of Missouri, told Live Science.

For many, the most likely candidate for an East African ancestor remains Lucy’s species, A. afarensis, which lived in what is now Ethiopia, Tanzania, and Kenya about 3.9 million to 3 million years ago. Mr Alemseged said this wide geographical distribution and persistence for almost a million years meant there was plenty of opportunity to spawn other species across Africa.

Scientists in the “Lucy” camp argue that A. afarensis’ fully upright gait, wide diet, early use of stone tools, and wide geographic range provide strong evidence for Lucy’s ancestor’s place in the human family tree.

This makes Spoor’s claim that Lucy’s species was not our direct ancestor significant. But he is not alone in this view.

Thomas Cody Prang, an assistant professor of biological anthropology at Washington University in St. Louis and co-author of the Nature study, said A. afarensis may have evolved human-like features completely independently of modern humans, just as bats and birds evolved wings on their own. Such convergent evolution has been proposed before in our family tree. For example, Plan’s team previously discovered that A. afarensis and modern humans independently evolved certain body proportions.

If this is true, other species that lived around the same time as Lucy’s species are likely the ancestors of later humans, Pran told LiveScience in an email.

Pran believes that A. dilemeda’s anatomy makes it a better candidate for our direct ancestor than Lucy. That’s because this species has a combination of old and new features. Furthermore, a 2015 analysis indicated that A. deiremeda is closer to the genus Homo than to Lucy’s species.

Others believe that the Nature paper reinstates A. africanus as a plausible ancestor of hominins.

In any case, many different human species evolved and intermingled across Africa over the past 3.5 million to 2 million years, said Lauren Schroeder, a paleoanthropologist at the University of Toronto Mississauga who was not involved in the study. That means our evolutionary history is a braided sequence of species separating and recombining, not a linear line of evolution.

“Early hominins may have emerged from within a broader pan-African australopith diversity. So, yes, Lucy’s species is still a candidate, but it’s no longer a candidate,” Schroeder told Live Science via email.

Even the authors of the new paper disagree on its meaning. Although Dr. Plan supports dethroning Lucy’s species as a direct ancestor of humans, the study’s lead author, paleoanthropologist Johannes Haile Selassie, director of the Institute of Human Origins at Arizona State University, maintains that Lucy’s species remains the best candidate for a direct ancestor of humans.

He told Live Science in an email that older features seen in A. dilemeda and A. africanus, such as legs adapted for climbing, contradict the idea that they are our direct ancestors. Lucy’s species, on the other hand, has more human-like legs, so A. afarensis is “likely an ancestor of what came later,” Haile Selassie said.

Of course, it is possible that conclusive evidence to resolve the debate may never be available.

“It’s almost certainly impossible to know who our direct ancestors are, and the more we learn about human evolution and how diverse our past has been, the more elusive that ancestor becomes,” Ward said.

But that doesn’t mean we’ll end up understanding less about our evolutionary past, Ward says. “Even though we may never know who our ancestors are, we can piece together a good deal of what they were like.”