NATIONAL GEOGRAPHIC
A year and a half after adding a puzzling new member to the human family tree, a team of researchers working in South Africa has offered an additional twist: the species is far younger than its bizarrely primitive body would suggest, and may have shared the landscape with early Homo sapiens.
First discovered in 2013 by two cavers exploring South Africa’s Rising Star cave system, a stunning trove of hominin remains—the single richest fossil site of its kind ever found in Africa—revealed a tiny-brained species with shoulders and a torso like an ape’s, but with some unshakably humanlike features as well. The mosaic’s name: Homo naledi, after the Sesotho word for “star.”
Now, the species’s star shines that much brighter. In papers published Tuesday in eLife, the team—led by University of the Witwatersrand (Wits) paleoanthropologist Lee Berger—provides an age range for the remains first reported in 2015: between 236,000 and 335,000 years old. The team also describes a second chamber within Rising Star that contains yet-undated H. naledi remains.
If these dates hold, it could mean that while our own species was evolving from other, large-brained ancestors, a little-brained shadow lineage was lingering on from a much earlier period, perhaps two million years ago or more. The proposed age range for the fossils also overlaps with the early Middle Stone Age, fueling a provocative, though unproven, possibility: that the stone-tool record in South Africa from that time wasn’t just the handiwork of anatomically modern humans.
“How do you know that these sites that are called [examples of] the rise of modern human behavior aren’t being made by Homo naledi?” says Berger, who is also a National Geographic explorer-in-residence. “You can imagine how disruptive that could be.”
A PUZZLE’S MISSING PIECE
When Homo naledi made its public debut in 2015, several key details about the species still lurked in the shadows. How was H. naledi related to other hominin species? Was it the “root Homo” at the base of our genus’s lineage, as elements of its body plan might suggest?
And as National Geographic reported at the time, the initial announcement frustrated scientists because of what it was missing.
“Without a date, these fossils are more curiosities than game-changers,” said William Jungers, a paleoanthropologist at the State University of New York, Stony Brook, in a 2015 interview. “Where they fit in the family tree will be influenced by their age—they're a twig, looking for a trunk.”
Some subsequent studies tried to fill the void by statistically estimating H. naledi’s age, based on how its skull and teeth compared to other hominins’. One pegged the species at about two million years old, give or take; the other, a study by Simon Fraser University researcher Mana Dembo, suggested it was about 912,000 years old ... plus or minus about a million years.
But all the while, Berger’s team was nursing a hunch that H. naledi was younger. “It’s been beautiful to watch, because as all these studies were being published, we essentially knew it was going to be less than half a million years old,” says Paul Dirks, a geologist at Wits and James Cook University.
Dirks says the team held off from going public earlier with a date because if the initial data had been wrong, skeptics would have seized on the misfire as evidence of the team’s recklessness—a criticism some already level at the team for its breakneck publishing pace. The chamber’s unusual lack of any faunal remains also meant the dating would require performing destructive sampling on the invaluable fossils themselves.
To start, they radiometrically dated some flowstones—layers of calcite laid down by running water—that had covered some of the H. naledi remains. Two labs independently showed that the flowstone was about 236,000 years old, meaning that the underlying H. naledi remains had to be older.
Getting a maximum age proved trickier; no flowstone sandwiched the fossils from below. The team arrived at an upper boundary—335,000 years old—by putting sediment grains and three H. naledi teeth through a battery of dating methods, including some based on the radiation dosage the materials had received after basking in the cave’s natural background radioactivity.
“In the end, we have tremendous confidence in the results,” says John Hawks, a University of Wisconsin-Madison paleoanthropologist who’s part of the H. naledi team. (Hawks and Berger discuss the dating, and the full story of discovering H. naledi, in the newly published National Geographic book Almost Human.)
Jungers, now a research associate at Madagascar’s Association Vahatra, says that the new dating once again emphasizes that fixing an age to fossils based on their shape poses risks. “Homo naledi (like Homo floresiensis) pounds another nail into that analytical coffin,” he says by email. H. floresiensis, nicknamed “The Hobbit,” was a diminutive, small-brained species that existed on the Indonesian island of Flores until as recently as 60,000-100,000 years ago.
Warren Sharp, a geochronologist at the University of California, Berkeley, who wasn’t involved in the study, applauds the team for the thoroughness of their effort. However, he emphasizes that by necessity, the team’s upper boundary requires modeling the cave’s radioactivity and behavior through time—a brutally difficult task.
“They may have done about as good a job as you can do with those techniques ... [but] the dates on the teeth are inherently less convincing,” says Sharp. “I’m not saying that’s the authors’ fault. I’m just saying you have to live with that.”
ANOTHER LIGHT IN THE DARK
Berger and his colleagues also announced Tuesday that Rising Star has yielded a second chamber containing H. naledi fossils, discovered during fieldwork in November 2013 by Steven Tucker and Rick Hunter, the same cavers who had found the first trove in a chamber called Dinaledi.
Marina Elliott, right, excavates the remains of Homo naledi inside the tight quarters of the Lesedi Chamber within the Rising Star cave system in 2014 when the bones were first discovered. She is being assisted by paleontologist Ashley Kruger.
About 130 additional specimens have been recovered from the Lesedi Chamber so far, representing two adults and at least one child. One of the two adult skeletons, likely a male, is remarkably complete, and includes a skull with many of its face bones preserved, filling in crucial information that was missing from the original find. Little wonder that the team named the individual Neo, after the Sesotho word for “gift.”
“[Neo] is really comparable in preservation to the Lucy skeleton,” says Hawks, referring to the famously complete 3.2 million-year-old skeleton of Australopithecus afarensis from Ethiopia. “We’re missing some parts that Lucy has; we have some parts that Lucy doesn’t have.”
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