When scientists unveiled the fossil remains of a newly discovered human species from South Africa called Homo naledilast September, the find electrified audiences around the world. It was an astonishing haul: some 1,550 specimens representing at least 15 individuals, recovered over just a few weeks of intensive excavation from the Rising Star Cave system outside Johannesburg. But it was the researchers’ favored explanation for how the remains ended up in the cave, more than the fossils themselves, that captured the public imagination and jolted the paleoanthropology community. They proposed that this creature—whose geologic age is unknown but who was clearly primitive; it had a brain the size of an orange—had deliberately disposed of its dead there. Many experts consider this behavior exclusive to our own far brainier species, H. sapiens.

Now an outside researcher has published the first formal critique of that provocative interpretation of the remains in a scientific journal. Members of the team that made the discovery dispute her claims, but other observers think that some of her criticisms are valid—and that the team has yet to make a convincing case that H. naledi deliberately disposed of the bodies in the cave.

Cavers discovered the H. naledi fossils in a chamber some 10 meters underground in Rising Star. To reach this inner sanctum, named the Dinaledi chamber, they squeezed through passages less than 25 centimeters across and climbed steep, jagged rocks in what would have been pitch darkness if not for their headlamps. How, the researchers wondered, did the fossils end up in such a remote part of the cave system?

To answer that question, geologist Paul Dirks of James Cook University in Queensland, Australia, and his colleagues analyzed the cave’s geology and features of the bones for clues. Bones can accumulate in caves by any number of mechanisms: For example, floodwaters can wash them in from their original resting place and carnivores can bring their kills in from outside. But such situations tend to produce fossil assemblages that contain a mix of animal species. And one of the most distinctive aspects of the Rising Star site is that H. naledi is the only medium or large animal species found there.

Hand of H. naledi is one of the most complete hands known for an extinct human species.

Credit: Kate Wong

In the absence of any of the telltale signs of things like flooding or carnivore activity, the researchers concluded that the best-supported explanation thus far was that H. naledi dragged its dead into the chamber, following at least part of the same arduous route the scientists took. The implication was that this extinct species with a brain a third the size of ours had an understanding of mortality—and a cultural tradition built around that concept.

That argument has met with skepticism from the outset. A number of experts expressed doubts in the popular press when Dirks, along with project leader Lee Berger of the University of the Witwatersrand in Johannesburg and their collaborators, went public with their findings in two papers published in the online journal eLife last September. But none had published their counterarguments in a peer-reviewed scientific journal—until now.

Aurore Val, a postdoctoral fellow at the University of the Witwatersrand, wrote the critique, which is in press at the Journal of Human Evolution and available online. In it she argues that it is impossible to establish—based on the evidence presented in the team’s paper on the geologic context of the H. naledi fossils and bone features that hint at their fate—that the complete bodies were disposed of inside the chamber or at its entrance in the manner the team proposes.

Val, who was Berger’s PhD student and who has published papers with Berger, Dirks and other members of the H. naledi team in the past, uses several lines of evidence described in the initial reports to make her case. Noting that the discoverers have yet to determine the fossils’ age, she contends that they cannot know what the cave was like when the remains entered the Dinaledi chamber. Caves can change dramatically over time, and Rising Star may have once allowed easier access to the chamber. Val also argues that the team did not analyze enough fossil material to rule out water transport or carnivore damage.

In a response submitted to the Journal of Human Evolution, Dirks, Berger and their teammates charged that many of Val’s criticisms are “spurious” and stem from misinterpretations of their published data. Mapping of the cave and surrounding rock indicates that there has never been a direct opening from the surface into the Dinaledi chamber, they counter, and although the geology shows the cave has changed over time, such changes have not fundamentally altered the way into the chamber.

Furthermore, Dirks and his co-authors wrote, the studies of the sediments in the chamber show that the fossils were not waterborne. And they noted that macroscopic examinations of all the fossil specimens, and microscopic inspections of more than a third of them representing all skeletal elements, did not reveal any carnivore tooth marks. Likewise, they wrote that analysis of the fractures in the fossils failed to identify a single one consistent with carnivore damage.

The fact that the Journal of Human Evolution has yet to publish the response has riled the authors, who were under the impression that it would appear simultaneously with Val’s critical commentary. According to Co-Editor in Chief Sarah Elton of Durham University in England, that was a misunderstanding on the authors’ part. She explains that publication of a response is not guaranteed. All content goes through peer review. If a response is accepted for publication, it will appear in the same print issue, but a critique may appear online before the associated response because of the journal’s production schedule. The response from Dirks and his co-authors is currently under consideration for publication, Elton says.

Outside researchers who have seen Val’s comment and the team’s response think some of Val’s claims have merit. “Caves are very dynamic systems, and it is difficult to reconstruct past structures,” says Jeffrey McKee of The Ohio State University, who has excavated other fossil human sites in South Africa. He also agrees with Val that the researchers have ruled out water transport and carnivore activity, among other possibilities, prematurely. Analysis of the taphonomy—what happened to the organisms between death and fossil discovery—“must be much more thorough,” he insists. The fact that H. naledi is the only medium-to-large animal species represented in the fossil assemblage, although unusual, is nonetheless consistent with scenarios other than deliberate interment. Another South African site, Taung, where he has worked, contains a fossil deposit that consists mostly of baboons—probably the work of leopards. Leopards often concentrate their hunting efforts on a single prey species, McKee explains. And they may do so without leaving any incriminating scratches or punctures on the bones. “Most carnivores take the guts first, so in many cases there are no marks at all,” McKee says, adding that the Taung baboon fossils show few carnivore marks. “Absence of evidence is not evidence of absence,” he says.

Martha Tappen of the University of Minnesota, who specializes in taphonomy, also shares some of Val’s skepticism. Although she endorses the team’s approach to figuring out how the bones entered the chamber—looking at competing hypotheses and seeing how their evidence stacks up against the predictions of those hypotheses—she finds their preferred scenario hard to swallow. “Their explanation of Homo naledi repeatedly carrying in their dead over years and putting them in the [chamber] is tortuous and incredible,” she asserts. Tappen wonders whether the extinct humans could have climbed down into the chamber—perhaps in search of water or to hide from a menacing carnivore or other humans—and then gotten trapped. “We may never know for sure,” Tappen muses. But one thing is clear in her view: “They need to keep digging.”

Dirks says the team is doing just that. “We are continuing exploring the cave and have found other chambers with [fossil human] remains, which could shed further light on the issue of why naledi remains occur at Rising Star,” he explains. “If every age group is equally represented in the deposit, the explanation for that is likely to be different from a situation where the age group is heavily skewed toward older and/or younger individuals, for instance. This may not necessarily prove or disprove deliberate disposal, but it will place further constraints.”

As for the burning question of how old the H. naledi remains are, Dirks says dating of the site is underway: “We are currently exploring five different techniques at seven different labs on several continents, conducting double-blind tests for three techniques to obtain maximum confidence in our results.” Although the team has been under intense pressure to ascertain the age of the material, the geology of the site is complex and the researchers want to get it right, he explains. “Hang in there,” he adds. “It won't be that long anymore.”