An extinct human species that lived hundreds of thousands of years ago may have deliberately buried its dead and carved meaningful symbols deep in a South African cave—advanced behaviors generally deemed unique to Neanderthals and modern Homo sapiens. If confirmed, the burials would be the earliest yet known by at least 100,000 years.
The claims, made today in two research papers uploaded to the preprint server bioRxiv, were also announced by paleoanthropologist Lee Berger at a conference at Stony Brook University in New York.
The publications come eight years after Berger first reported the discovery of a new hominin species inside the Rising Star cave system 25 miles northwest of Johannesburg. Named Homo naledi, the species is characterized by its small size—including a brain roughly a third the size of today’s humans—and a baffling mix of very old and relatively modern anatomical features.
The skeletal remains discovered in the cave are concentrated in a single, hard-to-reach subsystem and are dated to between 335,000 and 241,000 years ago—a period when modern humans were just beginning to emerge in Africa.
"We've found the cultural space of a non-[modern] human species," Berger says. The research is sponsored by the National Geographic Society, where Berger is Explorer in Residence.
Berger’s team raised the possibility of intentional burials in 2015 when it first announced the discovery of Homo naledi. That seemed the most plausible explanation for how more than 1,800 bone fragments ended up deep in an underground chamber reachable only by a four-story vertical drop through a 7.5-inch-wide slot—the length of a pencil—they dubbed the Chute.
Moreover, the position and intactness of some skeletal remains suggested that the dead may have been carefully laid out on the floor of the chamber rather than tossed down the Chute to collect as a jumble of bones at its base.
Many experts were skeptical that a small-brained hominin could engage in such human-like behavior, suggesting instead that the remains washed into the cave or were carried inside by predators. But the bone fragments showed no traces of gnaw marks, and analysis of the cave environment and sediments ruled out water deposition.
Other skeptics suggested that modern humans, who likely overlapped with Homo naledi in southern Africa for at least 50,000 years, may have carried the bodies in through the Chute or some other passage that has since collapsed. But the Rising Star team found no signs of modern humans and no evidence of a secondary entrance.
The researchers returned to Rising Star in 2017 and began making a series of discoveries that have not been fully revealed until now. They include concentrations of Homo naledi bone fragments that may belong to one or more individuals in shallow pits that cut through the layers of the cave floor and don’t follow its natural slope—evidence suggesting that the pits were dug. In addition, the composition of fill material in the pits differs from surrounding sediments.
One group of bones was excavated in complete blocks and stabilized in plaster. CT scans of the blocks revealed the remains of at least three individuals, including an older juvenile. The youth’s remains appear undisturbed and remarkably intact, including 30 teeth in the correct order, two series of partial ribs, a right foot, ankle, and lower limb bones. Near a partial right hand is a rock that researchers hypothesize may be a stone artifact or tool, but some outside experts dismiss the association outright.
The ‘peculiar’ primate
Arguments around deliberate interment of the dead often hinge on differences between what scientists call mortuary behavior and funerary behavior, says André Gonçalves, who studies how animals interact with the dead. Chimps and elephants, for example, display mortuary behavior when they keep watch over a dead body or physically interact with it expecting it to come back to life.
Funerary behavior, by contrast, involves intentional social acts by beings capable of complex thought who understand themselves to be separate from the natural world and who recognize the significance of the deceased. Until now, the earliest recorded evidence for funerary behavior and intentional burial among hominin species—including both modern humans and Neanderthals—was at least 100,000 years after Homo naledi.
“Humans are really peculiar as a primate because we bury our dead,” Gonçalves says. “No other primate seems to do it.”
External experts who reviewed the papers for National Geographic raised a variety of concerns around the evidence for deliberate burial. Some still maintain that water could have washed the bone fragments into natural depressions in the cave floor, which then filled with sediment over the years.
But, says anthropologist John Hawks, a Rising Star team member and co-author of the papers, “The strongest evidence we have is that the burials disrupt the existing stratigraphy in the cave.”
Another critique involves the state of the bones, most of which are dispersed and disconnected. “Most of the displacements can’t be explained by the natural course of decomposition,” says paleoanthropologist María Martinón-Torres, who studied the oldest-known human burial in Africa.
The new discoveries, however, have somewhat shifted the opinion of anthropologist Chris Stringer. “I might have been one of those people who's been skeptical about the idea that a small-brained creature like Homo naledi could be going deep into the cave to dispose of its dead,” he says. “But I have to say, on the amount I've seen so far, that yes, it does change my view on the balance of probability.”
For Gonçalves, who finds the discoveries “promising” but is also keeping a wait-and-see attitude, the idea that Homo naledi engaged in human-like behaviors isn’t especially surprising given how close in space and time the small hominins were to modern humans. “We’re separated from chimps and bonobos by six million years,” he says. “Three hundred thousand years is nothing.”
Writing on the wall?
In a second paper, researchers describe another new discovery: abstract shapes and patterns etched into the cave walls near the presumed burials. The inscribed surfaces appear to have been prepared with a substance and smoothed, and some of the markings seem to have been erased and engraved over, indicating that they were made over a period of time.
The nature of the cave’s dolomitic limestone walls make dating very difficult, and researchers concede that it will be “challenging to assess whether the engravings are contemporary with the Homo naledi burial evidence from only a few meters away.”
Archaeologist Curtis Marean notes that the particular cross-hatch designs that appear on the cave walls are “very similar” to designs found in later Homo sapiens sites in the region, as well as indigenous Khoi-San imagery.
While the researchers caution that further study is needed to identify and analyze all the engravings, they point out that the production of designs—whether painted, etched, or engraved—on cave walls or other surfaces is recognized “as a major cognitive step in human evolution.”
In a third paper, Berger and his colleagues synthesize their burial and rock-art data to challenge another long-held assumption: that bigger brains mean more complex behavior, such as making tools, managing fire, and creating symbols.
The fossil record shows that relative brain size in many hominin populations increased over the course of two million years, topping off with Homo sapiens. While a modern adult male brain has a capacity of roughly 1,500 cubic centimeters, Homo naledi’s brain was less than 600.
If this small-brained hominin did in fact engage in advanced behaviors such as deliberate burial and the creation of symbols associated with those burials, the researchers argue, then brain size shouldn’t be a major factor in determining whether a hominin species is capable of complex cognition.
Many key developments in human evolution, they point out, occurred among small-brained hominins, including the creation of distinct stone tools, the initial expansion out of Africa into Asia, and the use of fire. In addition, another small-brained species, Homo floresiensis, is known to have used tools and fire. Brain structure and wiring, they argue, may have played a more important role than brain size.
While evidence for fire in Rising Star is not specifically mentioned in the papers, Berger says the team has evidence for controlled fire in the cave system, including dozens of hearths. “That place is full of soot, fire, and burned bone. It’s everywhere” he says. Carbon-dating of the evidence is planned for the future.
A 'global human conversation'
The research team’s decision to go public with their extraordinary claims without first publishing in a peer-reviewed journal is a source of frustration for some paleoanthropologists, but Berger defends the decision. The papers will eventually appear in the online journal eLife, alongside reviews and an editorial summary, making the process “transparent,” he says.
“Your readers will be able to watch as the authors—our large team—interact with reviewers and editors as part of the open access policy,” Berger explains. The authors then have the choice to keep the papers as they are, or to incorporate comments from reviewers and other scientists. “Effectively, we're letting people in to watch the review process and the way peer review works.”
Experts who reviewed the papers agree that paleoanthropology is entering a new era with a growing awareness that there are other human species who have behaviors that until quite recently we thought were uniquely “modern human.”
With it come expectations of more discoveries of how Homo naledi lived, and how they’re related to us—or not. “If this species was adapted to living in caves and going deep into caves, which is the implication in Rising Star, then there must be more evidence of it in many other sites in South Africa,” notes Stringer.
“This deserves a global human conversation,” adds Berger. “What do we do next? How do we continue? We have just discovered a cultural space of another species that's not [modern] human, that’s not in our grade level. Not like us. How do we treat it? And I'm waiting to hear that.”