Ancient Human Relatives Buried Their Dead in Caves, New Theory Claims

In 2015, scientists reported an astonishing discovery from deep inside a South African cave: more than 1,500 fossils of an ancient hominin species that had never been seen before.

The creatures, named Homo naledi, were short, with long arms, curved fingers and a brain about one-third the size of a modern human’s. They lived around the time the first humans were roaming Africa.

Now, after years of analyzing the surfaces and sediments of the elaborate underground cave, the same team of scientists is making another splashy announcement: Homo naledi — despite their tiny brains — buried their dead in graves. They lit fires to illuminate their way down the cave, and they marked the graves with engravings on the walls.

Lee Berger, a paleoanthropologist at the University of Witwatersrand in Johannesburg and the leader of the project, said that the discovery that a small-brained hominin did such humanlike things was profound. It suggests that big brains are not essential for sophisticated kinds of thinking, he said, such as making symbols, cooperating on dangerous expeditions or even recognizing death.

“This is the ‘Star Trek’ moment,” he said. “You go out, you meet a species, it’s not human, but it’s equally complex to humans. What do you do? That’s our moment, right now.”

But a number of experts on ancient engravings and burials said that the evidence did not yet support these extraordinary conclusions about Homo naledi. The cave evidence found so far could have a range of other explanations, they said. The skeletons might have been merely left on the cave floor, for example. And the charcoal and engravings found in the cave might have been left by modern humans who entered long after Homo naledi became extinct.

“It seems that the narrative is more important than the facts,” said Maxime Aubert, an archaeologist at Griffith University in Australia.

Dr. Berger will describe the findings at a scientific meeting on Monday, and three papers detailing the evidence will be released by the journal eLife. The studies are currently under peer review, a journal spokeswoman said, and those reviews will be posted publicly when they are finished.

The Homo naledi remains were discovered in 2013 by two South African spelunkers exploring the Rising Star cave. Dr. Berger organized an expedition into the complex system of chambers and tunnels, which extends for miles underground.

“When you’re in there, it’s like you’re on a different planet,” said Tebogo Makhubela, a geologist at the University of Johannesburg who joined the team in 2014.

The researchers found a wealth of bones, but reaching them required some risky caving. Some passageways were so tight that only smaller members of the team could fit through.

All told, the researchers have found bones from at least 27 individuals. It seemed unlikely to Dr. Berger and his colleagues that they could have simply washed into the deep recesses of the cave.

In their 2015 report, the researchers suggested that Homo naledi brought the bodies there deliberately but left them on the cave floor rather than burying them, an act archaeologists call “funerary caching.” That was still a provocative claim, given how primitive Homo naledi appeared. Dr. Berger and his colleagues argued that the species belonged to a lineage that split from our own ancestors over two million years ago. While our lineage grew tall and gained a big brain, theirs did not.

At first, the scientists thought the fossils were spread out evenly across the chamber floors. But as they dug up more sediment in 2018, they observed that two fairly complete skeletons rested inside oval depressions.

And it didn’t look as if the skeletons had formed the depressions by sinking into the sediment. For example, an orange layer of mud surrounded the ovals, but it was not inside of them. Along the edges, the break looked clean.

This finding, as well as other lines of evidence, have led Dr. Makhubela and his colleagues to now conclude that the remains had been buried. “All of them seem to paint the same picture,” he said.

Until now, only humans were known to bury their dead, and the oldest known human grave dates back 78,000 years. Homo naledi lived much earlier than that. Dr. Makhubela said their fossils were at least 240,000 years old and might be as much as 500,000 years old.

The scientists also found bits of charcoal, burned bones of turtles and rabbits, and soot on the cave walls near the fossils. They proposed that Homo naledi used glowing coals to light their way into the caves and brought wood or some other fuel to burn fires. They might have cooked the animals as a meal, or perhaps as a ritual.

As these new discoveries came to light, Dr. Berger decided that he had to take a look for himself at one of the chambers, known as Dinaledi, that contained a purported grave. He had to lose 55 pounds before he could fit through the passageway. Last July, he was ready for the journey.

Dr. Berger went in alone and examined the fossils. As he made his way out, he passed a pillar. On its side, he noticed a set of hashtag-like grooves etched into the hard surface.

Getting out was harder than getting in. “I almost died,” Dr. Berger said, but managed to escape with a torn rotator cuff. Two members of the team, Agustín Fuentes of Princeton University and John Hawks of the University of Wisconsin, were waiting for him in the adjoining chamber. Dr. Berger showed them photos of the grooves he had taken.

The two scientists immediately went to their phones and pulled up the same image: an engraving in a cave in Gibraltar made by Neanderthals. It was strikingly similar to what Dr. Berger had just seen.

Based on the growing number of fossils scientists are finding in Rising Star, Dr. Fuentes said, it looks as if Homo naledi may have visited the cave for perhaps hundreds of generations, moving together into the dark depths to bury their dead and mark the place with art.

This type of cultural practice, he argued, would have demanded language of some sort. “You can’t do that without some complex communication,” he said.

But María Martinón-Torres, the director of Spain’s National Research Center on Human Evolution, said that such speculations were premature based on the evidence presented so far. “Hypotheses need to be built on what we have, not what we guess,” she said.

Dr. Martinón-Torres considered funerary caching more likely than burials, pointing out that the oval depressions did not contain full skeletons in complete alignment. If Homo naledi brought the bodies into the cave and left them on the cave floor, the bones could have become separated as the bodies decomposed. “Still, I think the possibility of having funerary caching with this antiquity is already stunning,” she said.

“I’m highly optimistic that they have burials, but they jury is still out,” said Michael Petraglia, the director of the Australian Research Center for Human Evolution. Dr. Petraglia wanted to see more detailed analysis of the sediment and other kinds of evidence before judging whether the ovals were burials. “The problem is that they’re ahead of the science,” he said.

And Paul Pettitt, an archaeologist at Durham University in England, said it was possible that Homo naledi did not bring the bodies in, either for caching or burying. The bodies might have washed in. “I’m not convinced that the team have demonstrated that this was deliberate burial,” he said.

As for the engravings and the fires, experts said it wasn’t clear that Homo naledi was responsible for them. It was possible they were the work of modern humans who came into the cave thousands of years later. “The whole thing is unconvincing, to say the least,” said João Zilhão, an archaeologist at the University of Barcelona.

One way to test these possibilities would be to collect samples from the engravings, charcoal and soot in order to estimate their age.

Dr. Hawks said that these experiments were on the team’s to-do list but could take years because there were so many samples to test. Rather than waiting, Dr. Hawks said, the team decided to present its data now and start a conversation with other scientists about how to proceed.

“For me, it’s much more important to document and to share than it is to be right,” Dr. Hawks said.

If the researchers are right, the findings will challenge some of the most important assumptions about human evolution. Humans and Neanderthals have huge brains compared with those of earlier hominins, and paleoanthropologists have long assumed that the bigger size brought major benefits. There would have to be some upside to outweigh the problems, evolutionarily speaking, of having big brains. They require a lot of extra calories to fuel, and an infants’ large heads put mothers at risk of dying during childbirth.

One benefit of a big brain might be complex thinking. Neanderthals have left behind an impressive record of cooperative hunting, tool use and other skills. And modern humans make symbols, use language and perform other feats of brainpower.

If a hominin like Homo naledi could make engravings and dig graves, it would mean brain size was not essential to complex thought, said Dietrich Stout, a neuroscientist at Emory University who was not involved in the studies.

“I think the interesting question moving forward is what exactly big brains are needed for,” Dr. Stout said.