But the findings, announced Monday at the Richard Leakey Memorial Conference at Stony Brook University in New York and in three papers posted online, triggered the kind of fierce debate that has followed H. naledi since its discovery made headlines in 2015. Experts not involved in the work are sharply divided over the evidence that H. naledi was burying its dead. Several also said there was no evidence that the engravings were thousands of years old.

Still, another expert said the extreme difficulty involved in entering the cave makes it highly unlikely that anyone but H. naledi would have carved the symbols.

The dispute reflects how much is at stake ― for our understanding of ancient history, and for the reporting of other potentially important discoveries. If true, the latest study would overthrow the dogma that burial of the dead is the sole province of our species, Homo sapiens (Latin for “wise human”).

“This is a great moment in human history,” declared Lee Berger, the South African paleontologist and National Geographic explorer-in-residence who co-wrote all three papers. People have wondered, Berger said, “‘What will we do when we meet another culture as complex as us?’ Well, you just did.”

Scientific tests have dated H. naledi bones found in the cave at between 236,000 and 335,000 years old. That’s much older than the 92,000-year-old graves at the Qafzeh cave in Israel, often cited as the earliest known human burials.

Neanderthals disposed of their dead 430,000 years ago at Sima de los Huesos (“Pit of Bones”) in Spain. But Berger stressed that burial is “massively more complex and significant” than mere disposal.

“What you are seeing here is a repeated ritual, a response to the dead that has meaning,” he said.

The implications are remarkable, because some scientists have connected larger brain size to the advanced behaviors that set humans apart from other primates. Yet H. naledi’s brain was about 450 to 600 cubic centimeters, or one-third the size of a modern human’s. For comparison, H. naledi’s brain was about the size of a grapefruit, and a human brain is the size of a medium cantaloupe.

Deliberate burial or accidental deaths?

At times throughout his 30-year career, Berger has drawn vigorous criticism for announcing or publishing research before gathering sufficient supporting evidence. He, in turn, has criticized the practice of waiting years to share discoveries with the public, calling it “elitist.” The new findings, he said, come “not just from Lee Berger. This is a massive group of some of the brightest minds on this planet.”

In the papers, Berger and his team describe how different layers of sediment are mixed in the areas around the bodies, which indicates the digging and filling of a grave. They also point to the discovery of a baby with intact vertebrae, including the parts of each vertebra, which are unfused in an infant. Those bones “would have almost certainly separated had the body not been buried,” said Frederick Grine, a professor of anthropology and anatomical sciences at Stony Brook University who was not part of the study team.

“Much to my delight, I found the burial paper to be convincing,” Grine said.

Michael Petraglia, director of the Australian Research Center for Human Evolution at Griffith University, said that while he doubts the new research will persuade all scholars, “my view is that the evidence for interments in pits is convincing. The pits are reminiscent of burials of Homo sapiens and Neanderthals.”

But Aurore Val, a full-time researcher at the French National Center for Scientific Research in Aix-en-Provence, said that while she was impressed by the amount of work that went into collecting data, “I wasn’t convinced back in 2015, and I’m not convinced today. … I think at this stage we are not at the point with the evidence where we can confidently exclude other scenarios that don’t involve mortuary practices.”

For example, Val said, rain or a mudflow could have carried bones from another part of the cave to the chamber where they were found, though researchers say they have seen no geological evidence to support either possibility.

Val said individuals could have entered the cave seeking refuge from predators or uncomfortable temperatures, and then been killed by an animal or a partial cave collapse. All would leave bones; none would amount to burial.

“I think the evidence for deliberate burial is interesting,” said Bernard Wood, university professor of human origins at George Washington University, “but I don’t think it’s decisive.”

One major point of contention has been the difficulty H. naledi would have had in transporting its dead to the cave chambers 300 feet below ground. The descent requires squeezing through passages as narrow as seven inches.

Berger, who had to lose 55 pounds to make the descent and then suffered a torn rotator cuff on the way out, said researchers have been able to take cameras and gear through the passages. Moreover, H. naledi had a smaller body better suited to the task ― just 5 feet in height with long legs, apelike shoulders and hands and feet similar to our own.

“Okay, they are smaller than us, but the spaces they have to crawl through are really tiny,” Val said. “And somehow they are pushing or dragging a dead person in front of them or behind them and doing that presumably for generations.”

Hash marks and hearths

Most experts interviewed were not persuaded by claims that engravings found on the cave walls were made by H. naledi between 241,000 and 335,000 years ago, which would make them among the oldest symbols ever discovered.

“In theory, those rock engravings could have been made by cavers in the 1930s,” Wood said. “They have jumped to the assumption that they were made by H. naledi.” And Petraglia wrote in an email, “I think it’s entirely possible that Homo sapiens was in these caves.”

However, researchers who have explored the cave system say there’s no evidence that anyone aside from a few modern cavers has entered the chambers since the time of H. naledi.

“I see no reason to doubt that these markings were made by the people who came into there and left the bodies,” said British archaeologist Paul Bahn, author of the 2016 book “Images of the Ice Age.”

Bahn pointed out that similar crosshatching marks were discovered in Gorham’s Cave in Gibraltar and are believed to have been left behind by Neanderthals more than 39,000 years ago. Moreover, he said, there is evidence that some hominins were willing to put themselves “through physical hell to leave an image for people to see.” He pointed to engraved figures discovered in a portion of Pergouset Cave in France that can only be reached by crawling about 425 feet through a narrow passage.

Berger said that he and his colleagues have adopted “the most plausible hypothesis” and that critics must prove that someone else entered the caves and made the engravings. “This does not say, ‘Bob was there’ or ‘Kilroy was there,’” he said.

“I am building with National Geographic a big global team of the very finest people who work on both interpretation of engravings, but also how to date them,” he added.

At a lecture last fall, Berger described finding fragments of charcoal, burned antelope bones, rocks arranged as hearths and soot marks on the cave walls, all indicating that the ancient hominins were using fire for light and cooking. These discoveries, however, have not been published, and Berger said efforts to date the soot marks will be difficult, because the radiocarbon method can only be applied to organic materials up to 60,000 years old.

Humans who have entered the cave describe a harrowing descent that must have required great skill and determination from ancient hominins.

“My adrenaline is pumping and my mouth is dry and I need that last sip of water,” said Keneiloe Molopyane, a National Geographic explorer and lead excavator, describing the entrance to a section of the cave called the chute labyrinth. “Essentially, it’s a crack in the ground with major dolomite points sticking out of it, grabbing onto you, preventing you from falling down to your death.”

The floor of the cave is damp, the air cool and dusty with a smell “like after it’s rained,” said Sarah Johnson, a graduate student at the University of Wisconsin-Madison who has made a dozen descents.

“It’s surreal sitting in this area where you know there were ancient hominins sitting thousands of years ago,” Johnson said. “It’s very humbling.”