South African paleoanthropologist and National Geographic explorer Lee Berger described finding soot-covered walls, fragments of charcoal, burned antelope bones and rocks arranged as hearths in the Rising Star cave system, where nine years earlier the team uncovered the bones of a new member of the human family, Homo naledi.

Control of fire is considered a crucial milestone in human evolution, providing light to navigate dark places, enabling activity at night and leading to the cooking of food, and a subsequent increase in body mass. When exactly the breakthrough occurred, however, has been one of the most contested questions in all of paleoanthropology.

“We are probably looking at the culture of another species,” said Berger, who dispensed with scientific convention by reporting the discoveries not in a peer-reviewed journal but in a press release and a Carnegie Science lecture at the Martin Luther King Jr. Memorial Library in Washington on Thursday. In an interview with The Washington Post, Berger, a professor at the University of the Witwatersrand in Johannesburg, said formal papers are under review and added, “There are a series of major discoveries coming out over the next month.”

He stressed that his team’s discoveries this summer answer a critical question raised when they announced the initial trove of 1,500 fossil bones: How did this ancient species find its way into a cave system about 100 to 130 feet below ground, a place that is devilishly hard to reach and, in his words, “horrifically dangerous”?

The research team now believes H. naledi used small fires in chambers throughout the cave system to light their way. Berger based the claim in part on his personal journey through the cave’s narrow passages, which required him to shed 55 pounds.

Moreover, he argued that use of fire by a human relative with a brain little bigger than a large orange upsets the traditional story of our development. For years, experts portrayed evolution as “a ladder” that moved ever-upward toward species with larger brains and greater intelligence, while leaving smaller-brained species to perish.

But evidence has been building that the process may have been messier than thought, a view that would be bolstered if indeed this smaller-brained contemporary of early Homo sapiens was advanced enough to use fire.

Berger’s lecture, accompanied by photographs from the cave but not by carbon dating and other traditional scientific methods, drew criticism, as have some of his previous assertions about the H. naledi fossils.

“There’s a long history of claims about the use of fire in South African caves,” said Tim D. White, director of the Human Evolution Research Center at the University of California at Berkeley, who is a past critic of Berger’s. “Any claim about the presence of controlled fire is going to be received rather skeptically if it comes via press release as opposed to data.”

Past reports of humankind’s early use of fire, even those accompanied by scientific evidence, have proved contentious. In 2012, archaeologists using advanced technology reported “unambiguous evidence in the form of burned bone and ashed plant remains that burning events took place in Wonderwerk Cave” in South Africa roughly 1 million years ago. Critics questioned that age estimate, and scientists revised the date to at least 900,000 years old after using a complex technique called cosmogenic nuclide dating.

White said rigorous studies must date both the evidence of fire and the H. naledi bones if Berger’s team is to demonstrate that both come from the same period. Other studies must show not just the presence of fire, but its controlled use. Testing would need to establish that the material believed to be soot actually is soot and not discoloration caused by chemicals or other factors.

Berger acknowledged that one of the major challenges facing him and his colleagues will be dating the materials they’ve found. So far, they’ve said the H. naledi bones date to between 230,000 and 330,000 year ago, though Berger stressed that those dates should not be viewed as the first or last appearances of the species.

White appeared most skeptical about the lack of stone tools found in the caves. He said archaeologists would expect to find thousands of stone tools in a place where human relatives were using fire for light and cooking.

“I will tell you at this stage there are no stone tools that we’ve found in the presence of a hearth,” Berger said in the interview. “That is an odd thing.” Nonetheless, he told the audience at the Carnegie Science lecture, “Fires don’t spontaneously start 250 meters into a wet cave, and animals don’t just wander into the fires and get burned.”

He said stone tools have been found in the general landscape outside the caves. He also pushed back against criticism that what the team has found does not constitute proof of an ancient hearth.

“We found dozens of hearths, not just one,” Berger said when asked about the evidence during the interview. “It’s 100 percent. There’s no doubt. … We’re now entering a phase where this goes from just bones to a rich understanding of the environment they lived in.”

Berger previously ran into controversy during the initial announcement of the discovery of H. naledi, when he suggested that these ancient relatives were deliberately using the caves as a place to lay their dead. Despite the debate, Berger repeated the claim at several points during the lecture, acknowledging that it was “perhaps not very well received by most of the academy.”

Other researchers said that even though much testing remains to be done, the latest finds at Rising Star are impressive.

“I think it’s terrific. It looks very convincing,” said Richard W. Wrangham, a professor of biological anthropology at Harvard University and author of the 2009 book “Catching Fire: How Cooking Made Us Human.”

“Of course it’s fascinating because of the small and generally mysterious nature of these people.”

Wrangham said that when the discovery of H. naledi was announced, he was discussing the dark caves where the bones were found with one of Berger’s colleagues and remarked, “Surely this must mean that they had light.”

However, Wrangham said he remained puzzled on one matter: “How did they put up with the smoke? Was there a draft that pulled smoke out of the cave?”

Wrangham said he is willing to take Berger at his word about the use of fire, based on the early evidence. He said the strongest evidence for early control of fire, however, comes from an archaeological site in Israel called Gesher Benot Ya’aqov, where experts say early human relatives used fire to cook fish about 780,000 years ago.

During the lecture, Berger also shared vivid descriptions of some of the 50 H. naledi individuals the team has found.

He described the fossil bones of a hand “curled into a death grip”; the skull of a child found sitting atop a shelf in the rock; and the skeleton of another child tucked into an alcove in one of the chambers. The dramatic images required an equally dramatic journey through a slit in the dolomite that narrows to just seven inches and requires extreme contortion of an explorer’s body.

“You’re basically kissing the ground,” said Keneiloe Molopyane, a 35-year-old researcher at the South African university’s Center for the Exploration of the Deep Human Journey. Explorers, she continued, emerge onto a perilous ridge about 65 feet above the cave floor. Inside, it’s pitch black, with “bats whizzing by you on either side. If you fall, you belong to the cave.”

The reward, however, is a feeling Molopyane vividly recalled from her first descent into the cave system: “Oh, God. I am the first person to see these remains in I don’t know how many thousands of years, and now I am touching them.”

Berger said roughly 150 scientists around the world are taking part in the effort to excavate, date and study the remains and artifacts found at the Rising Star cave system.

Asked to speculate on the interactions and possible conflicts that may have taken place between H. naledi and H. sapiens, Berger replied, “Everything you just asked, within the next 36 months, we will have answers.”