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Researchers have uncovered evidence that members of a mysterious archaic human species buried their dead and carved symbols on cave walls long before the earliest evidence of burials by modern humans.
The brains belonging to the extinct species, known as Homo naledi, were around one-third the size of a modern human brain.
The revelations could change the understanding of human evolution, because until now such behaviors only have been associated with larger-brained Homo sapiens and Neanderthals.
The findings are detailed in three studies that have been accepted for publication in the journal eLife, and preprints of the papers are available on BioRxiv.
Fossils belonging to Homo naledi were first discovered in the Rising Star cave system in South Africa during excavations in 2013. The cave system is part of South Africa’s Cradle of Humankind, a UNESCO World Heritage Site encompassing an area where scientists have found fossils of multiple ancient human ancestor species — remains that are helping to unlock the story of human evolution.
Paleoanthropologist and National Geographic Explorer in Residence Dr. Lee Berger and his team of “underground astronauts” have continued their work in the extensive, dangerous caves to better understand the extinct hominins, or ancient human ancestors.
Now, the research team has discovered the remains of Homo naledi adults and children that were laid to rest in the fetal position within cave depressions and covered with soil. The burials are older than any known Homo sapiens burials by at least 100,000 years.
During the work to identify the cave burials, the scientists also found a number of symbols engraved on the cave walls, which are estimated to be between 241,000 and 335,000 years old, but they want to continue their testing for more precise dating.
The symbols include deeply carved hashtag-like cross-hatchings and other geometric shapes. Similar symbols found in other caves were carved by early Homo sapiens 80,000 years ago and Neanderthals 60,000 years ago and were thought to have been used as a way to record and share information.
“These recent findings suggest intentional burials, the use of symbols, and meaning-making activities by Homo naledi. It seems an inevitable conclusion that in combination they indicate that this small-brained species of ancient human relatives was performing complex practices related to death,” said Berger, lead author on two of the studies and coauthor on the third, in a statement. “That would mean not only are humans not unique in the development of symbolic practices, but may not have even invented such behaviors.”
Crawling through caves
Exploring the labyrinth-like Rising Star cave system and its chambers isn’t for the faint of heart.
The team has mapped over 2.5 miles (4 kilometers) of the caves so far, which have a vertical depth of 328 feet (100 meters) and expand for more than 656 feet (200 meters) in length, said the studies’ lead geologist Dr. Tebogo Makhubela, senior lecturer of geology at the University of Johannesburg.
The cave system includes deadly steep drops and tiny passageways like Superman’s Crawl, a tunnel measuring 131 feet (40 meters) long and 9.8 inches (25 centimeters) across, requiring the researchers to belly crawl their way through, said Dr. Keneiloe Molopyane, National Geographic Explorer and lead excavator of Dragon’s Back Expedition (named for one of the cave’s features).
Berger said he had to lose 55 pounds (25 kilograms) to enter the cave’s precarious chambers in 2022.
“It was the most awful and wonderful experience in my life,” Berger said. “I almost died coming out of there, but it was obviously worth it to make some of these discoveries. But, I think an important part of that, though, is that the journey would not be nearly as difficult, I think, for Homo naledi.”
Homo naledi shared some similarities with humans, like walking upright and manipulating objects by hand, but members of the species had smaller heads, a shorter stature, and were thinner and more powerfully built, Berger said.
Homo naledi’s shoulders — which were oriented for better climbing — and teeth shared similarities with earlier hominins like Australopithecus, said Dr. John Hawks, professor of anthropology and paleoanthropology at the University of Wisconsin-Madison.
The researchers have found many Homo naledi fossils throughout the caves, including the remains of very young infants and aged adults, to help them understand Naledi as a population, Hawks said. And as the team continued deeper into the caves, it became clear that Homo naledi was very familiar with and using broad parts of the cave system.
When Berger and his team announced the discovery of Homo naledi in 2015, they suggested it was possible that the species deliberately disposed of their dead in the cave.
But the idea of a small-brained hominin doing so was considered to be a very controversial hypothesis.
In 2018, the team began to find evidence that supported the idea that Homo naledi intentionally buried their dead. The scientists found ovals dug into cave surfaces resembling holes, and the remains of bodies placed inside in curled positions.
Other burial sites were dug horizontally into slopes, with bodies placed inside, showing that the remains didn’t end up there by other, nondeliberate means, Berger said.
“It’s not a body that died in a depression or hole. It was a whole body that was covered in dirt and then decayed within the grave itself, in part demonstrating that it was buried at the time as a whole flesh entity, but not by some dramatic collapse or being washed in,” Berger said. “We feel that they’ve met the litmus test of human burials or archaic human burials and the most ancient human burials, and therefore describe them as graves or burials by the nonhuman species, Homo naledi.”
And then, the team found an artifact within a burial and discovered carvings on the wall.
Carvings on the wall
Within one of the graves is a tool-shaped rock, buried next to the hand of a Homo naledi adult. Within a passage above the burials, in an antechamber, is a wall covered with rock engravings.
The deeply carved geometric shapes appear on dolomite rock walls that reach 4.5 to 4.7 on Mohs Hardness Scale, which helps researchers assess the scratch resistance of minerals. Dolomite is halfway to a diamond (at the top of the scale) in terms of hardness, which means it would have taken an extreme amount of time and effort to carve into the walls, Berger said.
The team believes that Homo naledi, and not Homo sapiens, are responsible for the carvings because there is no evidence that humans have ever been inside the caves.
Homo naledi was able to see what they were doing inside the caves by using fire. There is evidence spread throughout the caves, including soot, charcoal and burnt bone, to show that they were actively setting fires, Berger said.
Both the burials and the symbols imply that Homo naledi was capable of engaging in meaningful behaviors, said Agustín Fuentes, National Geographic Explorer, on-site biocultural specialist and lead author of the third study.
The meaning of the symbols is unclear, and researchers can’t say whether they were used as a type of language or communication within the species.
“What we can say is that these are intentionally made geometric designs that had meaning for naledi,” Fuentes said. “That means they spent a lot of time and effort and risked their lives to engrave these things in these places where they’re burying bodies.”
The naledi findings suggest that larger brains can’t be the only connection with complex behavior that researchers once assumed related only to humans, Fuentes said.
“The challenge here, then, is that we now know that Homo naledi, in addition to Homo sapiens and Neanderthals and Denisovans and a few others, were engaging in the kind of behavior that we, even just a few decades ago, thought was unique to us,” he said. “That means we need to rethink the timing of fire use, of meaning-making and of the burial of the dead in hominin history.”
Chris Stringer, research leader in human origins at London’s Natural History Museum, said that although he was previously skeptical of claims of behavioral complexity in Homo naledi and its ape-size brain, “the considerable evidence presented now by Berger and colleagues for possible burials and wall engravings cannot be easily dismissed.” Stringer was not involved in the research.
“I would certainly like to see attempts at dating the evidence for the engravings and for the fire, but if these huge claims turn out to be well-founded, they have profound implications for our reconstructions of human evolution,” Stringer said.
The findings raise many questions, including whether the behaviors were already present in an ancient common ancestor that lived much earlier than Homo naledi or humans, and why we have such big brains “if human-like behavioural complexity can be achieved with a brain less than half that size,” Stringer said.
Berger and his colleagues’ work on the discovery of Homo naledi and how it potentially changes the human family tree will be shared in Netflix’s “Unknown: Cave of Bones” on July 17 and in a book coauthored by Berger and Hawks called “Cave of Bones: A True Story of Discovery, Adventure, and Human Origins,” available August 8.
The research team is continuing its work to better understand Homo naledi, including how old the species is, whether it existed closer in time to humans than previously thought, and if there is any DNA preserved in the bones found in the cave system.