Homo naledi may have made etchings on cave walls and buried its dead

A species of ancient human with a brain the size of a chimpanzee’s may have engraved symbols on cave walls and deliberately buried its dead. These new discoveries about Homo naledi, a supposedly primitive hominin, could potentially prompt a rethink of the origins of complex behaviours once thought to be only the domain of large-brained humans like us.

“It’s a remarkable thing. My mind is blown,” says Lee Berger at the National Geographic Society in Washington DC, who led the research. “Much of what we thought about the origin of intelligence and the cognitive powers of having a big brain clearly just died,” he says, though other researchers who spoke to New Scientist question this view. 

H. naledi was discovered in 2013 in the Rising Star cave system in South Africa when two cavers squeezed through an incredibly tight passage into a hitherto-unexplored chamber littered with fossil bones. In 2015, it was declared that these belonged to a new species. We now know that this hominin was around 144 centimetres tall and had a mix of primitive and modern features, with a brain a third of the size of ours.

It isn’t yet known how H. naledi fits in the hominin family tree, but its morphology suggests that its common ancestor with Neanderthals and modern humans dates back a million years or more. Dating of its fossil remains in 2017 showed that it lived relatively recently, from 335,000 to at least 241,000 years ago, so might have met Homo sapiens, which evolved in Africa around 300,000 years ago.

In 2021, the discovery of an infant skull in a narrow fissure that is almost impossible to access indicated that this hominin deliberately interred its dead. The finding also implied that H. naledi must have been able to control fire in order to navigate through the labyrinth of dark passages and, in December last year, Berger announced evidence of extensive use of fire in the Rising Star cave system, such as soot, hearths and burnt bones.

Now, Berger and his colleagues have published more remarkable findings from the Rising Star caves.

The team only discovered engravings in the caves in July last year, when Berger entered them for the first time. He had to lose 25 kilograms of weight in order to squeeze through passages in the rock as narrow as 17.5 centimetres wide.  “It was incredibly hard to get in, and I wasn’t sure I could get back out,” he says.

To his amazement, Berger spotted some engravings on a natural pillar that forms the entrance to a passage connecting the Dinaledi chamber – where H. naledi fossils were first discovered – and the Hill antechamber, where other remains had been found.

In three different areas of the walls, he saw geometric shapes, mainly composed of lines 5 to 15 centimetres long, deeply engraved into the dolomite stone. This is an incredibly hard rock, so the engravings would have taken considerable effort to make. Many of these lines intersect to form geometric patterns, such as squares, triangles, crosses and ladder shapes.

“There was this moment of awe and surprise in seeing these highly recognisable symbols carved into the wall,” says Berger. “Seeing these symbols was entirely unexpected.”

Aside from the 47 people who had recently accessed the caves, there is no evidence that anyone else except H. naledi had ever been inside, so the researchers argue that these extinct hominins must have carved the marks. However, this is only a preliminary report of the findings and the team hasn’t dated them yet.

We know that Neanderthals created similar symbols more than 64,000 years ago, as did modern humans in southern Africa from around 80,000 years ago. If the symbols in the Rising Star caves were indeed made by H. naledi, they could be far older.

Berger argues that to go to the effort of engraving this incredibly hard rock “in what appear to be important positions within these extraordinarily remote places, the interpretation is that they must have some meaning”.

Others are more cautious. “It is premature to conclude that symbolic markings were made by small-brained hominins, specifically H. naledi,” says Emma Pomeroy at the University of Cambridge. “While intriguing, exciting and suggestive, these findings require more evidence and analysis to support the substantial claims being made about them.”

Berger’s team has also detailed new evidence of what could be deliberate burials in the ground – a different mortuary practice to the internment of corpses in niches, such as the infant skull discovered in 2021. At one place in the Dinaledi chamber, the researchers found 83 bone fragments and teeth, seemingly from a single body, in an oval-shaped area of disturbed soil.

They also found another possible burial site in the Hill antechamber. In this instance, they encased an area of debris with a high concentration of bone fragments in plaster, allowing them to remove it from the cave system intact and use a CT scanner to reveal its contents.

This showed many bone and teeth fragments, mainly from one juvenile that seemed to have been in a fetal position, an arrangement also found in prehistoric H. sapiens burials, along with some fragmented remains of three other juveniles. Intriguingly, a single stone artefact – a distinctive crescent-shaped stone, 14 cm long, with striations on its surface – was found close to the hand of one of the bodies.

Although these analyses are only preliminary, the researchers argue that the orientation of the bones and patterns of soil disturbance indicate that bodies were interred in pits that had been deliberately dug out, then covered in sediment. If confirmed, these burials would predate the earliest known human burial in Africa by at least 160,000 years.

Other experts aren’t yet convinced. “This is an admirable attempt to demonstrate that the corpses of at least two individuals were deliberately buried in shallow pits, and one can certainly not rule this out,” says Paul Pettitt at the University of Durham, UK. “I’m not convinced that the team have demonstrated that this was a deliberate burial. Let’s walk before we can run.”

Silvia Bello at the Natural History Museum in London points out that the bones are fragmented, while skeletons that are deliberately buried usually show better preservation.

Remarkable behaviour

Further analysis, such as a more detailed scan of the Hill feature, should help clear up this issue. Nevertheless, the new studies are already building an ever-richer picture of H. naledi and its behaviour. “The evidence is impressive,” says Chris Stringer, also at the Natural History Museum.

“These humans were taking carcasses, bodies of fellow naledis, down deep into the cave, and they must have had artificial lighting,” he says. “This is remarkable behaviour for a creature that’s got an ape-sized brain. It suggests organisation, because this is not something a single individual would have done, it must have been a group activity. And it’s obviously happened multiple times. That implies the existence of what we might call a culture – a different species, not closely related to us.”

“There’s a lot of very intentional behaviour in that cave complex,” says Genevieve von Petzinger at the Polytechnic Institute of Tomar, Portugal. “It’s not like a bunch of people fell in a hole and scraped some marks.”

These kinds of sophisticated behaviours were only thought to be possible in hominins with large brains, such as Homo sapiens and Neanderthals. “These are challenging finds, and they certainly make us think about what it is to be human,” says Stringer, raising questions about why we developed such large brains.

In the meantime, further research at the Rising Star cave system will be limited while the researchers work out how best to investigate this site without destroying it.

“Homo naledi altered almost every single space. That has caused me to become incredibly cautious about allowing people into that space until we decide exactly how we’re going to approach it,” says Berger, who wants to engage the world’s scientific community in addressing this question. “We, as humans, have to decide how we’re going to approach the space of another species that they clearly saw as critically important to them.”

Reference: bioRxiv, DOI: 10.1101/2023.06.01.543133 and 10.1101/2023.06.01.543127