Homo naledi would have been the first human to bury his dead
A group of Homo naledi carry a dead body into the Rising Star cave in this artist's depiction. New evidence that these small-brained hominids may have practiced deliberate burial challenges our view of human evolution.
If the hypothesis is confirmed, the behavior adopted by this small-brained hominin would predate the oldest burials known to date by at least 100,000 years.
A human species extinct for several hundred thousand years would have voluntarily buried its dead and engraved significant symbols in the depths of a South African cave, a modern human behavior that current theories attributed only to Neanderthal Man and Homo sapiens . If the hypothesis is confirmed, the burial site would be at least 100,000 years older than the oldest known to date.
The hypothesis in question, introduced in two research articles published on June 5, 2023 on the bioRxiv prepublication server, was also the subject of an announcement by the paleoanthropologist Lee Berger during a conference given within from the State University of New York at Stony Brook.
These publications come eight years after the announcement, by this same specialist, of the discovery of a new species of hominin in the galleries of the Rising Star cave system, about 40 km from Johannesburg. Named Homo naledi , the species is characterized by its small size, including a brain three times smaller than ours, and an astonishing mix of very ancient and relatively modern anatomical traits.
The bones were discovered in a single sub-network that is difficult to access and the dating places them between -335,000 and -241,000 years ago. At this time, modern humans were only beginning to emerge in Africa.
“We unearthed the cultural space of a non-modern human species,” says Berger. The project is supported by a grant from the National Geographic Society, for which Berger is Explorer-in-Residence.
This composite skeleton of Homo naledi is surrounded by part of the fragments discovered by the hundreds in the cave. Scientists have identified more than 18 individuals, ranging from newborns to seniors.
The team led by Berger hypothesized the voluntary burial in 2015, at the same time as the announcement of the discovery of Homo naledi . This track seemed to offer the most plausible explanation for how more than 1,800 bone fragments could have ended up in an underground cave only accessible after a four-storey vertical descent, through a 19 cm wide chimney, dubbed The Chute (le toboggan, editor's note) by speleologists.
Additionally, the position and intact condition of some skeletal fragments suggest that the remains would have been carefully placed on the floor of the chamber, and not thrown from the top of The Chute, in which case they would have formed a heap of remains . humans at its core.
Skeptical about the possibility for a hominin with such a small brain to engage in such human behavior, several experts have preferred to hypothesize that the bones were deposited by water or predators. However, the analysis of the environment and the sediments of the cave made it possible to exclude the first hypothesis and the bones did not show any traces of bite.
Based on probable cohabitation with Homo naledi in South Africa for at least 50,000 years, other skeptics have suggested that the bones were deposited by modern humans who passed through the Fall or another passage that has since collapsed. Again, the Rising Star team found no trace of modern humans or signs of a possible secondary entrance.
In 2017, researchers returned to survey the Rising Star galleries. The series of discoveries that followed had not yet been fully revealed, it is now done . Among these revelations is notably the concentration of remains of Homo naledi belonging to one or more individuals in shallow cavities which cross the strata of the floor of the cave and do not follow its natural slope, which suggests that the cavities would have been dug . In addition, the composition of the backfilling of the cavities differs from the sediments present nearby.
A set of bones was exhumed in complete blocks and frozen in plaster. A scanner analysis identified at least three individuals, including a juvenile close to adulthood. The latter's remains were discovered in a remarkable state of preservation, including 30 teeth in the correct order, two sets of partial ribs, the right foot, ankle and other bones from the lower limbs. Faced with the discovery of a stone near a partial right hand, the researchers hypothesized an artifact or a tool, an idea quickly dismissed by various specialists outside the study.
A PRIMATE LIKE NO OTHER
The debate over the voluntary burial of the dead often revolves around the differences between mortuary behavior and funerary behavior, says André Gonçalves , who studies how animals interact with their dead. For example, chimpanzees and elephants adopt a mortuary behavior when they watch over the remains of a congener or interact physically with it expecting to see it come back to life.
Funeral behavior, on the other hand, involves social acts performed intentionally by complex-thinking beings who identify themselves as separate from nature and understand what death means. So far, the oldest evidence of funerary behavior and voluntary burial in hominin species, including both modern humans and Neanderthals, was 100,000 years after Homo naledi .
Nicknamed the “subterranean astronauts”, the team members make their way through a maze of narrow galleries to reach the isolated rooms that have yielded more than 1,800 fossilized bone fragments. The mission requires a lean physique and strong nerves.
“Humans are a special primate, because they bury their dead,” Gonçalves explains. “No other primate does this. »
At the request of National Geographic , independent experts have evaluated the scientific articles and raised various concerns about the evidence for voluntary burial put forward. Some still maintain that the water could have deposited the bones in the natural depressions of the cave, which would then have filled with sediment over time.
However, as John Hawks , an anthropologist on the Rising Star team and co-author of one of the articles, reminds us: “Our strongest evidence is that the burials disturb the existing stratigraphy of the cave. »
Another criticism concerns the state of the bones, mostly scattered and disconnected. "The natural process of decomposition is not enough to explain these movements," says paleoanthropologist María Martinón-Torres , who has studied the oldest known burial site in Africa .
That said, the recent discoveries have still managed to sow doubt in anthropologist Chris Stringer . "I too was skeptical of the idea that a creature with a small brain like Homo naledi could have ventured into the depths of a cave to deposit the body of a deceased," admits- he. "But I have to say in all honesty, with what I've been able to see at this point, it has indeed changed my opinion on the likelihood of this assumption. »
For his part, Gonçalves considers the series of discoveries "promising", but prefers to wait before commenting further. The idea that Homo naledi adopted noticeably human behaviors is not particularly surprising to him, given the temporal and spatial proximity between small hominins and modern humans. “Six million years separate us from chimpanzees and bonobos,” he adds. “Three hundred thousand years is nothing. »
Working in tight quarters, Marina Elliott and Ashley Kruger excavate fossil remains in one of the cave's chambers. Out of 60 applicants for the original 2014 expedition, six intrepid women were picked.
?A WALL OF EXPRESSION
In a second article, the researchers present another of their discoveries: a set of abstract shapes and patterns engraved in the walls of the cave , near the potential burials. The marked surface appears to have been prepared with a substance and smoothed. Moreover, some inscriptions seem to have been erased and then covered with other motifs, which suggests a repetition of the practice over an indefinite period.
The composition of the walls of the cave, in dolomitic limestone, constitutes an obstacle to the dating and the researchers concede that it will be “difficult to determine if the engravings are contemporary with the potential burials attributed to Homo naledi, distant from a few meters . »
Archaeologist Curtis Marean says the latticework patterns adorning the cave walls are "very similar" to those discovered at later sites associated with Homo sapiens in the region, as well as to Khoisan imagery.
While insisting on the need to go further in the identification and analysis of all the inscriptions, the researchers point out that the production of patterns, painted or engraved, on the walls of a cave or any other surface , is considered “a major cognitive step in human evolution. »
Various engravings of geometric shapes including squares, ladders, triangles and crosses were discovered on the cave walls in 2022. Their connection to nearby burials needs further investigation.
In a third paper, Berger and his colleagues synthesize their data on burials and rock art to tackle another long-standing assumption : the larger the brain, the more complex behaviors it gives rise to, such as tool making, firebending and symbol making.
The fossil record shows that relative brain size in hominin populations has increased over the past two million years, reaching its maximum value with Homo sapiens . While the brain of a modern human reaches an average volume of 1,500 cubic centimeters, that of Homo naledi did not exceed 600 cubic centimeters.
The polarizing filter used for this photograph revealed the imperceptible traces of previous engravings, erased then covered. These inscriptions could help demonstrate that the representatives of the species Homo naledi were much more evolved than science until now thought.
If this small-brained hominin did engage in evolved behaviors, such as voluntary burial and the creation of symbols associated with these burials, then brain size should no longer be considered a determinant of the ability to a species to enjoy complex cognition.
As the researchers point out, hominins with small brains are responsible for many major developments in human evolution, including the creation of stone tools, the first migrations out of Africa to Asia and the use of fire. Also, another small-brained species, Homo floresiensis , is known for its mastery of tools and fire. That's why the authors argue that brain structure and connections could have played a bigger role than brain size.
This artist's reconstruction of the face of Homo naledi from skull remains discovered in Rising Star Cave highlights the unique blend of primitive and modern features.
While the papers don't specifically mention evidence of the use of fire in Rising Star Cave, Berger says his team found traces of controlled fire within the underground network, including several dozen hearths. “The site is covered in soot and burnt bones. It's everywhere ! he said. Radiocarbon dating of the evidence is planned for further work.
A GLOBAL EXCHANGE
The decision by the researchers to make these disruptive statements public even before they are published in a peer-reviewed journal is a source of frustration for some paleoanthropologists, which does not prevent Berger from assuming this choice. The articles will appear in the online journal eLife , alongside the reviews and an editorial, making the process "transparent", explains the scientist.
“Your readers will be able to witness the interactions between authors (our great team), reviewers and editors thanks to the open access policy,” continues Berger. Authors will then have the choice of keeping the articles as they are or incorporating comments from reviewers and other scientists. “In fact, we give the public the opportunity to observe the evaluation process and understand how it works. »
The volume of the skull of this composite male skull of Homo naledi reaches only 560 cubic centimeters, less than half of the modern human skull visible in transparency.
The experts who have evaluated the articles agree on one point: paleoanthropology is entering a new era, an era marked by the awareness of the existence of other human species capable of behaviors still recently considered to be unique to “the modern human”.
This awakening raises expectations for discoveries about how Homo naledi lived and its possible connection to our species. “If this species was adapted to living in caves and exploring their depths, which the Rising Star implies, then there must be further evidence from many other sites in South Africa,” says String.
“The topic deserves a global human-to-human exchange,” adds Berger. “Where are we going next? How can we follow up on these discoveries? We have just uncovered the cultural space of another species that is not a [modern] human, that is not at the same stage as us. Different from us. How to react ? This is the answer I expect. »
Anthropologist Marina Elliott pauses at the entrance to Rising Star Cave, which is only just beginning to reveal its secrets. “We've only scratched the surface, literally,” Elliott says.
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