Did Neanderthals and modern humans take turns living in a French cave?

2022-02-09

A single, broken molar found buried within a windswept rock shelter in southeastern France could push back the first evidence of modern humans in Europe by nearly 10,000 years.

According to an international team, the tooth and dozens of stone tools from the same sedimentary layer belonged to a member of Homo sapiens who lived some 54,000 years ago, a time when Neanderthals were thought to have been the sole occupants of Europe. The findings also paint a remarkable picture of the intimacy of modern humans and their Neanderthal neighbors, suggesting they may have traded occupancy of the cave several times—once in as little as a year.

“I found this paper absolutely fascinating,” says Kristin Krueger, a dental paleoanthropologist at Loyola University Chicago. “From what I can tell, this is solid evidence … that modern humans made it to Europe earlier than thought.”

For others, the excitement is tempered with caution, especially given that the claim relies chiefly on a single modern human tooth. “There are several ‘ifs’ here,” says paleogeneticist Carles Lalueza-Fox of the University of Barcelona. “To make a stronger case, we should have at least more solid skeletal or genetic evidence.”

After arising in Africa, modern humans trekked into the Middle East as early as 180,000 years ago, where they may have first met and mated with the Neanderthals, who already lived in Europe and Asia. (Today, non-Africans carry the legacy of those matings, having inherited about 2% of their DNA from our closest cousins.) But moderns were late arrivals in Europe, where until now the earliest evidence of their remains and artifacts came from Bulgaria’s Bacho Kiro Cave, dated to about 45,000 years old.

The discoveries that could change that picture come from a rocky overhang known as Grotte Mandrin in the verdant Rhône River Valley. Since 1990, excavations of the cave’s floor have delved into 12 sedimentary layers deposited between 80,000 and 35,000 years ago.

In 2006, what archaeologists call layer E yielded a trove of precisely sharpened stone points and animal bones. Six years later, they found a partial molar—a baby tooth. “We spent 15 years excavating this layer,” says Ludovic Slimak, a paleoanthropologist at the University of Toulouse, Jean Jaurès. “We went slowly because it’s very rich and there’s a lot of very little material.”

Slimak and colleagues radiocarbon dated animal bones with butchery marks from layer E to between 57,000 and 52,000 years ago. Luminescence dating of sediments in the layer, which determines when quartz minerals were last exposed to sunlight, returned the same date range. Meanwhile Clément Zanolli, a paleoanthropologist at the University of Bordeaux, analyzed the layer E molar and eight other teeth found in other layers. Although the molar was broken, it retained a telltale part of its cusp called the talonid. The modern human talonid gives their teeth a squarer outline than those of Neanderthals. “[This] tooth is quite square,” Zanolli says, indicating it belonged to a young modern human. In contrast, all the teeth from layers above and below layer E had distinctly Neanderthal characteristics.

Layer E’s stone tools back up the tooth’s identification, Slimak says: They are smaller, more precisely made, and more standardized than the tools from the layers bearing Neanderthal teeth, which resemble Neanderthals’ characteristic Mousterian tools. “With Neanderthal tools, every tool is a creation,” Slimak says. “If you look at 1000 tools, each will be completely different. But with a Homo sapiens industry … it’s superstandardized, superregular.”

Tools and teeth from the next few layers above layer E suggest Neanderthals at some point reoccupied Grotte Mandrin. Then, in layers dated to about 42,000 years ago, the tools once again appear to have been made by modern humans; they resemble the “proto-Aurignacian” tools found in other modern human sites from the same time period, Slimak says.

Together, that evidence shows modern humans had reached southern France about 54,000 years ago, the researchers report today in Science Advances. The settlers probably came from the east and traveled up the Rhône River Valley from the Mediterranean coast, Slimak says. After modern humans first moved into the shelter, they and Neanderthals took turns for another 10,000 years.

“The authors make a strong case for the dates,” Krueger says. “They provide the stratigraphic sequence, completed over many, many years of excavation, [and] also use multiple methods for dating each layer.”

But the layer E molar is key to the argument. Shara Bailey, a dental paleoanthropologist at New York University who developed methods to distinguish modern and Neanderthal teeth, isn’t certain the molar came from a modern human. Too much of it is missing, she says. “It would be so cool if it were true … but it’s not a slam dunk.”

If members of our species really did make an early appearance in the cave, they may have had close contact with Neanderthals, one intriguing data point suggests. Thin layers of mineral deposits form along the walls of rock shelters like Grotte Mandrin, recording the passage of wet and dry seasons, a bit like tree rings. These mineral layers can trap soot from fires burning inside the caves, offering microscopic records of campfires past. This new technique, known as fuliginochronology, was first described in 2018 by one of the paper’s co-authors, Ségolène Vandevelde, an archaeologist at the University of Paris-Saclay.

Tiny chunks of mineral fell from the cave wall into each excavated layer. The sequence of soot layers in wall chunks from layer E overlapped with the sequence in chunks from the layer immediately beneath it, which held Neanderthal tools. That suggests only a brief period passed between the formation of the layers and therefore between the exit of the Neanderthals and the entrance of moderns—perhaps no more than a single calendar year. “They probably met at some point, but we cannot say for sure they met in the cave,” Slimak says.

Rachel Wood, a radiocarbon scientist at Australian National University, calls the study “remarkable.” But she isn’t confident the researchers can pinpoint the timing of soot layers from 54,000 years ago to within 1 year given the evidence they’ve presented so far. “Given the uncertainties … I’d be skeptical about this supporting a short transition between the two [layers].”

The paper’s findings could be revolutionary for our understanding of the transition between the last Neanderthals and the first moderns in Europe, says Francesco d’Errico, an archaeologist also at the University of Bordeaux. But he and others want far more evidence. “If the pattern proposed is confirmed by future discoveries, we will certainly need to change our view of this transition,” he says. “Such a paradigm shift is entirely possible but requires … more sites and more unequivocal evidence.”

science.org, 9 February 2022
; https://science.org