Modern humans and Neanderthals met—and made love, or at least babies—at some point in prehistory. But how long and exactly where the two species intermingled has been a mystery. Now, a reevaluation of radiocarbon dating at archaeological sites in France and northern Spain indicates that some 40,000 years ago, our ancestors overlapped with Neanderthals in the region for up to 2800 years, sharing not just genes with each other, but potentially culture as well.
“The time span is insignificant on a geological scale,” says Antonio Rodríguez-Hidalgo, an archaeologist at the Catalan Institute of Human Paleoecology and Social Evolution who was not involved with the study. “But on a human scale, there is enough time for very interesting things to happen.”
Other scientists, however, say the wide margins of error for many of the dates analyzed in the study undercut strong claims about the identities of the inhabitants and whether they indeed overlapped. It’s “a good starting point,” but the conclusions could change based on more accurate dating, says Sahra Talamo, a chemist who directs a radiocarbon laboratory at the University of Bologna.
Radiocarbon dating estimates the age of organic objects such as bones and charcoal based on the steady radioactive decay of their carbon-14 isotopes. Scientists have used the method for decades—and they’ve been refining it for just as long.
A major revision came in 2020, when radiocarbon scientists announced that a brief reversal of Earth’s magnetic field about 42,000 years ago, known as the Laschamp event, had temporarily supercharged the amount of carbon-14 in the atmosphere. Anything living at the time incorporated extra amounts of the isotope as a result, throwing off modern efforts to radiocarbon date their remains.
“It pushed dates that were around 40,000 years further back in time and made things that were older than 43,000 or 44,000 years appear younger in time,” says Igor Djakovic, an archaeology Ph.D. student at Leiden University.
That period coincides with a critical era in human history. One major kind of stone tool technology in Europe known as the Châtelperronian industry—consisting of palm-size scrapers and knives and traditionally associated with Neanderthals—was replaced by a kit of more sophisticated tools called the proto-Aurignacian industry, which featured smaller, more precisely worked blades traditionally associated with modern humans. (There is debate, however, over whether either industry was truly exclusive to humans or Neanderthals.) That time span is also when Neanderthals began to disappear from their longstanding European strongholds and modern humans started moving into the continent.
Hoping to clarify the dates for sites containing Châtelperronian and proto-Aurignacian artifacts, Djakovic and colleagues reran the radiocarbon analyses performed by other teams representing 17 sites across France and northern Spain. They also recalculated the dates for 10 Neanderthal skeletons from the same range. But this time they applied a recently developed calibration standard known as IntCal20, which accounts for the heightened carbon-14 caused by the Laschamp event. None of the dates changed drastically, but as a whole, the oldest dates shifted a bit younger whereas the youngest dates got a bit older, compressing the estimated ranges.
Next, they plotted those refined dates using a statistical approach called optimal linear estimation, which aims to predict when a particular technology may have begun and ended based on the intervals in between the ages of known artifacts.
The researchers found that modern human associated proto-Aurignacian tools show up in the region between 42,200 and 42,600 years ago, whereas the Neanderthal-associated Châtelperronian tools disappear about 40,800 to 39,800 years ago. That indicates that the two toolmaking industries overlapped in time and space for anywhere from 1400 to 2800 years, the team concludes today in Scientific Reports. The dates—though based on a relatively small number of sites—also suggest proto-Aurignacian tools spread from south to north over time, hinting at the possible route of modern humans through the continent, Djakovic says.
It's not the first time researchers have proposed modern humans and Neanderthals overlapped in Europe during this period. But the revised dates offer a narrower, more geographically constrained window into one such possible event, Djakovic says.
This overlap would have still provided time for generations of humans and Neanderthals to meet, interbreed, and share toolmaking tips with one another. And that in turn could explain why later Châtelperronian tool caches appear to borrow proto-Aurignacian elements, such as small, precisely made blades, Djakovic says.
Still, he admits such interpretations are speculative and controversial. Except for in the small number of instances in which modern human or Neanderthal remains have been found alongside these tools, no one knows for certain which species made which tools at most sites.
The study’s agnosticism over which species made the tools is actually a strength, says Emmanuel Discamps, an archaeologist at CNRS, the French national research agency. “Who knows if the Châtelperronian or the proto-Aurignacian were made by Neanderthals, modern humans, hybrids, or a bit of all of that, depending on chronology and geography,” he says. By not assuming the identities of these toolmakers, he adds, researchers can envision more complex histories that might better match what really happened.
But the uncertainties in the radiocarbon record still give Shara Bailey, a paleoanthropologist at New York University, pause. Radiocarbon dates are only reliable out to about 50,000 years ago. Dates measured toward the end of that range tend to have large margins of error, she says, making it difficult to make firm conclusions about which objects are older than others. “I take the results of all these types of studies with a grain of salt.”
Katerina Douka, an archaeologist at the University of Vienna who helped collect some of the radiocarbon dates reanalyzed in the new study, says it’s great to see her team’s data reused to refine our understanding of when and where Neanderthals and modern humans may have lived side by side. “Western Europe is a cul-de-sac, and many have predicted that this could be a region where the two populations coexisted and interacted more intensely.”
But she suspects that in other parts of Europe, such overlaps between different human species and cultures may have been even more complex, and happened at different times and tempos, than the new paper suggests. “There is a lot more to find out about this key period in human evolution.”
