A cave in the northern Caucasus Mountains may hold a key to the long-standing mystery of why the Neandertals, our closest relatives, went extinct. For nearly 300,000 years the heavy-browed, barrel-chested Neandertals presided over Eurasia, weathering glacial conditions more severe than any our own kind has ever faced. Then, starting around 40,000 years ago, their numbers began to decline. Shortly after 28,000 years ago, they were gone. Paleo­anthropologists have been debating whether competition with incoming modern humans or the onset of rapidly oscillating climate was to blame for their demise. But new findings suggest that catastrophic volcanic eruptions may have doomed the Neandertals—and paved the way for modern humans to take their place.

Researchers led by Liubov Vit­a­lien­a Golovanova of the ANO Laboratory of Prehistory in Saint Petersburg studied the deposits in Mezmaiskaya cave, located in southwestern Russia. First discovered by archaeologists in 1987, the cave once sheltered Neandertals and, later, modern humans. Analyzing the various stratigraphic layers, the scientists found layers of volcanic ash that, based on the geochemical composition of the ashes, they attribute to eruptions that occurred in the Caucasus region around 40,000 years ago. Because the cave preserves a long record of Neandertal occupation preceding the ash layers but no traces of them afterward, the team surmises that the eruptions devastated the locals.

Moreover, looking more broadly at sites across Eurasia, the investigators noted that the eruptions coincided with the disappearance of the Neandertals across most of their range, save for a few groups that took refuge in the south. In a paper published in Current Anthropology, they propose that the eruptions precipitated a so-called volcanic winter that may have resulted in mass deaths of Neandertals and their prey. The misfortune of the Neandertals, however, was a boon for modern humans, who lived in southern locales unaffected by the volcanic activity. Once the Neandertals were gone, so the theory goes, moderns could move north unchallenged.

The team’s interpretation of the data from the cave has elicited criticism from some researchers, such as Francesco G. Fedele of the University of Naples in Italy, who complained in commentaries published alongside the paper that the age of the ashes is not firm enough to draw such conclusions. But others, including Paul B. ­Pettitt of the University of Sheffield in England, called the new extinction and replacement scenario plausible. The riddle of the Neandertals’ downfall is far from solved, but the volcanic eruption theory may turn up the heat on the competition.