Why the Neandertals disappeared is one of archaeology’s longest-running debates. Over the years, opinions have shifted back and forth between climate change, competition with modern humans, and combinations of the two. Earlier this year, the climate change contingent got a boost when a European team determined that the Italian eruption, known as the Campanian Ignimbrite (CI), was two to three times larger than previous estimates. The researchers calculated that ash and chemical aerosols released into the atmosphere by the eruption cooled the Northern Hemisphere by as much as 2°C for up to 3 years.

Modern humans entered Europe from Africa and possibly the Middle East around the time of the eruption and Neandertals’ demise, give or take several thousand years. The timing is critical. If Neandertals began disappearing before the eruption, it could not be responsible for their extinction; if their demise began at the same time or shortly afterward, the correlation with climate might still hold.

With these issues in mind, a team of more than 40 researchers from across Europe, led by geographer John Lowe of Royal Holloway, University of London in Egham, U.K., used a new technique for detecting volcanic ash across a much larger area than previously possible. The new method relies on deposits of cryptotephra, tiny particles of volcanic glass that are invisible to the naked eye. Unlike visible ash deposits, which are found over a more limited range, the much lighter cryptotephra can penetrate and be recovered from far-flung archaeological sites as well as marine, lake, and marsh environments. Moreover, by analyzing the chemical composition of the microscopic particles, researchers can trace them back to specific volcanic eruptions, in this case the CI.