About 40,000 years ago, a huge volcanic eruption west of what is now Naples, Italy, showered ash over much of central and Eastern Europe. Some researchers have suggested that this super-eruption, combined with a sharp cold spell that hit the Northern Hemisphere at the same time, created a "volcanic winter" that did in the Neandertals. But a new study of microscopic particles of volcanic glass left behind by the explosion concludes that the eruption happened after the Neandertals were already mostly gone, putting the blame for their extinction on competition with modern humans.

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.

The team collected samples containing CI cryptotephra from four central European caves where stone tools and other artifacts typical of Neandertals and modern humans have been found. They also gathered the particles from a modern human site in Libya and from marshland and marine sites in Greece and the Aegean Sea. The results, the team argues in a paper published online this week in the Proceedings of the National Academy of Sciences, are incompatible with the hypothesis that the CI was responsible for Neandertal extinction, at least in central Europe. The CI cryptotephra lie above, and so postdate, the transition from Neandertal to modern human stone tool types at all four central European sites, indicating that modern humans had replaced Neandertals before the catastrophic events of 40,000 years ago.

Moreover, analysis of tree pollen and other climatic indicators from the marsh and marine sediments confirmed that the CI was contemporaneous with a sharp cold spell called a Heinrich event, which is also often cited as a contributor to Neandertal extinction. So the data suggest that the eruption and the cold snap happened after the Neandertals had already vanished from central Europe.

"Climate was probably not directly responsible for Neandertal extinction, and catastrophic events most certainly were not," says co-author William Davies, an archaeologist at the University of Southampton, Avenue Campus, in the United Kingdom. That leaves competition with modern humans as the most likely culprit, the team contends.

Nevertheless, the authors concede that their results are only directly applicable to central and probably Eastern Europe, and not to Western Europe, where some researchers have claimed that Neandertals hung on until at least 35,000 years ago in Portugal and Spain. Because the team has not been able to find cryptotephra that far west, "we cannot rule out the survival of Neandertals post-CI and post Heinrich ... in refugia like the Iberian Peninsula," says co-author Chris Stringer of the Natural History Museum in London. "But it must have been a very limited survival at best, as they headed to physical extinction."

The team's techniques offer new clues to the eruption, says Clive Finlayson, director of the heritage division at the Gibraltar Museum and head of the excavations at Gibraltar's caves, at the southern tip of Spain, where Neandertals may have survived until as late as 30,000 years ago. But Finlayson, an advocate of climate change as the key factor in Neandertal extinction, says the researchers have not proven their case. "We can only conclude from this that the eruption and subsequent climatic changes had no effect on Neandertals that were already extinct. To pretend that these results speak to other factors that may have generated the Neandertal extinction, which was a protracted process, is utter nonsense."