The stocky, heavy-browed Neandertals ruled Europe for hundreds of thousands of years. And then, around 40,000 years ago, their population began to decline sharply; shortly after 28,000 years ago or so they were gone. In their place stood anatomically modern humans. Did the Neandertals die at the hands of the invading moderns? Did the moderns outcompete them? Or was catastrophic environmental change the culprit? Researchers have debated the nature of the Neandertals’ mysterious demise for decades.

In recent years scenarios implicating rapid swings in climate and/or a massive volcanic eruption in southern Italy some 40,000 years ago that ostensibly led to a volcanic winter have gained prominence. The idea is that these disastrous events pushed Neandertals out of vast areas of Europe, opening up these territories to moderns, or that the events fostered adaptive changes among moderns that enabled them to overtake the resident Neandertals. But a new study casts doubt on those environmental explanations and blames the downfall of our closest evolutionary cousins on modern humans instead.

John Lowe of the Royal Holloway University of London and his colleagues set out to test the theory that environmental change precipitated the decline of the Neandertals 40,000 years ago. They located microscopic layers of volcanic ash known as cryptotephra deposits from that mega eruption at a number of archaeological sites--including ones located well north and south of the Mediterranean--containing Neandertal and modern human remains. Having evidence of this well-dated event in all these places allowed the team to synchronize the archaeological and paleoclimatic records from the sites, eliminating some of the dating uncertainties that can hinder studies of cultural responses to environmental change.

The researchers studied the record of Neandertal and early modern human artifacts underlying (and thus predating) and overlying (and thus postdating) the ash from the Italian eruption, which occurred just after the start of a brutally cold and dry climate phase. They found that the transition to the more advanced technologies of the so-called Upper Paleolithic cultural traditions associated with modern humans began before the eruption. This indicated to the team that neither that blast nor the concurrent climate deterioration drove the cultural changes, the dispersals of moderns, or the regional extinction of Neandertals in northern and eastern Europe during this time, and that Neandertals were probably mostly gone long from these places before then. (The Neandertals managed to hang on in Iberia and possibly elsewhere for thousands of years longer though.)

“Our evidence indicates that, on a continental scale, modern humans were a greater competitive threat to indigenous populations than the largest known volcanic eruption in Europe, even if combined with the deleterious effects of climatic cooling,” Lowe and his colleagues conclude in the report detailing their findings, published online this week by the Proceedings of the National Academy of Sciences USA. “We propose that small population numbers and high mobility may have initially saved the Neandertals, but that they were ultimately outperformed in this capacity by [anatomically modern humans].”

When I read this paper, I wondered what Clive Finlayson of the Gibraltar Museum would have to say about it. Finlayson has been a leading proponent of the theory that climate change was the main cause of the Neandertals’ demise, which I wrote about in 2009. I emailed him asking he thought about the new study and he had this to say:

“Since the Neanderthals were extinct in the region at the time of the eruption it is clear that it was not a cause, neither was the subsequent climate change. That’s fine. But it does not mean that the conclusions which the authors come to from this are any closer to what may have occurred, for the following reasons:

1) The Neanderthal extinction was not a punctual event but a long drawn process, as I have often explained and written about, of attrition of population levels with partial recoveries along the way.

2) The authors claim that “modern human” presence before and after the eruption shows they were unaffected by the event. Not necessarily because all they have is presence. We have no idea of population levels before and after. We don’t know either if those that show up after might have come in from a distant region to fill a vacuum. We just cannot say.

3) The authors claim evidence of competition from modern humans as the cause of the Neanderthal extinction. This is the default argument – we think we didn’t find evidence of climate or volcanic activity on the Neanderthal extinction, therefore it must have been modern people. Why? Show it!

4) That this event and its aftermath did not cause the Neanderthal extinction – which is hardly surprising given 1) above – does not mean that other climatic and environmental changes did not!”

Finlayson was not previously convinced that the eruption did in the Neandertals, and obviously he’s not persuaded that the authors of the new study have eliminated climate change as the cause.

If Lowe and his colleagues are right, however, and competition with modern humans was what doomed the Neandertals, it’s going to be really interesting to see if paleoanthropologists can home in on what gave them that competitive advantage. Once upon a time Neandertals were seen as brutes who could barely find their way out of their caves. But then archaeologists started finding evidence of Neandertal sophistication—a piece of jewelry here, a fancy tool there. Discoveries made over the past few years have further blurred the divide between Neandertals and modern humans. Sequencing of ancient DNA has shown that Neandertals interbred with early modern humans—often enough that up to 4 percent of the DNA in people outside Africa today comes from Neandertals. More recently researchers have shown that Neandertals used medicinal plants and might have made cave art.

Man oh man what I wouldn’t give to journey back to the last glacial stage to see how it all went down. In a climate-controlled, volcanic eruption-proof time machine, of course.