About 40,000 years ago, Neanderthals began disappearing from Europe, but exactly why they died out is a mystery. Some paleoarchaeologists have hypothesized it’s possible they simply couldn’t reproduce fast enough to keep up with the modern humans moving into Europe around that time. Others suggest modern humans slaughtered any bands of Neanderthal they came across or infected them with novel diseases. And some suggest that an environmental catastrophe, like a volcanic eruption in Europe, killed off many plants and animals.

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Researchers propose a new hypothesis this week that suggests our bipedal brethren weren’t equipped to stand a cold spell that accompanied two long periods of extended climate change that took place around the time the species began its decline, Malcolm Ritter at the Associated Press reports.

To investigate the climate of central Europe during the age of Neanderthals, researchers looked at stalagmites in two Romanian caves. According to a press release, like trees, stalagmites grow thin new layers each year. Temperature influences the size and chemical composition of the calcium carbonate layers. Each layer includes isotope data about rainfall, soil bacteria that reveals the fertility of the land and other information that can help create a detailed annual climate record. In this case, the cave formations provided the most detailed record of climate change in Europe available so far.

Ritter reports that the new palaeoclimate records show that a particularly cold, dry period began about 44,000 years ago and lasted 1,000 years. Another cold dry period began, 40,800 years ago, lasting about 600 years. It was cold enough that average temperatures dropped to below zero, creating year-round permafrost.

Those climate disruptions correspond to the archaeological record, which shows that at the same time Neanderthals began to disappear from the Danube River Valley and in France, the heart of their territory, while early signs of modern humans begin to appear. The paper appears in the journal Proceedings of the National Academy of Science.

“For many years we have wondered what could have caused their demise. Were they pushed ‘over the edge’ by the arrival of modern humans, or were other factors involved?” co-author Vasile Ersek of the University of Northumbria in England says in the release. “Our study suggests that climate change may have had an important role in the Neanderthal extinction.”

The double-dose of super-cold weather likely radically changed the environment, transforming the open woodlands of central Europe into Arctic-like steppes, reports Ariel David at Haaretz. Early humans with more adaptable strategies likely moved into former Neanderthal territory and did not actively kill the species off.

“It seems we are off the hook for that one,” says lead author Michael Staubwasser of the University of Cologne, Germany.

The researchers aren’t necessarily suggesting that modern humans didn’t have a hand in the end of Neanderthals. There is some evidence that there was violence between the species. But David reports that in 2014 the latest known Neanderthal bones were re-dated and found to be 40,000 years old, not 30,000 years old as previously believed.

So, instead of having a 15,000 year window to outcompete and exterminate Nenderthals, humans, who only entered Europe 45,000 years ago, only had a few thousands years to make contact and wipe out the species. That scenario is unlikely, meaning that another factor, like climate change, probably also had a hand in reducing Neanderthal numbers.

It’s possible that the Neanderthal population crashed during that first cold period. When the second one happened, the remaining small bands of Neanderthals were likely absorbed into human populations, as evidenced by the Neanderthal DNA in the genome of modern humans.

So why did Neanderthals die out during these climate shifts while modern humans survived? The researchers suggest that because Neanderthals relied heavily on protein from large game animals they had trouble adapting when climate change impacted populations of those animals. Homo sapiens, on the other hand, were more adaptive, eating a variety of plants, fish and meat, meaning they could survive on the cold steppe.

Rick Potts, a human origins expert at The Smithsonian’s National Museum of Natural History, tells Ritter that the paper suggests a different dynamic between humans and our close cousins. “As has been said before, our species didn’t outsmart the Neanderthals,” he says. “We simply outsurvived them. The new paper offers much to contemplate about how it occurred.”

Not everyone is convinced by the research. Israel Hershkovitz, a physical anthropologist at Tel Aviv University, tells David that Neanderthals went through a lot of cold snaps before the ones 45,000 years ago and weathered them fine, so it doesn’t make sense that this one event would impact them so heavily. He also questions whether the climate record from caves in Romania can accurately represent all of Europe, saying there is evidence that other parts of the continent had a mild climate in the same period.

However, the researchers point out that the cold spells didn’t just impact Neanderthals. They continued to ice out modern humans after the Neanderthals disappeared; each time one culture of ancient humans disappeared in the face of a changing climate, another culture replaced them when the world warmed up again.