Who killed off the Neanderthals? We usually suspect ourselves, but while Homo sapiens has had a hand in the extinction of countless species, a study published on Monday appears to exonerate our distant ancestors in this prehistoric whodunit. Neanderthal populations were already dwindling by the time of their extinction some 40,000 years ago but what killed off our close evolutionary cousins seems to have been climate change.

Open gallery view Reconstruction of human, hobbit and Neanderthal women, from left Credit: Ismoon

The researchers reconstructed the climate of prehistoric central Europe by analyzing stalagmites from two caves in Romania. They discovered that the region went through two periods of extremely cold temperatures, even by Ice Age standards, around the time when Neanderthal populations dwindle and then disappear from the fossil record in what used to be the heartland of their territory.

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By correlating their geological results with archaeological findings, the team concluded that these periods likely delivered a deadly one-two punch to Neanderthals, with humans being guilty, at most, of moving into the lands formerly inhabited by the extinct hominins.

“It seems we are off the hook for that one,” jokes Michael Staubwasser, a professor of isotope geochemistry at the University of Cologne, in Germany, and the lead author of the paper published in the Proceedings of the National Academy of Sciences (PNAS).

Neanderthals and Homo sapiens split off from their last common ancestor around 500,000 to 700,000 years ago. Homo sapiens went on to evolve in Africa. Neanderthals developed in Eurasia and the Levant, inhabiting an area that stretched from Europe to Siberia and the Middle East.

Scholars have been debating for over a century how much of a part we played in their demise.

With their stocky constitution and a brain that was larger than Homo sapiens’, Neanderthals seem to have been well adapted to the conditions of Ice Age Europe, though their domain was south of the ice sheet, which led to suspicions about our ancestors.

Some of the evidence is damning. Neanderthals did go extinct shortly after anatomically modern humans migrated massively from Africa into the Middle East and later Europe, starting some 65,000 years ago.

What happened when the two groups came in contact? Archaeologists have found evidence of sapiens-on-Neanderthal violence and even cannibalism. Another theory, which doesn't negate the possibility of violence, is that we interbred with Neanderthals until our larger numbers meant they were genetically obliterated as a separate group. There is strong evidence for that too, given that the genome of all non-African populations today contains around 2 percent of Neanderthal DNA – as well as genetic material from other hominins.

Open gallery view Reconstruction of a Neanderthal family, based on finds in Krapina, Croatia Credit: Nikola Solic, Reutera

In addition, or alternatively, we may have simply outcompeted them thanks to our supposedly superior social skills or cultural development, other researchers have suggested.

The weather report, in stone

Open gallery view Does this Neanderthal skull look very familiar? It should: You share 99% of this proto-human's genes. Credit: Anagoria, Wikimedia Commons

But some scientists don't think modern humans were the primary cause of Neanderthal extinction. This view was strengthened when, in 2014, the most recent Neanderthal bones found so far were re-dated and found to be around 40,000 years old, rather than 30,000 as previously thought.

Since modern humans began to arrive in Europe around 45,000 years ago, this left a relatively small window, evolutionarily speaking, of a few millennia for sapiens to single-handedly exterminate or push out the Neanderthals from a vast territory. Other factors must have been at work.

The idea that climate change may be to blame is not new, but so far lacked firm scientific evidence to back it up, note the scientists in the newly-published PNAS paper.

The stalagmites they analyzed – one from a cave near the Danube valley and a second in the Carpathian Mountains in the north of Romania – contain a record of the weather conditions that occurred during the millennia over which they slowly formed, says Bogdan Onac, a paleoclimatologist from the University of South Florida and a member of Staubwasser’s team.

When calcium carbonate precipitates to create stalactites and stalagmites, the rock formations retain the oxygen isotopes from the rain that fell on the surface above the cave, as well as the carbon isotopes the water collected from the soil as it slowly flowed underground, Onac explains. The oxygen signatures can give us information about temperature changes, and even whether the rain came from the Atlantic Ocean or the Mediterranean; while the carbon tells us if the soil was rich and filled with critters, bacteria and plants – or arid and barren, he says.

The researchers found that Central and Eastern Europe underwent two major periods of very cold and dry conditions, from 44,300 to 43,300 years ago, and then again from 40,800 to 40,200.

Climatologists had already studied these extremely cold periods, also called stadials, in ice cores dug up in Greenland, but they didn’t know how much they affected Europe. It is now clear that the changes throughout the continent were extreme, Staubwasser says.

For example, based on his team’s results and other paleoclimate studies from the area, he estimates that the average temperature in the entire Danube region dropped from an already bracing 4 degrees to sub-zero levels, creating permafrost or near-permafrost conditions on the ground throughout the year.

Open gallery view A human (left) and Neanderthal skull. Credit: Wikimedia Commons

“What’s important is not just the temperature,” Staubwasser tells Haaretz. “Because of the cold, it also gets very dry, and this changes the entire ecosystem of Europe: you go from an open woodland to a cold steppe.”

Europe, the frozen wasteland

The transformation of Europe into a frozen, arid wasteland would have depleted the herds of large game that prehistoric hunter-gatherers lived off, quickly causing entire areas to be depopulated. This can be seen in most Neanderthal sites in Eastern and Central Europe, where archaeologists have turned up “sterile” layers – almost devoid of bones, tools and other signs of habitation – that date to the two stadial periods, the PNAS paper notes.

After the end of the first stadial, Homo sapiens sites begin to show up in the archaeological record in Europe. Also a few Neanderthal sites were repopulated, suggesting that while depleted in numbers, our hardy cousins had survived the first climatic onslaught. They would not survive the second.

“The picture that is emerging is that between the two cold periods you might have had Neanderthal and humans living together, not in the same sites but in the same region, close enough to meet and interbreed,” says Staubwasser. In fact, the DNA analysis of one of the oldest Homo sapiens skeletons in Europe, also found in Romania and dated to around 40,000 years ago, has shown that this individual’s ancestors had interbred with Neanderthals between four to six generations earlier, meaning there still had to be Neanderthals around to have sex with.

But by the end of the second stadial, the only people left in Europe were Homo sapiens. Even if a few Neanderthals did survive the second cold spell, they would have been quickly absorbed into the much larger sapiens groups, Staubwasser notes.

“We didn’t actively replace them,” he hypothesizes. “They retreated, and we followed.”

How could the newcomer sapiens emerge relatively unscathed from Europe’s double deep freeze, while the Neanderthals, who after all were the region’s original inhabitants, ultimately succumbed?

In the race for survival, Homo sapiens may have had an edge thanks to a more varied diet, which included a larger share of plants and fish than that of the Neanderthals, who relied more heavily (although not exclusively) on the large mammals that would have been most affected by the change in climate, Staubwasser postulates.

This theory is supported by multiple past studies of tooth wear, bone isotopes and fossilized feces of the two hominin groups. One 2016 a study by Tel Aviv University archaeologists suggested that Neanderthals had evolved their large chests and pelvises precisely so they could better metabolize the high amounts of protein they were getting from their diet. As often happens in evolution, the high level of specialization that made the Neanderthals into lean, mean meat-eating machines may have contributed to their doom when conditions changed and there were fewer animals around to hunt.

Still, we should not think too much of our adaptation skills, as anatomically modern humans were not immune from the effects of the deadliest times of the last Ice Age. In the roughly six millennia after the Neanderthals went extinct, there were at least two more extreme cold spells, the PNAS paper notes. After each stadial, the continent’s human population was replaced by a new one, likely immigrating from the south-east, with a completely different genetic makeup, the study concludes.

“The pattern of genetic turnovers in Europe continued,” Staubwasser says. “Whenever one of these cold intervals occurred, it seems to have triggered a full cycle of depopulation and repopulation by different people with a different culture.”

Not all researchers believe the PNAS study closes the case on the Neanderthals’ disappearance.

"I agree that weather plays a major role in human demography, but I doubt it brought about the extinction of the Neanderthals,” says Israel Hershkovitz, a physical anthropologist at Tel Aviv University. "The Neanderthals experienced lots of cold episodes prior to 45,000 years ago, why would they affect their demography only at the very end?"

Hershkovitz questioned whether the results of the stalagmite analysis could be applicable to the entire continent, noting that “there is great variability among European sites in regard to climatic records: some testify to severe cold while others suggest a comfortable climate.” So, at least for some, it seems that the jury is still out on this ancient mystery.