Humans weren’t always alone on our branch of the evolutionary primate tree. Among our closest cousins, none are better known than the Neanderthals.
Formerly the name was a insult we applied to our comrade Homo sapiens – we viewed Neanderthals as a less sophisticated and less intelligent version of ourselves. This perspective has changed over time, as has the nature of investigating the mysterious disappearance of ancient humans.
While we still can’t be certain, it’s possible the Neanderthals escaped by chance. Their small, isolated populations scattered across Eurasia were still on the brink of extinction.
Our evolutionary vision of Neanderthals
Hominids known as Neanderthals had a long run. Evidence suggests they emerged in Eurasia at least 400,000 years ago and may have persisted until 35,000 years ago in some areas, such as Gibraltar, which is located at the southern tip of Spain.
The first Neanderthal fossil was discovered in Germany in 1856, just three years before Charles Darwin’s publication About the origin of species. This discovery sparked a wave of controversy: some suggested that the remains belonged to an unknown human species; others have argued that the body came from a sick human. One of Germany’s most famous professors suggested that it may have been a Russian Cossack who suffered from rickets and crawled into a cave to die in Napoleonic times, Gerrit Dusseldorp explains, archaeologist from the University of Leiden. But the discovery in 1886 of similar remains in Belgium invalidated this theory.
In fact, our understanding of our closest cousins has changed dramatically over time, says Dusseldorp. After deciding that Neanderthals were their own unique creature, we formed the common perception that they were like us, but much less intelligent. William Golding’s 1955 novel The Heirs popularized a meeting between humans and our gentle but obscure cousins. “In the end, it doesn’t end well for them,” says Dusseldorp.
Our assumptions about the fate of Neanderthals have also changed over time. The oldest theory is that we outmatched them: we were smarter or stronger, and we either wiped them out directly by killing them or by taking control of the resources. The relatively close time between our arrival in Eurasia and their demise lends weight to this theory, he explains, as modern humans seem to have largely started getting there around 40,000 years ago.
Modern evidence has also complicated the picture. Genetic studies and fossil remains suggest humans entered Neanderthals 210,000 years ago; our own DNA indicates that we crossed paths with Neanderthals, suggesting peaceful contact; and we found evidence of the use of sophisticated tools, including a prehistoric glue made from birch bark tar, as well as jewelry and rock art.
In Sapiens: a brief history of humanity, explains Dusseldorp, author and historian Yuval Noah Harari refers to a cognitive revolution in modern humans preceding our foray into Eurasia 40,000 years ago. “Just as we have discovered evidence of sophisticated Neanderthal behavior over time, some of the behaviors associated with this revolution also have a much longer temporal depth,” says Dusseldorp. “The first humans probably had them.”
But a 2019 PLoS A paper posed a very different theory, which has gained ground among archaeologists: Neanderthals may have become extinct because their often-tenuous population fell below sustainable levels – by entirely natural forces.
Study author Krist Vaesen, associate professor of philosophy and applied biologist at Eindhoven University of Technology in the Netherlands, explains that he and his colleagues aimed to study the null hypothesis of the extinction of Neanderthals: A null hypothesis assumes no difference between sample populations (in this case, modern humans and our Neanderthal relatives), and that any difference between them is due to chance or error.
Previous theories of Neanderthal extinction were based on assumptions of inferiority to modern humans. Yet no researcher had tested the idea that they were just as capable of surviving as we are. So, rather than assuming that there were differences between us and Neanderthals, Vaesen and his colleagues assumed that we were functionally interchangeable. Were they going to disappear again?
By applying conservation biology simulations used to track endangered species such as tigers, they highlighted three key factors that often determine the fate of endangered species: inbreeding, Aisle effects and stochasticity. Essentially, they wanted to use estimates of Neanderthal population counts to determine whether demographic factors such as inbreeding, random variations in sex ratios, random events such as floods or droughts, and difficulty finding a mate. could explain the disappearance of the Neanderthals.
And depending on their model, it could. Neanderthals have spent much of their existence with a population near extinction, says Vaesen. Random variations in reproductive potential and population size, combined with inbreeding reducing both fertility and survival, could sufficiently explain the disappearance of our cousins.
Maybe humans played some role, but it might not have been competitive, says Vaesen. If humans blocked the migratory routes used by Neanderthals, we may have contributed to their extinction by forcing more inbreeding without directly interfering with them, he says.
This theory of the disappearance of Neanderthals is gaining ground. A Vaesen and Dusseldorp survey published earlier this year at Scientific reports found that among 216 paleoarchaeologists demography was the dominant theory, followed by environmental factors and then competitive factors.
Consider the simple factor of geography: Neanderthals likely lived in smaller groups than the dozens that make up modern hunter-gatherer groups among humans, says Dusseldorp. Rough estimates of the Neanderthal population drop to 10,000 individuals at a time.
Now consider that Neanderthal fossils have been found in areas as far away as England, Iraq, and Siberia. Spreading 10,000 people over an area of 21.14 million square miles results in a population density of about 0.0005 per square mile. “I find it increasingly difficult to see a population of only a few thousand individuals spread over such a large area, being Darwin’s intelligent experience of nature,” says Dusseldorp. “Basically a tropical primate [living] in a gigantic steppe in such a small number, it seems a precarious existence at the best of times.
This happened in one of the most precarious climatic environments on Earth, in which the Ice Ages passed at times warm enough for hippos to live in the Rhine and vice versa, he explains.
The definitive cause of the Neanderthals extinction, however, is far from clear. Dusseldorp would like to see further evidence of modern human demography in Africa and investigation of evidence of interconnection, or lack thereof, in different geographic populations of Neanderthals. “I’m going into demographics, but there are quite a few unanswered questions,” says Dusseldorp. “I think in reality it will always be a combination of factors,” he says.