Terrestrial civilisation has developed with only one human species. But it could have been otherwise. It is not yet certain how many species of our genus have existed; currently the count is between ten and eleven, if we include the recently described Homo longi and Homo daliensis, not yet accepted by the entire scientific community. But we do know that until about 40,000 years ago, the blink of an eye in geological time, at least one other close relative shared this planet with us. They were the Neanderthals, Eurasian natives whose brutish reputation has been rehabilitated by scientific findings that have rediscovered them as a species very similar to us in many ways. But there is one thing we still don’t know about them: why they are no longer with us. And who they were may help to explain why they went extinct, and this is why the knowledge provided by new technologies is key to unravelling this chapter of our common past.
We have known about Neanderthals since the 19th century. For decades they were considered a primitive species, rightly extinguished when they came up against the intellectual superiority of Homo sapiens. It was around this time that the German anatomist Gustav Schwalbe introduced a linear view of human evolution: Pithecanthropus erectus (now Homo erectus) discovered in Java by Eugène Dubois would have evolved into Neanderthal or Homo primigenius, and this in turn into modern humans. This idea of a linear evolution would later be represented in the famous drawing of the “march of progress,” erroneous but still very popular today. The press of the time went so far as to portray Neanderthals as beasts that walked on all fours and lacked the ability to smile. Signs of cannibalism found in 1899 in Krapina, Croatia— a disputed hypothesis—reinforced their reputation as savage barbarians.
But even from the earliest finds of Neanderthal remains, the linearity proposed by Schwalbe was disputed: other scientists did not see Neanderthals as an ancestor of modern humans, but rather as a parallel line. Over time, evidence and analysis proved this second option to be correct: the lineages that would give rise respectively to Neanderthals and sapiens separated their evolutionary destinies more than 800,000 years ago, and also their geographical domains: our origins are in Africa, while Neanderthals are the true Eurasian natives.
At the same time, excavations have been unearthing much evidence that has improved the image of the Neanderthal people. Like us, they made tools, wore clothing, controlled fire and buried their dead. It is possible that they made music. They shared with us certain mutations in the FOXP2 gene, involved in language, although whether they spoke like us depends on other traits and is still being studied. They engraved symbols on European cave walls 20,000 years before modern humans arrived on this continent. And as for cannibalism, now confirmed, it seems that may have had a ritual rather than a dietary purpose, as we sapiens have done in many cultures. And some Neanderthals were also devoured by our ancestors.
But even if the latter is true, it doesn’t mean that we ate the Neanderthals to extinction. The annihilation of our European relatives still has no clear explanation: if they were so similar to us, why did they disappear? Given that Neanderthals and modern humans overlapped in Europe for a few thousand years after the arrival of the latter from Africa—between 3,000 and 5,000 years, although recent research puts the figure up to perhaps 8,000 or even more than 10,000 years—the traditional hypothesis assumed that in the competition for resources only one human species could remain. The Neanderthals were the losers, either through direct conflict or perhaps due to climate change that affected them to a greater extent because of their more restricted diet and greater energy needs.
Organizational capacity may have been key
In recent years, a new theory has been added, linking the decline of Neanderthals to their own characteristics that differentiated them from modern humans. Several clues, both anatomical and archaeological, point to the possibility that Neanderthals had less capacity for social organization than sapiens, which would have made them more vulnerable in times of scarcity. In 2014, an analysis of Neanderthal genomes led by the Max Planck Institute for Evolutionary Anthropology (Germany) and published in the journal PNAS revealed that our relatives had low genetic diversity and lived in small, isolated groups. Compared to sapiens, Neanderthals possessed less variety in certain genes related to certain behaviours, in particular traits such as hyperactivity and aggressiveness. This possibility of learning more about an extinct species through the DNA found in remains has become a reality in recent decades thanks to the development of paleogenomics , a new technology whose main proponent, the Swedish geneticist Svante Pääbo, received in 2022 the first Nobel Prize in Physiology or Medicine awarded for the study of human evolution.
Could these genetic differences explain different behaviour that would have been detrimental to the survival of Neanderthals? The study’s co-lead author, Sergi Castellano, is decidedly cautious: “We don’t know the phenotypic effect of these genetic variants, so they don’t support any behavioural theories,” he tells OpenMind. The difficulty, he adds, lies in inferring behavioural traits from genes. According to the researcher, work is currently being done along these lines by introducing the Neanderthal and sapiens variants separately in mice, “but it will take years of experiments” to reach any conclusions, he stresses.
However, there is someone who has attempted to advance our knowledge using another approach, that of evolutionary psychology. Glenn Geher, of the State University of New York at New Paltz, based on the established fact that most of today’s humans, except those of African origin, retain in our genome around 2% of Neanderthal DNA, the result of ancient crossbreeding between the two species which occurred over a long period of time and until the Neanderthals were almost extinct. Geher recruited volunteers willing to provide a genetic analysis of their “Neanderthaleness” and subjected them to an extensive behavioural and personality test. Using classical methodology in psychology, the researcher then correlated the greater or lesser presence of Neanderthal variants with behavioural traits.
Using this approach, Geher found a “small but statistically significant” correlation between the percentage of Neanderthal genes and personality facets, as he explained to OpenMind. And interestingly, the results were “consistent with this basic theme regarding the sociability of Neanderthals.” Specifically, the psychologist observed that individuals with a higher degree of genomic “Neanderthaleness” are averse to strangers and more prone to nervousness and anxiety, traits possibly related to lower sociability. According to Geher, his study may open “a brand-new avenue to explore the nature of these ancestral cousins of ours—as well as the reasons for their ‘demise’.”
No conclusive evidence
The socialisation hypothesis still has a long way to go, if new evidence does not invalidate it before then. But the social capacity of Neanderthals has continued to be the subject of study and debate. In 2016, French and Belgian researchers published in the journal Nature the discovery deep in Bruniquel cave, in southwest France, of a set of large circles built with pieces of broken stalagmites. At around 176,000 years old, these stone rings attributed to Neanderthals are among the oldest examples of human construction. The authors of the study wrote: “Our results therefore suggest that the Neanderthal group responsible for these constructions had a level of social organization that was more complex than previously thought for this hominid species.”
Study co-author Jacques Jaubert of the University of Bordeaux, France, told OpenMind that in his opinion there is no reason to imagine major differences in the way of life between Neanderthals and modern humans who lived in the same period, even if the two groups changed over time. In any case, Jaubert stressed that the Bruniquel circles, built 120,000 years before the extinction of Neanderthals, are too old to shed any light on the question.
Other evidence for a somewhat complex social organisation has also come from paleogenomics. In 2022, a genetic study of 13 Neanderthal genomes found that related individuals formed communities, as expected, but also that young women probably migrated to other groups to start their own families. While this custom may have been aimed at avoiding inbreeding, it does reveal a certain level of social structure.
Other clues about possible Neanderthal behaviour have emerged from an even newer technology than paleogenomics: brain organoids, tiny clusters of neurons created in culture from stem cells that attempt to replicate basic brain development. The use of new genetic cut-and-paste tools such as CRISPR has allowed researchers to create mini-brains with Neanderthal genes and see how they differ from those made to resemble modern humans.
The Neanderthal presence in our genome
The results are interesting: the Neanderthal version of a gene called TKTL1 reduces the production of neurons. When scientists introduced this gene variant into mice and ferrets, they observed that the neuronal shrinkage mainly affects the frontal neocortex, a brain region involved in higher cognitive processes. In another study, Neanderthal versions of certain proteins involved in cell division function worse than those of modern humans, resulting in more errors in the creation of neurons. These experimental approaches suggest that Neanderthals may not have reached the cognitive sophistication of the brain of modern humans.
On the other hand, studies have shown that the Neanderthal presence in our genome has decreased over time, from 10% 45,000 years ago to 2% today. It seems that our genome has been shedding some Neanderthal genetic heritage. And while it is still an open question whether this might have improved our cognitive ability, it does at least appear to have been linked to an increased burden of some ailments in our extinct relatives. The possibility that disease played a role in the extinction of Neanderthals also stems from the hypothesis that perhaps we sapiens are better adapted to the infections they suffered, while they succumbed to ours. Competition? Reduced social or intellectual abilities? Climatic changes? Disease? Perhaps weaponry worse than ours, as some studies suggest? Or a bit of all of these? “There is certainly not only one reason that caused the demise of the Neanderthals,” concludes Jaubert, and nothing seems to indicate that this prehistoric mystery will be solved any time soon. For Geher, however, there is another interpretation of the matter. Given that billions of humans carry Neanderthal heritage in our genes, they are in a sense still present, leading the psychologist to paraphrase Mark Twain: “I would argue that the news of the extinction of the Neanderthals is greatly exaggerated.”
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