Humans may not have been responsible for the ultimate extinction of the Neanderthals according to a new study by Dutch researchers who wanted to know if inbreeding and other demographic problems alone could have wiped them out.
Many different viable theories have been put forward by scientists to explain the disappearance of Neanderthals. Some think that climate change did them in, while others assume Homo sapiens (Anatomically Modern Humans) were superior, thus resulting in Neanderthals not being able to compete. Inbreeding likely also played a role. And genetic studies have shown that Neanderthals mated with humans, which is why their DNA lives within us today, meaning this blending could have hastened their demise as well.
But Krist Vaesen, an associate professor at Eindhoven University of Technology, and a team of researchers from other universities wanted to focus on inbreeding and demographic problems to find out if those alone could account for the extinction of the Neanderthals 40,000 years ago.
So, they developed computer models to eliminate all other possible factors except demographic factors, with inbreeding being one of the prime factors.
In a study published by PLOS ONE, the team pointed out that Neanderthals, as well as other hominid species, had a relatively small population, with estimates ranging from 5,000 to 70,000 individuals spread out in small clans across Europe into Asia. And because Neanderthals may have had a more difficult time finding suitable mates outside the gene pool of their small clans, they were forced to choose mates inside the gene pool, resulting in health problems that made their situation even worse in the long run.
According to the study:
The models presented here implement three basic factors that, according to conservation biology (the field from which our models were drawn), would put such small populations at risk of extinction: inbreeding, Allee effects and stochasticity. Inbreeding depression refers to the reduction in fitness of individuals that arise from matings between genetic relatives, matings thus that are more likely to occur in small populations. Inbreeding, which seems to have been common in Neanderthals, might lead to a lower fitness because it increases the chances of the expression of recessive, deleterious traits and because homozygotes often have a general disadvantage relative to heterozygotes. Allee effects refer to the effects that population density has on reproduction and, thus, on population growth. At lower densities, the case we are concerned with here, growth rates might drop due to problems in mate-finding, and to several problems that highly cooperative species, such as Neanderthals, are particularly susceptible to, including low availability of helpers in cooperative hunting, defending kills from kleptoparasites, and allo-parenting. Finally, stochastic, annual fluctuations in births, deaths and sex ratio are more likely to place smaller populations on a trajectory towards extinction than bigger ones.
After running the models, inbreeding over the course of 400,000 years did, indeed, result in extinction.
Our results support the hypothesis that the disappearance of Neanderthals might have been the result of a demographic factors alone, that is, the result merely of the internal dynamics that operate in small populations…Our results indicate that the disappearance of Neanderthals might have resided in the smallness of their population(s) alone: even if they had been identical to modern humans in their cognitive, social and cultural traits, and even in the absence of inter-specific competition, Neanderthals faced a considerable risk of extinction. Furthermore, we suggest that if modern humans contributed to the demise of Neanderthals, that contribution might have had nothing to do with resource competition, but rather with how the incoming populations geographically restructured the resident populations.
Our models indicate that these factors alone could have resulted in Neanderthal extinction, even if Neanderthals and AMHs were identical in terms of individual-level traits that are deemed relevant to persistence or extinction (e.g., cognitive and technological ability, sociality).
There are currently 7 billion humans living on this planet, and we currently face resource shortages that were, frankly, non-existent over 40,000 years ago. There would have been enough plants and animals for both humans and Neanderthals to gather and hunt. Enough wood and stone to make weapons and tools, and more than enough water to drink. Humans may have just been better at getting these resources and utilizing them, but they did not necessarily have to compete for them.
But the study asks a key question and provides an answer.
If Neanderthals lived in small populations since ~400 kya, why did it take so long for them to become extinct? A first relevant consideration concerns demographic stochasticity. We have seen that annual fluctuations in births, deaths and sex ratio might determine whether and when a small population disappears. So our results are consistent with a scenario in which a small population of Neanderthals persists for several thousands of years, and then, due to a stroke of bad luck, disappears…It is not implausible that, despite regular local extinction events, a small metapopulation manages to survive over prolonged stretches of time but eventually dies out due to its overall size and stochasticity. Noteworthy, there is nothing unusual about the persistently small size of Neanderthal populations. Hominin populations likely were small throughout the Pleistocene
“Small populations can survive over long stretches of time, and then suddenly disappear due to random fluctuations in births, deaths, sex ratio, and environmental pressures,” Vaesen said in a statement.
Indeed, the same is true of any species. For instance, a small population of mammoths continued to persist on a remote island off the coast of Russia in the Arctic for several thousand years after the main population had already died out. But inbreeding and later environmental factors resulted in their ultimate extinction as well.
Related: Massive ancient tree reveals clues about polar shift that may have ended the Neanderthals
Another scenario the study suggests is that humans may have only contributed to Neanderthal extinction rather than caused it.
“Our results suggest another plausible scenario: a peaceful co-existence of the two species, a scenario in which the mere presence of bands of modern humans reduced the opportunities for Neanderthals to find mates, intra-breed and migrate,” Vaesen said.
Faced with these difficulties, some Neanderthals mated with humans, and as their species died out, their genetic code lived on through generations of humans over tens of thousands of years into the present day. In a way, that means the Neanderthals did not go entirely extinct but rather saved themselves by interbreeding instead of inbreeding when given the opportunity.
Still, inbreeding is not the only reason why Neanderthals were wiped out. Multiple factors combined over hundreds of thousands of years and the small population of Neanderthals simply could not overcome them in the end. That is why one of Charles Darwin’s main natural laws is “survival of the fittest.”
Humans were more fit for long term survival as a species than the Neanderthals and did not have to rely on inbreeding to grow the population.
“Regardless of whether external factors or resource competition played a role in the extinction of Neanderthals, our study suggests that any plausible explanation also needs to incorporate demographic factors as key variables,” Vaesen concluded.
The extinction of the Neanderthals highlights all too well how vulnerable any species can be to becoming extinct if the right conditions and factors are present, from lack of resources to population isolation, to small numbers, environmental disaster, and competition with other species. Their demise should help us learn how to prevent future extinctions, including our own.
Related: Study suggests Neanderthals were wiped out by a common childhood ear infection
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