The extinction of the Neanderthals may have been caused by sex, not fighting
A new paper argues that Homo sapiens may have been responsible for the extinction of Neanderthals, not through violence, but through sex.
Making love, not war, may have been responsible for putting the Neanderthals on the path of extinction.
While about 2% of the genome of all living people from outside Africa come from Neanderthals, there is very little evidence that this process went the other way.
A new article, published in the magazine PalaeoAnthropologyraises the prospect that interbreeding with our ancestors would have reduced the number of Neanderthals breeding with each other, leading to their eventual extinction.
While only 32 Neanderthal genomes have been sequenced so far, leaving it possible that the lack of Homo sapiens DNA in their genome is actually a quirk of sampling, the authors hope advances in DNA sequencing technology will solve this hypothesis. by making more genomes available.
Professor Chris Stringer, the museum’s research leader in human evolution, co-authored the new paper with colleague Dr. Lucile Crete.
Chris says, “Our understanding of the interaction between Homo sapiens and Neanderthals has become more complex in recent years, but it’s still rare to see scientific discussions about how the interbreeding between the groups actually occurred.”
“We propose that this behavior could have led to the extinction of Neanderthals if they had regularly breeding with Homo sapiens, which would have eroded their population until they disappeared.”
The first encounters of Neanderthals and Homo sapiens
Neanderthals and Homo sapiens separated from each other about 600,000 years ago and evolved in very different parts of the world.
Fossils of Neanderthals have been found throughout Europe and Asia, as far as southern Siberia. They are believed to have evolved in this environment for at least 400,000 years, adapting to a predominantly cooler climate than is found today.
Meanwhile, our own ancestors types evolved in Africa. It is currently uncertain whether Homo sapiens are the direct descendants of a group of ancient African hominids or are the result of admixture between different groups scattered across the continent.
From genetic datait appears that the two species first met when Homo sapiens began to migrate occasionally from Africa about 250,000 years ago.
“Without knowing exactly how Neanderthals looked or behaved, we can only speculate what Homo sapiens might have thought of their relatives,” Chris says.
“The language differences would probably have been greater than we could imagine, given the time depth of the separation, and would have been much greater than those between any modern languages.”
The language barrier may have been enhanced by the individual characteristics of both species, with comparisons of Neanderthals and Homo sapiens suggesting that the species’ brain and vocal apparatus were different.
Neanderthal genomes also show that nearly 600 genes were expressed differently between our species and theirs, particularly those associated with the face and voice.
Another notable difference would have been the forehead, with Neanderthals having a prominent brow ridge that could have been used for social communication.
However, the signals these ridges were trying to convey may have been lost on our ancestors. Some research suggests that with reduced brow ridges, Homo sapiens could instead turn to the eyebrows to convey a range of subtler, temporary signals.
In any case, these encounters eventually led to breeding between the two species, but exactly how this happened is also shrouded in mystery.
Neanderthal and Homo sapiens interbreed
We know that our species has interbred with Neanderthals since the first genomes of our relatives were sequenced.
However, the Neanderthal genes we have in us today are not the result of these early sporadic interactions that Homo sapiens had when they first left Africa. Instead, they come from the much larger migrations that modern people about 60,000 years ago.
The crossing at this point may have been the result of courtship or may have been less friendly. Encounters between separate groups of our closest living relatives, chimpanzees, show evidence of both behaviors.
Whether or not the crossing was successful seems to depend on the exact pair that was breeding. So far, there is no evidence of Homo sapiens genetics in late Neanderthal genomes dating to between 40 and 60,000 years ago.
It is possible that this is due to the process of hybridization itself, as some species are only capable of producing offspring in certain directions. For example, pollen from the Capsella rubella plant can successfully fertilize Capsella grandiflora seeds, but not the other way around.
The lack of mitochondrial DNA, which is inherited by females, from Neanderthals in living humans has been suggested as evidence that only male Neanderthals and female Homo sapiens were able to mate, but there is also evidence that male hybrids may have been less fertile than females.
With fewer Neanderthals breeding with each other and group sizes already small and scattered due to environment, hybridization outside of Neanderthal family groups could have helped drive the species into decline. At this point, however, there isn’t enough evidence to decide either.
“We don’t know if the apparent one-way gene flow is because it just didn’t happen, if the breeding happened but wasn’t successful, or if the Neanderthals we have are not representative,” Chris says.
“As more Neanderthal genomes are sequenced, we should be able to see if nuclear DNA from Homo sapiens has been passed on to Neanderthals and show whether this idea is correct.”
Future research could also explore similar questions regarding another hominin species known as the Denisovans, giving us a better idea of how our species interacted with its closest relatives.
Chris Stringer et al, Mapping Interactions of H. neanderthalensis and Homo sapiens from the Fossil and Genetic Records, PaleoAnthropology (2022). DOI: 10.48738/2022.iss2.130
Natural History Museum
This story has been republished courtesy of the Natural History Museum. Read the original story here.
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