02 November 2022

Make love not war

Interbreeding with humans could have caused the extinction of Neanderthals

Ilya Dochar, ERR

A new article published in the journal PalaeoAnthropology (Stringer, Crété, Mapping Interactions of H.neanderthalensis and Homo sapiens from the Fossil and Genetic Records) raises the question that interbreeding with our ancestors reduced the number of Neanderthals reproducing with each other, which at one point led to their disappearance.

Although only 32 Neanderthal genomes have been sequenced to date (which leaves the possibility that the absence of Homo sapiens DNA in their genome is actually a sampling quirk), the authors hope that advances in DNA sequencing technology will be able to refute this hypothesis.

"Our knowledge of the interaction between Homo sapiens and Homo neanderthalensis has become more complex over the past few years, but it is still rare to see a scientific discussion of how interbreeding actually occurred between these groups," says one of the authors of the article, scientific director of the Museum of Human Evolution, Professor Chris Stringer. — We assume that this behavior could lead to the extinction of Neanderthals. If they bred with Homo sapiens, this could lead to a blurring of the population until their disappearance."

The first meetings of Neanderthals and Homo sapiens

Neanderthals and Homo sapiens separated from each other about 600,000 years ago and evolved in different parts of the world.

Neanderthal fossils have been found in Europe and Asia, as far south as Siberia. They are thought to have spent at least 400,000 years evolving in this environment, adapting to a predominantly cooler climate than today.

At the same time, the ancestors of our own species evolved in Africa. It is currently unclear whether Homo sapiens are direct descendants of one group of ancient African hominids or the result of a mixture of different groups spread across the continent.

Judging by the genetic data, Homo sapiens and Homo neanderthalensis first met each other when our ancestors began raiding from Africa from time to time about 250,000 years ago.

"Without knowing exactly how Neanderthals looked and behaved, we can only assume that Homo sapiens would have mistaken them for their relatives," says Stringer. — The language differences were probably greater than we can imagine, given the temporal depth of the separation. They would be much larger than between any modern languages."

The language barrier may have been reinforced by the individual characteristics of both species. A comparison of Neanderthal and homo sapiens suggests that the brain and vocal apparatus of these species were different. The genomes of Neanderthals also show that almost 600 genes manifested differently in our species and their species, especially those related to the face and voice.

Another notable difference was the forehead: Neanderthals had a prominent brow arch that could be used for social communication. However, the signals they could transmit could be incomprehensible to our ancestors. Some studies suggest that the reduced brow ridges allowed Homo sapiens to use eyebrows instead to transmit a number of more specific signals.

In any case, these encounters eventually led to the crossing of two species between both species, but exactly how this happened is shrouded in mystery.

The crossing of a Neanderthal and a reasonable person

We have known that our species interbred with Neanderthals since the first genomes of our relatives were sequenced.

However, the Neanderthal genes available today are not the result of the earliest single interactions with humans when they first left Africa. They originate from later times, when about 60,000 years ago modern humans began to migrate en masse to other parts of the world.

Then the crossing could be the result of either mutual courtship or less friendly actions. Collisions between separate groups of our closest living relatives, chimpanzees, indicate both types of behavior.

Whether the crossing was successful or not seems to depend on each particular pair that bred. There is still no evidence of Homo sapiens genetics in the genomes of late Neanderthals dating back 40-60 000 years ago.

Perhaps this is due to the process of hybridization itself, since some species are able to produce offspring only in certain directions. For example, the pollen of the Capsella rubella plant can successfully fertilize the seeds of Capsella grandiflora, but not vice versa.

The absence of mitochondrial DNA in living humans, which is inherited from Neanderthals through females, has been proposed as evidence that only Neanderthal males and Homo sapiens females could mate. There is also some evidence that male hybrids may have been less fertile than females.

Due to the smaller number of Neanderthals breeding with each other, and the size of groups that were already small and scattered due to the environment, hybridization outside of Neanderthal family groups could push this species into decline. However, at the moment there is not enough evidence to talk about an unambiguously correct theory.

"We don't know if the obvious one—way gene flow is caused by the fact that nothing else just happened, that reproduction took place but was unsuccessful, or whether the genomes of Neanderthals that we have are unrepresentative," Stringer says. "As more Neanderthal genomes are sequenced, we will be able to see if any nuclear DNA was passed from Homo sapiens to Neanderthals, and demonstrate whether this idea is true."

Future research may also explore similar questions regarding another hominid species known as Denisovans, which will give us a more complete picture of how our species interacted with its closest relatives.

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