Early human hybridization was affected by Ice Age cycles?

It is certain that the first human crosses involved two now-extinct human species: Neanderthals and Denisovans. Usually, these two “cousins” rarely intersect. In fact, they had different environmental preferences. However, changes in climate have led to migrations of both species. Then they managed to meet and mingle. This, however, is what a new study published in science.

Neanderthals and Denisovans, distant cousins

Homo sapiens, our current human species, has not always been the only species of the human race. on the contrary ! Several thousand years ago, during Middle ice age (which extends from 780,000 to 128,000 BC) other forms of humans appeared. In Eurasia in particular, two species are evolving. To the west the Neanderthals settled, while to the east the Denisovans.

Currently, mainly Neanderthals that we find in the form of fossils. And so our distant cousin disappeared nearly 40,000 years ago. It is still our closest relative today and looks a lot like us. However, Neanderthals still differed from us, especially in the skull. The latter thus has a protrusion of the supra-orbital ridge, broad teeth and large eyes. Their muscles were compressed and massive to adapt to the harsher and colder climates of the time.

for Denisova, There are still many mysteries to be cleared up… The discovery of this new human species is relatively recent. In 2010 a team of researchers discovered the remains in Denisova Cave in Siberia. These fossils did not belong to Neanderthals, so scientists concluded that there was a new member of the human family! Denisovans lived around the same time as Neanderthals. Moreover, they are closer to this kind than to ours.

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Neanderthals and Denisovans lived in different regions

Despite living at the same time, the two species preferred different climatic conditions. Thus Denisovans were better adapted to cold environments. Thus, they settled in the boreal forests or tundra regions of northeastern Eurasia. In contrast, Neanderthals preferred the warmer forests and grasslands of the Southwest. So, probablyThe two cousins ​​rarely met.

However, we do know that modern humans carry in their genome small amounts of DNA that come from Neanderthals and Denisovans. In addition, previous study known religious, Daughter of a Denisovan father and a Neanderthal mother. This tiny baby could have been born 90,000 years ago, as a result of the first human crossings.

So hybridization appeared a long time ago. The two species certainly didn’t have much opportunity to meet. However, when possible, the “cousins” did not appear to have any prejudice towards each other and mated without concern. These first human crossings are found today in our DNA. Denisova left less than 1% of its genome in Asian and Native American populations and up to 5% in Aboriginal Australians or Papua New Guineans. Also, all modern humans except the population in Africa have about 2% of the Neanderthal genome.

However, scientists still know little about when, where, and how often this human interbreeding took place.

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Changes in climate that may have affected these human crossings

So the researchers wanted to know more about the frequency and locations of these matings. For this, Korean and Italian scientists worked side by side. They used fossil data, computer simulations of past climate, and genomic information.

Dr. Jiaoyang Ruan is a postdoctoral researcher at the IBS Center for Climate Physics, South Korea. It indicates that ” We tried to understand the possibility of mixing between Neanderthals and Denisovans. For this, we used species distribution models. These combine many fossil, archaeological and genetic data. These models ran into transient simulations of the combined general circulation model of global climate and biome. “.

Studies have shown that although Neanderthals and Denisovans did not cohabit together, they did occasionally intersect. Indeed, at that time, the Earth was experiencing displacements in its orbit. This led to changes in climatic conditions and, consequently, in the distribution of vegetation. All of these modifications had an effect on the diet of both types of humans. So they had to move to other overlapping habitats at times. This thus increased the chances of miscegenation.

For more detail, the team looked at the change in vegetation patterns in Eurasia over the past 400,000 years. Thus the researchers saw that during this period, the concentration of carbon dioxide in the atmosphere was high and interglacial conditions were mild. This caused the temperate forest to expand eastward into Eurasia. So the Neanderthals “followed” the forest to meet the Denisovans. On the contrary, when the climate cooled, the two species separated and thus the chances of mating between them decreased.

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Human crossings that occur in “hotspots”

The scientists took their analyzes even further. They wanted to identify the “hot spots” of communication between the two human species. That means the areas where Neanderthals and Denisovans had the best chance of meeting.

Thus, they identified several places in Eurasia. It corresponds to Central Eurasia, the Caucasus, the Tian Shan, and the Changbai Mountains. Tian Shan is a mountain range that stretches across China, Kazakhstan and Kyrgyzstan. As for the Changbai Mountain Range, it extends from China to North Korea. These different models allowed the researchers to place Denny in the climatic context of the time and also confirmed other previously observed mating episodes.

Finally, the researchers also note that the two human species had high odds of encounters in the Siberian Altai at different times: between 340 and 290 thousand years ago, between 240 and 190 thousand years ago, and between 130 and 80 thousand years ago.

Therefore, climate change has severely affected human interbreeding and is still leaving its genetic traces withinHomo sapiens.

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