Scientists say climate change played big role in early human interbreeding

By Jim Leffman via SWNS

Climate change played a big part in the early interbreeding of human species, scientists claim.

It is known that modern man is also bred with extinct species such as Neanderthal and Denisovan as we retain traces of them in our DNA.

Now researchers believe that warm periods in between ice ages allowed different species adapted to different climates to mingle and interbreed.

The team from South Korea and Italy studied rare bones from a girl with a Denisovan father and Neanderthal mum.

The remains, nicknamed Denny, from 90,000 years ago were discovered in 2018 in a cave in Siberia, the only place where Denisovan remains have been found.

Up to six percent of the genes in modern Papua New Guineans and between three and five percent of aboriginal Australian DNA is Denisovan.

Because Neanderthals developed after hominim species left Africa, the percentage of Neanderthal DNA in modern African humans is zero or close to zero but about one to two percent in people of European or Asian background.

In a new approach to study hybridisation the two teams looked at climate rather than genetics to explain how it happened.

The study, published in the journal Science, concluded Denisovans were much more adapted to cold environments, such as boreal forests and even tundra, compared to their Neanderthal cousins who preferred temperate forests and grassland.

They used a supercomputer to simulate the climate during interglacial periods when Earth’s orbit around the Sun was more elliptic and northern hemisphere summer occurred closer to the Sun.

As the earth warmed up their habitats overlapped and allowed the hominims to interbreed.

The scientists also looked at how vegetation patterns changed over Eurasia during the past 400,000 years.

They discovered that elevated atmospheric CO2 concentrations and mild interglacial conditions caused an eastward expansion of temperate forest into central Eurasia which created dispersal corridors for Neanderthals into Denisovan lands.

Dr. Jiaoyang Ruan, at the IBS Center for Climate Physics (ICCP), South Korea and lead author of the study said: "Their habitats of choice were separated geographically, with Neanderthals typically preferring southwestern Eurasia and Denisovans the northeast.

“It is as if glacial-interglacial shifts in climate created the stage for a unique and long-lasting human love story, whose genetic traces are still visible today."

Professor Axel Timmermann, corresponding author of the study and director of the ICCP and professor at Pusan National University added: “When Neanderthals and Denisovans shared a common habitat, there were more encounters and interactions among the groups, which would have increased the chance of interbreeding."

The simulation of past habitat overlaps puts the first-generation Neanderthal/Denisovan hybrid Denny into a climatic context.

It also agrees with other known episodes of interbreeding. Future paleo-genetic reconstructions can be used to test the robustness of the new supercomputer model-based predictions of potential interbreeding intervals around 210,000 and 320,000 years ago.

Professor Pasquale Raia from the University of Naples, Federico II in Italy, co-author of the study said: "To deal with the very sparse Denisovan dataset, we had to devise new statistical tools, which could also account for known ancestral relationships amongst human species.

“This allowed us for the first time to estimate where Denisovans could have lived.

"To our surprise, we found that, apart from areas in Russia and China, also northern Europe would have been a suitable environment for them”

Whether Denisovans ever lived west of the Altai mountains is unknown but it can be tested using large-sample genetic analyses of Denisovan ancestry in European populations.

Such analysis is expected to shed new light on the relationship between early dispersal, habitat encroachment and human genetic diversification.