Atmospheric CO2 Levels and Historical Fluctuations as Factors Influencing Interbreeding in Early Human Species

Wednesday - September 20 2023, 19:49 UTC - 1 year ago

A new study published in the journal Science has revealed that the fluctuation in atmospheric CO2 levels during pre-historic times and the subsequent changes in climate and vegetation were significant factors influence when and where early human species interbred. Using existing paleo-anthropological evidence, genetic data, and supercomputer simulations. It was concluded that interglacial warm periods increased the likelihood of interbreeding between Neanderthals and Denisovans and the Monroe Interglacial period of Earth (~400,000 years ago) was the optimal period for hominin expansion and interbreeding due to increased levels of CO2 in the atmosphere.

A new study published in the journal Science by an international team reveals that historical fluctuations in atmospheric CO2 levels and the subsequent alterations in climate and vegetation were significant factors influencing when and where early human species interbred. Modern-day humans possess a small proportion of DNA in their cells that originated from other hominin species, specifically the Neanderthals and the elusive Denisovans. Back in 2018, scientists announced to the world the discovery of an individual, later nicknamed Denny, who lived 90,000 years ago and who was identified as a daughter to a Denisovan father and a Neanderthal mother [Slon et al. 2018]. Denny, along with fellow mixed-ancestry individuals found at Denisova cave, testifies that interbreeding was probably common among hominins, and not limited to our own species Homo sapiens.

To unravel when and where human hybridization took place, scientists usually rely on paleo-genomic analysis of extremely rare fossil specimens and their even scarcer ancient DNA content. In the new Science paper, the team of climate experts and paleo-biologists from South Korea and Italy pursued a different approach. Using existing paleo-anthropological evidence, genetic data, and supercomputer simulations of past climate, the team found that Neanderthals and Denisovans had different environmental preferences. More specifically, Denisovans were much more adapted to cold environments, characterized by boreal forests and even tundra, compared to their Neanderthal cousins who preferred temperate forests and grassland.

"This means that their habitats of choice were separated geographically, with Neanderthals typically preferring southwestern Eurasia and Denisovans the northeast," says Dr. Jiaoyang Ruan, a postdoctoral researcher at the IBS Center for Climate Physics (ICCP), South Korea and lead author of the study. However, according to their realistic computer simulations, the scientists found that in warm interglacial periods, when Earth’s orbit around the Sun was more elliptic and northern hemisphere summer occurred closer to the Sun, the hominin habitats began to overlap geographically. "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", adds Prof. Axel Timmermann, corresponding author of the study and director of the ICCP and professor at Pusan National University.

The simulation of past habitat overlaps does not only put the first-generation Neanderthal/Denisovan hybrid Denny into a climatic context, but it also agrees with other known episodes of interbreeding ~78, 120 thousand years ago. Future paleo-genetic reconstructions can be used to test the robustness of the new supercomputer model-based predictions of potential interbreeding intervals around 210 and 320 thousand years ago.

To further determine the climate drivers of the east-west interbreeding seesaw, the scientists looked more closely at how vegetation patterns changed over Eurasia during the past 400 thousand years. They discovered that elevated atmospheric CO2 levels, driven by millennial-scale climate cycles, decreased the latitudinal temperature gradient and shifted vegetation fronts northwards, enabling Homo sapiens to expand, intermix, and later dominate in most of the old Neanderthal and Denisovan territories.