A new study published in the journal Science by an international team of researchers shows that historical fluctuations in atmospheric carbon dioxide levels and subsequent changes in climate and vegetation were important factors that influenced when and where early human species mated. Modern humans have a small amount of DNA in their cells derived from other humanoid species, notably Neanderthals and the elusive Denisovans.
Photo of the remaining skeletal fragments of Denisova 11 (Denny), the daughter of a Neanderthal mother and a Denisovan father, from Denisova Cave in Russia. Source: Institute of Basic Science
As early as 2018, scientists announced to the world that they had discovered an individual who lived 90,000 years ago and was later nicknamed Danny. After identification, her father was a Denisovan and her mother was a Neanderthal [Slonetal.2018]. Danny and the other hybrid individuals found in Denisova Cave prove that interbreeding between humans may be common and not limited to our own species, Homo sapiens.
To reveal when and where human interbreeding occurred, scientists often rely on ancient genome analyzes of extremely rare fossil specimens and their even rarer ancient DNA content. In the new Science paper, a team of climate experts and paleoanthropologists from South Korea and Italy took a different approach. Using existing paleoanthropological evidence, genetic data and supercomputer simulations of past climates, the team found that Neanderthals and Denisovans had different environmental preferences. More specifically, Denisovans were more adapted to colder environments, characterized by boreal forests and even tundra, while their Neanderthal cousins preferred temperate forests and grasslands.
Schematic diagram of habitats favored by Neanderthals (red scale) and Denisovans (green scale). Potential hybridization regions in Central Asia and Northern Europe are represented by overlapping colors and baby shapes. Source: Institute of Basic Science
"This means that the habitats they chose were geographically separated, with Neanderthals generally favoring southwestern Eurasia and Denisovans favoring the northeast," said Dr. Jiaoyang Ruan, a postdoctoral researcher at the Center for Climate Physics (ICCP) at the Institute of Basic Science in Korea and first author of the study.
However, according to their realistic computer simulations, the scientists found that during warm interglacial periods, when the Earth's orbit around the sun was more elliptical and the northern hemisphere's summers were closer to the sun, the apes' habitats began to overlap geographically. Professor Axel Timmermann, corresponding author of the study, director of ICCP and professor at Pusan National University, added: "When Neanderthals and Denisovans shared a common habitat, there would have been more encounters and interactions between the two groups, thereby increasing the chances of interbreeding."
Simulations of past habitat overlap not only place the first-generation Neanderthal/Denisovan hybrid Denny into a climate context, but also fit with other known hybridization events dating back about 78,000 and 120,000 years ago. Future paleogenetic reconstructions could be used to test the robustness of new supercomputer model-based predictions of potential hybridization intervals around 210,000 and 320,000 years ago.
To further determine the climate drivers of the east-west hybridization seesaw, the scientists took a closer look at changes in Eurasia's vegetation patterns over the past 400,000 years. They found that rising atmospheric carbon dioxide concentrations and mild interglacial conditions caused temperate forests to expand eastward into central Eurasia, creating a dispersal corridor for Neanderthals into Denisovan lands. Dr. Nguyen commented: "The glacial-interglacial climate changes seemed to have set the stage for a unique and enduring human love story, the genetic traces of which are still clearly visible today."
One of the main challenges the researchers faced in their study was estimating the climate conditions that Denisovans would have preferred. "To deal with the very sparse Denisovan data set, we had to devise new statistical tools that could also account for known ancestral relationships between human species," said study co-author Professor Pasquale Raia of the University of Naples Federico II in Italy. "This gave us the first estimate of where Denisovans might have lived. We were surprised to find that, in addition to Russia and China, northern Europe was also a suitable environment for Denisovans," he added. "
Whether Denisovans once lived west of the Altai Mountains is unknown, but it can be tested through genetic analysis of large samples of Denisovan ancestors in European populations. This analysis is expected to shed new light on the relationship between early dispersal, habitat encroachment and human genetic diversification.