Looking at habitat overlap for Neanderthals and Denisovans, Pusan National University researcher Jiaoyang Ruan and colleagues found patterns of interbreeding between these two hominins that correlate with climate and environmental change in Eurasia.
“Contemporary humans carry in their cells a small amount of DNA derived from Neanderthals and Denisovans,” Dr. Ruan and co-authors said.
“The 90,000-year-old hominin individual named ‘Denny’ — recently identified as the daughter of a Denisovan father and a Neanderthal mother — bears testimony to the possibility that interbreeding was quite common among early human species.”
“But when, where, and at what frequency did this interbreeding take place?”
Using fossil data, supercomputer simulations of past climate, and insights obtained from genomic evidence, the authors were able to identify habitat overlaps and contact hotspots of these early human species.
“Little is known about when, where, and how frequently Neanderthals and Denisovans interbred throughout their shared history,” Dr. Ruan said.
“As such, we tried to understand the potential for Neanderthal-Denisovan admixture using species distribution models that bring extensive fossil, archeological, and genetic data together with transient Coupled General Circulation Model simulations of global climate and biome.”
The researchers found that Neanderthals and Denisovans had different environmental preferences to start with.
While Denisovans were much more adapted to colder environments, such as the boreal forests and the tundra region in northeastern Eurasia, their Neanderthal cousins preferred the warmer temperate forests and grasslands in the southwest.
However, shifts in the Earth’s orbit led to changes in climatic conditions and hence vegetation patterns.
This triggered the migration of both these hominin species towards geographically overlapping habitats, thus increasing the chance of their interbreeding.
The scientists further used insights gained from their analysis to determine the contact hotspots between Neanderthals and Denisovans.
They identified Central Eurasia, the Caucasus, the Tianshan, and the Changbai mountains as the likely hotspots.
Identification of these habitat overlaps also helped them place ‘Denny’ within the climatic context and even confirmed the other known episodes of genetic interbreeding.
They also noted that the Denisovans and Neanderthals would have had a high probability of contact in the Siberian Altai during 340,000-290,000 years ago, 240,000-190,000 years ago, and 130,000-80,000 years ago.
To further elucidate the factors that triggered the ‘east-west interbreeding seesaw,’ the researchers examined the change in vegetation patterns over Eurasia over the past 400,000 years.
They observed that elevated atmospheric carbon dioxide concentrations and mild interglacial conditions caused an eastward expansion of the temperate forest into central Eurasia, and the dispersal of Neanderthals into Denisovan lands.
On the contrary, lower carbon dioxide concentrations and corresponding harsher glacial climate potentially caused a fragmentation of their habitats, leading to lesser interactions and interbreeding events.
“Pronounced climate-driven zonal shifts in the main overlap region of Denisovans and Neanderthals in central Eurasia, which can be attributed to the response of climate and vegetation to past variations in atmospheric carbon dioxide and northern hemisphere ice-sheet volume, influenced the timing and intensity of potential interbreeding events,” said Pusan National University’s Professor Axel Timmermann.
The study was published in the journal Science.
Jiaoyang Ruan et al. 2023. Climate shifts orchestrated hominin interbreeding events across Eurasia. Science 381 (6658): 699-704; doi: 10.1126/science.add4459