New research suggests a key role of ice age cycles in early human interbreeding

Contemporary humans carry in their cells a small amount of DNA derived from Neanderthals and Denisovans. “Denny,” a 90,000-year-old fossil individual, 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? 

In a recent study published in Science on 10 August 2023, researchers from Korea and Italy have joined hands to answer this question. Using fossil data, supercomputer simulations of past climate, and insights obtained from genomic evidence, the team was able to identify habitat overlaps and contact hotspots of these early human species. Dr. Jiaoyang Ruan, Postdoctoral Researcher at IBS Center for Climate Physics (ICCP), South Korea, explains, “Little is known about when, where, and how frequently Neanderthals and Denisovans interbred throughout their shared history. 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 researchers 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 the researchers place ‘Denny’ within the climatic context and even confirmed the other known episodes of genetic interbreeding. The researchers also noted that the Denisovans and Neanderthals would have had a high probability of contact in the Siberian Altai during ~ 340–290, ~240–190 and ~130–80 thousand years ago.

To further elucidate the factors that triggered the ‘east-west interbreeding seesaw,’ the team examined the change in vegetation patterns over Eurasia over the past 400 thousand years. They observed that elevated atmospheric CO₂ 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 CO₂ 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 CO₂ and northern hemisphere ice-sheet volume, influenced the timing and intensity of potential interbreeding events,” remarks senior author Axel Timmermann, Director, ICCP and Professor at Pusan National University, South Korea.        

In summary, the study shows that climate-mediated events have played a crucial role in facilitating gene flow among early human species and have left lasting impressions on the genomic ancestry of modern-day humans.

***

Reference

DOI: https://doi.org/10.1126/science.add4459

Author : Jiaoyang Ruan^1,2*, Axel Timmermann^1,2*, Pasquale Raia³, Kyung-Sook Yun^1,2, Elke Zeller^1,2, Alessandro Mondanaro⁴, Mirko Di Febbraro⁵, Danielle Lemmon^1,2, Silvia Castiglione³, Marina Melchionna³

Affiliations:

¹Center for Climate Physics, Institute for Basic Science; Busan, South Korea.

²Pusan National University; Busan, South Korea.

³DiSTAR, Monte Sant’Angelo, Napoli Università di Napoli Federico II; Naples, Italy.

⁴DST, Università degli Studi di Firenze; Florence, Italy.

⁵Department of Biosciences and Territory, University of Molise, C. da Fonte Lappone, Pesche (IS) Italy

About the institute

The Institute for Basic Science (IBS) Center for Climate Physics (ICCP) is an international climate research center. It provides a world-class international research environment for improving the basic understanding of climate and its variability, developing new research frontiers in earth system science, and performing skillful decadal earth system forecasts and improved long-term projections using high-performance computer simulations. ICCP’s mission is to enhance the understanding of natural climate variability and man-made climate change, and to improve the ability to predict climate impacts on the hydrological cycle, ice-sheets, sea level, and regional processes.

About Dr. Jiaoyang Ruan

Dr. Jiaoyang Ruan, lead author of this study, is a Postdoctoral Researcher at IBS Center for Climate Physics, at Pusan National University. He is interested in the history of Earth’s climate and human evolution. He assesses climate effects on human evolution by supercomputer model simulations, statistical methods, and synthesis of existing fossil data.

About Professor Axel Timmermann

Prof. Axel Timmermann is the Founding Director of the IBS Center for Climate Physics. He is a Distinguished Professor at Pusan National University, South Korea. Prof. Timmermann’s research group studies human evolution and climate, climate/carbon cycle interactions, glacial dynamics, climate predictability, abrupt climate change, sea level rise, ice-sheet instabilities, and the El Niño phenomenon. Prof. Timmermann is a highly cited award-winning researcher with over 200 publications.

Lab Website: https://ibsclimate.org/

ORCID IDs: https://orcid.org/0000-0003-4733-1125; https://orcid.org/0000-0003-0657-2969

Corresponding author email: jiaoyangruan@pusan.ac.kr; axel@ibsclimate.org.