Key role of ice age cycles in early human interbreeding

A study published in Science indicates that climatic shifts over the past 400,000 years have influenced Neanderthal and Denisovan interbreeding.

Recent paleogenomic research revealed that interbreeding was common among early human species. However, little was known about when, where, and how often this hominin interbreeding took place. Using paleoanthropological evidence, genetic data, and supercomputer simulations of past climate, a team of international researchers has found that interglacial climates and corresponding shifts in vegetation created common habitats for Neanderthals and Denisovans, increasing their chances for interbreeding and gene flow in parts of Europe and central Asia.

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.

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Central Asian horse riders played ball games 3,000 years ago

 Researchers have investigated ancient leather balls discovered in the graves of horse riders in northwest China. According to the international research team, they are around 3,000 years old, making them the oldest balls in Eurasia. The find suggests amongst others that the mounted warriors of Central Asia played ball games to keep themselves fit.

Today, ball games are one of the most popular leisure activities in the world, an important form of mass entertainment and big business. But who invented balls, where and when? The oldest balls that are currently known about were made in Egypt about 4,500 years ago using linen. Central Americans have been playing ball games for at least 3,700 years, as evidenced through monumental ball courts made of stone and depictions of ball players. Their oldest balls were made of rubber. Until now, it was believed that ball games in Europe and Asia followed much later: In Greece about 2,500 years ago and in China about 300 years after that.

Eurasia's oldest known balls

Researchers from the University of Zurich, together with German and Chinese researchers, have now examined in more detail three leather balls found in graves in the old Yanghai cemetery near the city of Turfan in northwest China. The balls, measuring between 7.4 and 9.2cm in diameter, have been dated at around 2,900 to 3,200 years old. "This makes these balls about five centuries older than the previously known ancient balls and depictions of ball games in Eurasia," says first author Patrick Wertmann of the Institute of Asian and Oriental Studies of the University of Zurich. "Unfortunately, however, the associated archaeological information is not sufficient to answer the question of exactly how these balls were played."

The earliest illustrations from Greece show ball players running, and depictions from China show riders using sticks. Comparable curved sticks were also found in Yanghai, but there was no apparent direct connection with the balls. Moreover, they are dated to a more recent period. "Therefore, the leather balls from Yanghai are not connected to early forms of field hockey or polo, even though two of the balls were found in the graves of horsemen," says Wertmann.

New era of Central Asian equestrian warfare

In one of the riders' graves, the preserved remains of a composite bow and a pair of trousers (1) were found, which were made in the region at that time and are among the oldest in the world. Both are signs of a new era of horse riding, equestrian warfare and fundamental societal transformations which accompanied increasing environmental changes and a rising mobility in eastern Central Asia. The current study shows that balls and ball games were part of physical exercise and military training from the very beginning. In addition, just like today, sport also played a central role in society and was a widespread leisure activity. The study's findings once again highlight that this region was a center of innovation within Eurasia several millennia ago.

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