A groundbreaking study published in the journal Science by an international team of researchers reveals that the interbreeding of early human species was influenced by changes in atmospheric CO₂, climate, and vegetation in the past.

Modern-day humans carry traces of DNA from other human species, including Neanderthals and Denisovans, in their cells.

In 2018, the discovery of an individual known as Denny, who lived 90,000 years ago, provided evidence of interbreeding between Neanderthals and Denisovans. This finding suggests that interbreeding among hominins was widespread and not limited to Homo sapiens.

Traditionally, scientists rely on paleo-genomic analysis of rare fossils and ancient DNA to determine when and where human hybridization occurred. However, in this study, the team of climate experts and paleo-biologists took a different approach.

By using existing paleo-anthropological evidence, genetic data, and supercomputer simulations of past climate, the researchers discovered that Neanderthals and Denisovans had different environmental preferences. Denisovans were better adapted to cold environments, such as boreal forests and tundras, while Neanderthals preferred temperate forests and grasslands.

The scientists found that during warm interglacial periods, when the Earth’s orbit was more elliptic and northern hemisphere summers were closer to the sun, the habitats of Neanderthals and Denisovans overlapped geographically. This increased the likelihood of encounters and interbreeding between the two groups.

The researchers also found that these simulated habitat overlaps align with known episodes of interbreeding that occurred approximately 78,000 and 120,000 years ago. They suggest that future paleo-genetic reconstructions can validate their predictions of potential interbreeding events around 210,000 and 320,000 years ago.

To understand the climate drivers of this east-west interbreeding pattern, the team examined changes in vegetation patterns across Eurasia over the past 400,000 years. They determined that elevated atmospheric CO₂ concentrations and mild interglacial conditions led to the expansion of temperate forests into central Eurasia, creating pathways for Neanderthals to move into Denisovan territories.

One of the primary challenges of the study was estimating the preferred climatic conditions for Denisovans due to limited data. To overcome this, the researchers developed new statistical tools that accounted for ancestral relationships among human species. Their findings suggest that apart from regions in Russia and China, northern Europe would have been a suitable environment for Denisovans.

Further research using genetic analyses of Denisovan ancestry in European populations can provide more insights into the relationship between early human dispersal, habitat encroachment, and genetic diversification.