Sex Surge Between Early Human Species Explained in New Study

Shifts in the earth's climate made sex between Neanderthals and Denisovans more common, a new study has found.

The study by researchers at South Korea's Pusan National University, which had been published in Science, found that over the past 400,000 years, breeding between the two extinct subspecies of human was influenced more by the climate than originally thought.

And the fossil of a 90,000-year-old human—found to have a Denisovan father and a Neanderthal mother, according to DNA testing—suggests that sex between the two subspecies was actually quite common.

The Denisovans, or Denisova hominins, are an extinct subspecies of human that lived from 500,000 to 30,000 years ago. Neanderthals, also an extinct subspecies, lived in Eurasia until about 40,000 years ago.

Although the two subspecies favored different habitats most of the time—with Denisovans preferring cooler environments, and Neanderthals preferring the warmth—shifts in the Earth's orbit changed the distribution of vegetation, and the overall environment.

This caused an overlap in the two subspecies migration patterns, increasing the chances of them meeting and breeding.

The Earth's orbit around the sun determines seasonal fluctuations as well as long-term climate. For example, when the Earth is going through a cycle where it is closer to the sun, we experience a warmer climate.

Serbian scientist Milutin Milankovitch believed that periodic changes in the Earth's position relative to the sun determined long-term climate fluctuations, NASA reports. These changes were responsible for Ice Ages—periods of extreme cold driven by a reduction in the planet's temperature.

Scientists already knew that Neanderthal and Denisovans had sex, however they do not know when, where or how often it took place.

To find out more, researchers analyzed paleoanthropological evidence, genetic data and supercomputer simulations of past climate to assess if it had any impact, the study reported. They found that interglacial climates, and the way this shifted vegetation in certain habitats, meant there were more opportunities for the two species to encounter each other.

"Little is known about when, where, and how frequently Neanderthals and Denisovans interbred throughout their shared history," Jiaoyang Ruan, Postdoctoral Researcher at the IBS Center for Climate Physics in South Korea, said in a press release. "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 scientists were also able to determine several hotspots, where breeding was more common. Central Eurasia, the Caucasus, the Tianshan and the Changbai mountains, were all likely to have been inhabited by both species at some point.

It was determined that the two species will have had a high probability of meeting and breeding in the Siberian Altai—in Russia— from 340,000 to 290,000 years ago; from 240,000 to 190,000 years ago and from 130,000 to 80,000 years ago.

"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," senior author Axel Timmermann, Director, ICCP and Professor at Pusan National University, said in a press release.

Modern humans carry a small amount of DNA from Neanderthals and Denisovans to this day. This study shows that changes in climate may have played a vital role in facilitating gene flow between the two. These may have even left lasting impressions on the human genome today.

Update 08/15/23, 7:45 a.m. ET: This article has been updated with a new headline.

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