The timeframe when Neanderthal genes entered our lineage has become clear. Until now, it was estimated that Neanderthals coexisted with modern humans' direct ancestors, Homo sapiens, for 10,000 to 20,000 years before going extinct about 40,000 years ago, mixing their blood during that period. This latest finding has significantly narrowed the timeframe of hybridization to a range of thousands of years. Paleoanthropologists expect that Neanderthal genes will provide clues about the pathways through which our ancestors left Africa and spread across the globe.
An international research team led by Arev Sümer, a researcher at the Max Planck Institute in Germany, noted on the 13th that they found evidence suggesting that hybridization between modern humans' direct ancestors and Neanderthals occurred slightly later than previously known, around 49,000 to 45,000 years ago, as published in the international journal Nature.
Neanderthals, one of humanity's relatives, left Africa about 400,000 years ago and first settled in Eurasia. They are believed to have encountered and coexisted with Homo sapiens, who began leaving Africa around 47,000 to 65,000 years ago, in Europe.
The fact that modern humans' ancestors had close interactions with Neanderthals is confirmed by genetics. Today's human genes vary by region, but about 2% derive from Neanderthals. This means that the two species interbred and that traces of this interaction remain in cellular DNA.
Though rare, evidence of hybridization has also been found in fossils of early humans. A typical example of hybridization between modern humans' direct ancestors and Neanderthals is a 45,000-year-old Homo sapiens skeleton discovered in the Bachkovo Cave in Bulgaria. This skeleton contained genes from Neanderthals that had been present for at least 10 generations. Consequently, it has been difficult to determine the exact timeline of hybridization between modern humans and Neanderthals. Until now, it was estimated that the first interbreeding occurred between 47,000 and 65,000 years ago.
The research team discovered fossilized bone fragments of early humans in the eastern German region of Rannis. The dating of these fragments is estimated to be about 41,000 to 49,500 years ago, and they are believed to belong to modern humans' direct ancestors. However, it is unknown what connections these individuals had with other groups that were present in Europe at that time.
To find out what connection these individuals had with Neanderthals and human species that lived in Eastern Europe, the research team extracted and analyzed the DNA. They also compared it with the 45,000-year-old human DNA discovered in Zlaty Kunt in the Czech Republic. The fossils from Zlaty Kunt, consisting of a female skull discovered in 1950, are recognized as evidence of early hybridization between Homo sapiens and Neanderthals. However, it remains unclear which group they belonged to.
As a result of the genetic analysis, the bone fragments found in Rannis, Germany, were confirmed to belong to the early group of Homo sapiens that first left Africa, along with the Zlaty Kunt skull. The researchers analyzed Neanderthal genes within their genomes to determine the timing of the first hybridization between the two groups.
As time goes on, the Neanderthal genes in Homo sapiens diminish in length, and by comparing these lengths, the period when the Neanderthal genes first intermingled can be determined. Recently mixed Neanderthal genes were found among these samples. The researchers estimated that the period when hybridization between the two human species occurred was around 49,000 to 45,000 years ago. This is a significant narrowing of the previous estimate, which suggested the timeframe of coexistence and hybridization between the two species ranged as wide as 20,000 years.
The research team also published results in the international journal Science regarding a comparison of the DNA of 334 individuals from early human periods and modern humans. They investigated the distribution of Neanderthal genes among 59 early humans who lived from 45,000 years ago to 2,200 years ago and modern humans. By tracing the process of how Neanderthal genes mixed and evolved in Homo sapiens, it is possible to find the timing of the first hybridization.
Similarly, this study confirmed that hybridization between the two human species occurred during a similar timeframe. The findings published in Nature and Science imply that only a specific early group that left Africa in a short period interbred with Neanderthals.
Researcher Sümer stated, 'This research allows us to examine the processes through which early humans who left Africa spread globally and challenges the conventional belief that hybridization occurred multiple times between Homo sapiens and Neanderthals. It appears that hybridization took place just once among early population groups, and traces of that remain today.'
Reference material
Nature (2024), DOI: https://doi.org/10.1038/s41586-024-08420-x
Science (2024), DOI: https://doi.org/10.1126/science.adq3010