Scientists have compared the genomes of 2,000 living individuals, three Neanderthals, and one Denisovan to trace the flow of genes between these groups over the last quarter of a million years, as reported by Ancient Origins.
In the course of the study, researchers employed a genetic tool called IBDmix, which utilizes machine learning techniques to decode genomes. This time, the scientists aimed to find Neanderthal DNA not in modern humans, but rather the DNA of early humans within the Neanderthal genome.
Using the new method, they were able to identify several waves of contact between Neanderthals and humans: the first wave occurred around 200,000 to 250,000 years ago, the second wave was 100,000 to 120,000 years ago, and the largest wave happened 50,000 to 60,000 years ago.
"The vast majority of genetic studies have focused on how mating with Neanderthals affected the phenotypes of modern humans and our evolution. But the same questions are interesting in the reverse direction," says the study leader from the Lewis-Sigler Institute for Integrative Genomics at Princeton, Joshua Aikey.
The team of researchers discovered that the offspring from those initial waves of Neanderthal mating with modern humans must have remained with the Neanderthals, which is why there are no traces left in the genomes of contemporary humans.
The final piece of the puzzle revealed that the Neanderthal population was actually even smaller than previously thought. The research indicated that the genetic diversity of the Neanderthal population largely derived from DNA sequences of early humans, whose population was significantly larger.
Scientists believe that the Neanderthal population consisted not of 3,400 reproducing individuals, but was closer to about 2,400.
Overall, the new findings paint a picture of how Neanderthals disappeared from the records around 30,000 years ago.
"I don't like to use the term 'extinction' because I think that Neanderthals were largely absorbed by the human population," emphasizes Aikey.
The scientist's idea is that Neanderthal populations gradually dwindled until the last survivors became part of human communities.
This model of assimilation was first proposed by Fred Smith, a professor of anthropology at the University of Illinois, in 1989. The new data strongly supports this theory.
"In the end, humans simply demographically overwhelmed the Neanderthals and incorporated them into modern human populations," concluded the expert.
It is worth noting that Neanderthals interbred with an unknown species of humans even before Homo Sapiens. Neanderthals likely not only mixed with our ancestors but also with a now-extinct species of early humans.