When the human-Neanderthal babies existed

 Modern humans and Neanderthals shared Eurasia for tens of thousands of years. But they did not produce offspring together for the entire period.

The replica of a Neanderthal girl is in the Neanderthal Museum in Mettmann. Little is known about the clothing of the group of people who became extinct around 40,000 years ago.© OLIVER BERG / DPA / PICTURE ALLIANCE (DETAIL)

Around 60,000 years ago, the Neanderthals had unusual new neighbors: Homo sapiens had come from Africa and reached the Eurasian landmass, in spektrum. Over time, both groups of people repeatedly produced offspring together. Experts have now investigated exactly when this genetic mixing took place and how long the encounter lasted as part of a current study. It turned out that the phase in which modern humans and Neanderthals had children together began later than previously thought and only lasted for a relatively short time. The scientists calculated a time window of around 7,000 years for this.

This was the result of the analysis of hundreds of DNA samples. Leonardo Iasi, evolutionary geneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, and his colleagues examined the DNA of 58 individuals who lived between 2,200 and 45,000 years ago and compared it with that of 231 people from the present day. The data provide information about when certain Neanderthal DNA sequences appeared in the genome of Homo sapiens, whether they were preserved, and when they disappeared again.

The study by Iasi and colleagues has not yet been peer-reviewed and published in a specialist journal. However, a preliminary report is already available online on the preprint server »bioRxiv«. The authors did not want to comment on questions about the study.

The study initially confirmed the picture that had long been known: the events in Paleolithic Eurasia left genetic traces up to modern times. All people except those with purely African ancestry today carry a significant proportion of Neanderthal DNA in them, around one to two percent. People whose ancestors came from sub-Saharan Africa are excluded from this, as their ancestors never came into contact with Neanderthals.

The large-scale comparison of hundreds of ancient and modern genomes showed that the influx of Neanderthal genes was limited to a single surge that began around 47,000 years ago and lasted for around 6,800 years. At the end of that phase, the Neanderthals were already close to extinction.

However, Homo sapiens and Homo neanderthalensis also mostly went their separate ways during these almost seven millennia. Gene transfer was limited to individual episodes. At least for modern humans, the genetic material of the neighbors does not seem to have been particularly beneficial, as the researchers discovered. Many adopted DNA sequences were removed from the genome of Homo sapiens at a remarkable speed over time through evolutionary processes.

As a result, human genomes today contain vast "deserts" that are completely devoid of Neanderthal DNA. The research team found these cleared areas in ancient genomes from the last phases of human-Neanderthal interaction, so the process of removal began very early on. According to Emilia Huerta-Sanchez, an evolutionary biologist at Brown University in Providence, Rhode Island, this suggests that many Neanderthal sequences were detrimental to humans and were therefore quickly selected out by evolution.

For Huerta-Sanchez, this work fills some key gaps in the history of early humans because, thanks to the inclusion of Paleolithic genomes, it reveals more about the time course of the interaction and the evolutionary forces that acted on Neanderthal DNA in anatomically modern human populations.

However, some gaps in knowledge remain. For example, much less is known about the genetic history of people from Oceania and East Asia than about that of people from Europe, partly because there is little reconstructed genetic data from this region. Why modern people there have retained slightly higher proportions of Neanderthal DNA - about 20 percent more than their European neighbors - therefore remains an unsolved question.