Around 40,000 years ago, Neanderthals, who had occupied large portions of Europe and western Eurasia for hundreds of thousands of years, were gradually replaced by Homo sapiens, who had migrated from Africa. However, this replacement was not immediate. Instead, the two species coexisted for several millennia, during which interbreeding occurred, leaving traces of Neanderthal DNA in the genome of modern Homo sapiens. This interaction between the two species is a key focus of research today, as it provides valuable insights into the shared history of our evolutionary ancestors.
A recent study conducted by researchers at the University of Geneva (UNIGE) has analyzed the distribution of Neanderthal DNA within modern human genomes over the past 40,000 years. The study, published in Science Advances, revealed subtle variations in the proportion of Neanderthal-derived DNA across different periods and regions, offering new insights into how Neanderthal genes have persisted in human populations. By examining genetic data from over 4,000 genomes of individuals who lived in Eurasia, the researchers have provided a clearer picture of how Neanderthal DNA has influenced the genetic makeup of Homo sapiens over millennia.
Genome sequencing and comparative analysis have already established that Neanderthals and Homo sapiens interbred, leading to the presence of roughly 2% of Neanderthal DNA in the genomes of present-day Eurasians. However, the percentage of this genetic contribution is not uniform across populations. In particular, Neanderthal DNA is slightly more prevalent in the genomes of Asian populations than in those of Europeans. This finding has prompted scientists to consider different hypotheses to explain these variations.
One possibility is that natural selection may have acted differently on Neanderthal genes in various regions. For example, certain Neanderthal genes may have been more advantageous in Asian populations, leading to their higher prevalence there. However, Mathias Currat’s research team, based at UNIGE, has proposed an alternative hypothesis involving migratory flows. Currat’s previous work, based on computer simulations, suggests that when a migrating population hybridizes with a local population, the proportion of local genetic material tends to increase as one moves away from the point of migration. This hypothesis could help explain the varying proportions of Neanderthal DNA across different regions.
The researchers tested this hypothesis by examining genetic data from over 4,000 genomes of individuals who lived in Eurasia over the last 40 millennia. The results showed that in the period following the migration of Homo sapiens out of Africa, Paleolithic hunter-gatherers living in Europe had a slightly higher proportion of Neanderthal DNA than those living in Asia. This finding contrasts with the current situation, where Asian populations typically have a higher percentage of Neanderthal DNA. However, it aligns with paleontological evidence, as Neanderthals were primarily located in western Eurasia, with no Neanderthal fossils found further east than the Altai region of Siberia.
The study also examined changes in Neanderthal DNA in European populations during the transition to the Neolithic period, roughly 10,000 to 5,000 years ago. This period saw the arrival of farmers from Anatolia (modern-day Turkey) and the Aegean region. These Anatolian farmers, who had a lower proportion of Neanderthal DNA than the indigenous European populations, intermingled with the European hunter-gatherers. As a result, the proportion of Neanderthal DNA in European genomes decreased slightly during this period. By the time of the Neolithic, the percentage of Neanderthal DNA in European populations had become somewhat lower than in Asian populations, a trend that persists in present-day populations.
This decline in Neanderthal DNA during the Neolithic is linked to the spread of agriculture and the mixing of populations. As Anatolian farmers migrated into Europe, their genetic contributions diluted the Neanderthal DNA that had been more prevalent in the hunter-gatherer populations. The integration of these two groups, with different proportions of Neanderthal DNA, created a more genetically diverse population, which is reflected in the lower levels of Neanderthal DNA in modern European populations.
The study’s findings illustrate the importance of analyzing ancient genomes in combination with archaeological data to trace the complex history of interbreeding between Homo sapiens and Neanderthals. By studying the proportion of Neanderthal DNA in the genomes of humans from different periods, scientists can begin to identify trends and understand the role of Neanderthal genes in the evolution of modern humans.
Mathias Currat, the lead author of the study, explains that these findings provide a valuable reference for future research. “We are beginning to have enough data to describe more and more precisely the percentage of Neanderthal DNA in Homo sapiens at certain periods of prehistory,” he notes. “This work can therefore serve as a reference for future studies to more easily detect genetic profiles that deviate from the average, which may reveal advantageous or disadvantageous effects.”
The integration of Neanderthal DNA into the human genome has had lasting effects on the biology of modern humans. Some of these genetic contributions may have been beneficial, helping Homo sapiens adapt to new environments and challenges. Other Neanderthal genes may have been less advantageous, potentially contributing to health issues or other traits. As research continues and more ancient genomes are sequenced, scientists will gain a deeper understanding of the complex genetic legacy left by Neanderthals and how it has shaped the evolution of Homo sapiens.
Source: University of Geneva