Neanderthals used to occupy most of Eurasia before the migration of Homo sapiens around 70,000 years ago. This lineage then disappeared around 40,000 years ago.
Mysterious Human Ancestor
While not much is known about the ensuing relationship between Neanderthals and humans, it was likely contentious.
It is known that during the overlap of the Late Pleistocene period in Eurasia, the two interbred at times. This is clear since several humans today still have Neanderthal DNA traces within them.
Now, based on a new study, this connection goes even further back than thought as there are traces of a long-forgotten chapter found in the Neanderthal genome.
The study posits that when modern humans arrived in Eurasia during the Late Pleistocene, Neanderthals present in the area already had traces of human DNA within them. This DNA may have come from a previously unknown and much longer run-in with an older human lineage.
This suggests that Homo sapiens reached Eurasia over 250,000 years ago, long before the earliest evidence of modern humans in the continent. The fossil record documents that modern humans evolved in Africa just 300,000 years ago.
Evolutionary biologist Alexander Platt from the University of Pennsylvania explains that they discovered the ancient interbreeding reflection where ancient modern human genes flowed into Neanderthals. These individuals left Africa roughly 250,000 to 270,000 years ago. They are cousins to humans that are alive today and they were more like modern humans compared to Neanderthals.
These earlier modern humans died out later on. Neanderthals then kept on dominating the area for 200,000 years. Remnants of this prehistoric encounter can be found in the Neanderthal genome.
Traces of Unknown Human Ancestor's DNA Found in Neanderthal Genome
To examine this, the researchers first followed the clues that another study uncovered. This study found that chunks of Neanderthal-like DNA, known as Neanderthal-homologous regions (NHRs), are present in several human populations across Africa.
The findings were surprising since the majority of Neanderthal and modern human interbreeding took place in Eurasia. It posits questions regarding the seeming abundance of Neanderthal DNA in Africa.
However, NHR prevalence across Africa remains unclear. Earlier research depended on a limited genome set with a common ancestry from West and Central Africa that was relatively recent.
To fill this gap, the researchers of the new study looked into a broader spectrum of present-day genomes. This covered 180 individuals from populations that are genetically diverse across Ethiopia, Cameroon, Botswana, and Tanzania. They then compared these with the Neanderthal genome of an individual that dwelled in the Altai Mountains roughly 120,000 years ago.
Geneticist Sarah Tishkoff, the senior author of the study from the University of Pennsylvania, explains that the study stresses the significance of covering geographically and ethnically diverse populations in studies pertaining to human genetics and genomes.
The researchers utilized a statistical technique to look into how and when the entry of NHRs took place among these populations. The study found that it was mainly modern humans that gave Neanderthals the genes in the first place, though modern humans may have also inherited Neanderthal genes.
In every population assessed, NHRs were detected. This shows their widespread abundance across Africa.
The study also discovered that in particular populations, people who migrated from Eurasia back to Africa also introduced Neanderthal genes.
Homo sapien gene variants are likely present in the Neanderthal genome's noncoding regions. This suggests that natural selection may have weeded them out because modern human DNA did not significantly help Neanderthals. The researchers also note that the human genome is gradually shedding out Neanderthal gene variants as well.
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