The Summary
- Many people have a tiny slice of Neanderthal DNA, evidence of interbreeding between the species and ancient human ancestors.
- Two new studies suggest that interbreeding occurred during a limited period of time as ancient humans left Africa.
- Clarifying that timeline narrows the possible range of time when humans spread to new continents.
A little fragment of Neanderthal DNA that remains tens of thousands of years after the extinction of the species is hidden in the genetic codes of many humans, posing a conundrum that has long fascinated scientists.
About 1% to 2% of the DNA of most non-Africans can be traced back to Neanderthal ancestors.
The specifics of that evolutionary history, however, are yet unknown. How frequently did Neanderthals and ancient humans breed together? When exactly did that occur? Why did modern people survive while Neanderthals became extinct? Now, what do we get out of that Neanderthal DNA?
Regarding some of those fundamental questions, two study teams have independently examined sets of ancient genomes and reached similar findings. Neanderthals and ancient humans may have interbred for a brief time after humans left Africa and moved to other continents, according to studies published Thursday in the journals Nature and Science.
The results show that the wave of interbreeding occurred between 43,500 and 50,500 years ago. The majority of Neanderthal DNA was then eliminated over the course of the following 100 generations, but not all of it. The DNA that is still present today is connected to characteristics including metabolism, immunological response, and skin pigmentation.
The new results show that the interbreeding event occurred more recently than some earlier estimates indicated, which changes and reduces the range of time that humans may have spread to areas like modern-day Australia and China.
It also explains the importance of human fossilized remains found outside of Africa, such as in Europe, that are over 50,000 years old. According to the current research, those populations vanished and turned into evolutionary dead ends.
The history of humanity is not solely one of achievements. According to Johannes Krause, a professor at the Max Planck Institute for Evolutionary Anthropology in Germany and the author of the Nature research, humanity truly went extinct multiple times. We now know of several lineages that did not contribute to subsequent generations.
The results also reveal the increasing proficiency of anthropologists in reconstructing ancient DNA and using it to draw conclusions about the evolution of human history.
Priya Moorjani, an assistant professor of molecular and cell biology at the University of California, Berkeley and one of the authors of the Science research, said, “It’s just really cool that we can look at these events from the past and really reconstruct what our paths look like.” Although 50,000 years ago is a long time ago, the ability to obtain genetic data from these samples allows us to gradually construct a more detailed picture.
The two research teams approached their task in various ways.
Moorjani’s team created a database of genomic data from 275 modern people and 59 ancient people who lived between 2,000 and 45,000 years ago. The scientists then examined how the length and distribution of Neanderthal DNA in those genomes changed throughout time.
According to their findings, the introduction of Neanderthal genes into humans took place approximately 47,000 years ago and lasted for a maximum of 7,000 years. These results are consistent with archeological evidence that indicates a geographic overlap between Neanderthals and humans as they left Africa. Although it has not been verified, many scientists believe the two species may have met in the Middle East.
Following the interbreeding, natural selection eliminated many Neanderthal features while retaining a few.
Leonardo Iasi, a postdoctoral researcher in the evolutionary genetics department at the Max Planck Institute for Evolutionary Anthropology and a co-author of the Science paper, stated that most of the selection, both positive and negative, on Neanderthal ancestry occurred within about 100 generations following the gene flow.
The examination of six genomes from bones found in Ranis, a cave site beneath a medieval castle in modern-day Germany, was the basis for the other recent study published in Nature. The remains date back about 45,000 years, and the DNA suggests two of the six individuals were a mother and daughter.
They re now the oldest nuclear genomes that we have of modern humans, Krause said.
The researchers also analyzed the DNA of an individual found in a cave site in present-day Czechia, about 140 miles from Ranis. The two sites date back to roughly the same period. The results indicated that two of the individuals found in Ranis were closely related to the one in the Czechia cave within five or six degrees in family relation.
They concluded that the individuals found at the two sites were most likely part of a small isolated population of perhaps just 200. From a genetic perspective, they didn t make it the population died out.
They present a genetic lineage that has no descendants that actually, later on, got extinct, Krause said.
However, those individuals DNA shares the same traces of Neanderthal influence as the remains that the other team of researchers analyzed. That strengthens the idea of a single event of admixture or interbreeding.
It s always nice to have two independent studies that are working with independent data using independent methods that come up with basically the same answer. That engenders a lot of confidence, said Joshua Akey, a professor at the Lewis-Sigler Institute for Integrative Genomics at Princeton University, who wasn t part of either research group.
Chris Stringer, a professor and research leader on human evolution at the Natural History Museum in London, said pinpointing the interbreeding event helps line up other key parts of the human evolution timeline. The findings constrain the timing of the arrival of populations in regions like China and Australasia [Australia, New Zealand and Papua New Guinea] that gave rise to present-day people in those areas to less than 50,000 years ago, because their genomes share the same interbreeding event, he said.
The studies also clarify the time frame in which humans interbred with Denisovans, another extinct species, Stringer added that took place after the introduction of Neanderthal DNA.
Akey said questions remain, though. It s not clear how often humans and Neanderthals mated. And there s more to be learned about the traits humans got from Neanderthal and Denisovan DNA. Plus, there s the mysterious disappearance of Neanderthals 39,000 years ago.
Akey said he thinks mating between humans and Neanderthals might have led to the latter s vanishing.
My inclination is to think that mating was pretty frequent, Akey said. And that there was enough mating that it contributed to the disappearance of Neanderthals by incorporating them into human populations. But that s still speculative.
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