According to some studies, humans may be as susceptible to environmental change as animals. Our new researchAnalysis of genetic data from more that a thousand people who lived in Europe and Asia during the past 45,000 Years.
We found traces of more than 50 “hard sweeps” in which a rare genetic variant rapidly swept through a population – most likely after a change in conditions in which those lacking the variant died out. The most notable sweep was among Anatolian farmers, which occurred in an area associated with MHC-III.
Although hard sweeps are often seen in other species, they have not been observed in humans. Over the last 8,000 years, the hard sweeps were often hidden by the frequent mixing of populations.
Our research shows that even though humans are known for their ability to adapt and develop new tools, techniques and behaviors, it is not enough to withstand difficult times.
How natural selection works
Modern humans live in an incredible range of natural environments, from the Arctic to the tropical rainforest.
Unlike most animals, humans can draw on cultural innovations – such as fire and clothing – to overcome the challenges these environments present.
However, innovations may not always be enough to adapt to new environmental conditions. This is where genetic variability between individuals becomes important.
People with genetic variants that help them adapt to new situations will be more likely to have more children. These beneficial variants are more likely to be found in the future.
Charles Darwin, nearly 200 years ago, called this “natural selection.”
How people adapt
Researchers used statistical tools for the search for evidence of hard sweepings. They found plenty of evidence for past adaptive events, but very little in human genomes. Hard sweeps are more common in humans.
As a consequence, Some have speculatedIt is possible that human genetic adaptation is not common due to cultural innovations. Others believe selection has occurred in a variety of moderately beneficial genetic variants. This could have led to subtle signals that are difficult to detect.
Hidden signals
Nearly 40 years ago, technology was developed to extract DNA from archaeological bones. This technology has made it possible to Study the genomes of ancient peoplesThis has changed how we see the relationship between ancient civilisations and human groups.
The ancient DNA research showed that intermixing of genetically diverse populations in Eurasia has been a frequent occurrence over the past 10,000 year.
We thought these events might have erased historical sweep signals from modern human genomes – but that ancient genomes predating these intermixing events may still retain traces of the signals.
It was 10,000 years ago that the last ice age ended. More genetic diversityThere are more hunter-gatherers in Europe today than there are humans.
Actually, genetic differences among ancient European hunter-gatherers was as great as those between modern populations in east Asia and western Europe.
This extreme genetic differentiation was wiped out over the past 8,000 year by several migrations, mixing events, and modern Europeans are now much more genetically homogeneous.
‘Hard sweeps’ in human history
Our new research reveals: Publié in Nature Ecology & EvolutionWe revisited the question by scanning over a thousand ancient human genomes, sourced from all across Eurasia.
We were curious: Could these recent mixing events have obscured historical selective sweeps so that they were not visible in modern human genomes.
We first tested this hypothesis by running computer simulations that were based on genetic mixing estimates derived from ancient Eurasian genomes. Simulation results showed that ancient selection signals might be very weakly diluted in modern genomes.
Next, we collected and analyzed genetic data from over 1,000 ancient human remains. The oldest of these samples is approximately 45,000 years old.
We compared selection signals found in ancient genomes and those found in modern genomes. The modern data contained more hard sweep signals than those in the ancient data. Because of their rarity or absence in at least one mixing population, more recent sweeps are particularly vulnerable to erasure.
Our results show that hard sweeps are indeed part of human genetic adaptation. This suggests that our genetic adaptation may not be different from those of other animal species.
The genetic basis for adaptation
There is increasing genetic evidence to support historical mixing between different populations. This is not just a matter of humans, but also other species. It suggests that such mixing could be quite common in nature.
Our study suggests that hard sweeps could have been more frequent than we think, if these mixing events are common. We may be biased in our understanding of how species adapt to environmental pressures.
We will need new statistical methods that can disentangle signals from hard sweeps and other selection events to better understand the mechanisms of adaptation at the genetic level.
Yassine Souilmi, Group Leader Genomics and Bioinformatics Australian Centre for Ancient DNA University of Adelaide; Christian HuberAssistant Professor of Biology Penn State?, and Ray Tobler, Postdoctoral fellow, Australian National University
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