Humans may be just as vulnerable to environmental change as other animals, according to our new research analyzing genetic data from more than a thousand people who have lived in Europe and Asia over the past 45,000 years.
We found traces of more than 50 “hard sweeps” in which a rare genetic variant quickly swept through a population – most likely after a change in conditions in which those without the variant disappeared. The most striking sweep occurred among early Anatolian farmers, in a genetic region associated with the immune system called MHC-III.
Hard sweeps have often been observed in other species, but until now there have been few signs of them in humans. The traces of the hard sweeps had been masked by frequent mixing between populations over the past 8,000 years.
Our results show that humans’ famous ability to adapt our behavior and develop new tools and techniques was not always enough to survive when times got tough.
How Natural Selection Works
Modern humans live in a wide variety of natural environments, from the frozen arctic to the sweltering rainforest.
Unlike most animals, humans can rely on cultural innovations – such as fire and clothing – to overcome the challenges these environments present.
However, these innovations may not always have been sufficient to cope with new environmental conditions. This is where genetic variability between individuals comes into play.
Read more: We’ve found traces of humanity’s centuries-old arms race with coronaviruses written into our DNA
Individuals with genetic variations that make them better equipped to deal with new conditions will tend to leave more offspring. As a result, these beneficial variants become more common in future generations.
This process of genetic adaptation was dubbed “natural selection” by Charles Darwin almost 200 years ago.
How humans adapt
Using statistical tools to search for evidence of hard sweeps, the researchers found ample evidence of past adaptive events in many animals and plants, but little in human genomes. Specifically, hard sweeps are remarkably rare in humans.
As a result, some have speculated that genetic adaptation in humans is rare, perhaps because cultural innovations have rendered it largely unnecessary. Others have suggested that selection has occurred in many moderately beneficial genetic variants, leading to signals that are subtle and difficult to detect.
hidden signals
Almost 40 years ago, new technologies for extracting tiny amounts of DNA from archaeological skeletal remains were developed. This made it possible to study the genomes of ancient populations and changed our view of how human groups and ancient civilizations relate to each other.
Ancient DNA research has revealed that over the past 10,000 years in Eurasia, mixing between genetically divergent populations has been particularly common.
We thought that these events could have erased historical scavenging signals from modern human genomes – but ancient genomes that predate these shuffling events may still retain traces of the signals.
Read more: The origin of ‘us’: what we know so far where we humans came from
About 10,000 years ago, after the end of the last Ice Age, there was far more genetic variety among hunter-gatherers living in Europe than there is among humans living there today. .
In fact, the genetic differences between groups of ancient European hunter-gatherers were as great as the differences now seen between contemporary Western European and East Asian populations.
This extreme genetic differentiation has collapsed over the past 8,000 years due to several migrations and admixture events, making modern Europeans much more genetically homogeneous.
The “hard knocks” in the history of humanity
In our new research, published today in Nature Ecology & Evolution, we revisited this question by scanning more than a thousand ancient human genomes from across Eurasia.
We wondered: Could these relatively recent mixing events have masked historical selective sweeps, so that they were invisible in modern human genomes?
To test this idea, we first performed computer simulations based on genetic admixture estimates from studies of ancient Eurasian genomes. The simulation results suggest that ancient selection signals could indeed be greatly diluted in modern genomes.
Read more: Nobel Prize: Svante Pääbo’s ancient DNA discoveries offer clues to what makes us human
Next, we compiled and analyzed genetic information from over 1,000 ancient human remains, with the oldest sample being approximately 45,000 years old.
We compared the selection signals of ancient genomes with those of modern genomes. The old data contained many more harsh sweep signals than the modern samples. Newer scans were particularly prone to erasure, as they were sparse or absent in at least one of the mixture populations.
Our results confirm that hard sweeps were indeed part of the repertoire of human genetic adaptation. This suggests that we may not be so different from other animal species after all.
The genetic basis of adaptation
Genetic evidence of historical mixing events between different populations is growing. This is not only in humans but also in other species, suggesting that such a mixture may be reasonably common in nature.
If these mixing events are widespread, our study suggests that hard sweeps may also have been more frequent than we currently think. Overall, we may have a biased view of how species have genetically adapted to environmental pressures.
To better understand how adaptation works at the genetic level, we will need to develop new statistical methods to disentangle signals from hard sweeps and other selection events.
Read more: What’s next for ancient DNA studies after Nobel Prize honors groundbreaking field of paleogenomics
#Ancient #DNA #reveals #hidden #history #human #adaptation