Humans may be just as vulnerable to the effects of environmental change as other animals, according to a new study analyzing the genetic data of more than 1,000 people who have lived in Europe and Asia over the past 45,000 years.
Team Leader Yassin Swelmy, a genomics and bioinformatics scientist at the Australian Center for Ancient DNA, University of Adelaide; Christian Huber, assistant professor of biology at the University of Pennsylvania, and Ray Tobler, research fellow at the Australian National University, traced more than 50 “complex scans” in which a rare genetic variant rapidly spread through a population – likely after a change in circumstances in which those lacking variable. The most astonishing leap of the first farmers in Anatolia occurred in a genetic region associated with the immune system called MHC-III.
Complex scans have often been observed in other species, but so far they have been few in humans. The effects of heavy raids have been masked by the frequent mixing of the population over the past eight thousand years.
The results show that our famous ability of humans to adapt their behavior and develop new tools and technologies has not always been enough to survive difficult times.
How natural selection works
Modern humans live in a wide variety of natural environments, from the Arctic to tropical rainforests.
And unlike most animals, humans can rely on cultural innovations like fire and clothing to overcome challenges in these environments.
However, these innovations may not always be enough to work in new environmental conditions. This is when genetic variation between individuals comes into play.
They make people with genetic variations more ready to cope with new conditions and tend to leave more offspring. As a result, these beneficial options become more common in future generations.
Charles Darwin called the process of genetic adaptation “natural selection” almost 200 years ago.
How do people adapt?
Using statistical tools to search for evidence of complex scans, the researchers found ample evidence of past adaptive events in many animals and plants, but little in the human genome. In particular, complex scans are notably rare in humans.
As a result, some have suggested that genetic adaptation is rare in humans, perhaps because cultural innovation has made it largely unnecessary. Others have suggested that selection occurred between several fairly useful genetic variants, resulting in subtle, hard-to-detect signals.
Nearly 40 years ago, new methods were developed to extract DNA traces from archaeological skeletal remains. This allowed us to study the genomes of ancient populations and changed our understanding of how ancient human groups and civilizations were related to each other.
Studies of ancient DNA have shown that over the past 10,000 years, the mixing of genetically disparate populations of Eurasia has occurred especially often.
We thought that these events may have erased the historical data of modern human genome scans, but the ancient genomes that preceded these convoluted events may still retain signal traces.
About 10,000 years ago, after the end of the last ice age, there was far more genetic diversity among the hunter-gatherers living in Europe than among the people living there today.
Indeed, the genetic differences between ancient European hunter-gatherer groups were as great as the differences now observed between modern populations in Western Europe and East Asia.
This intense genetic differentiation has collapsed over the past 8,000 years due to numerous migrations and mixing events, making modern Europeans more genetically homogeneous.
Complex research in human history
In a new paper published in the journal Nature Ecology & Evolution, researchers return to this question by scanning more than a thousand ancient human genomes from across Eurasia.
And they said: Could relatively recent admixture events have obscured historical selective scans so that they were invisible in the modern human genome?
To test this idea, they first ran computer simulations based on genetic admixture estimates from ancient Eurasian genomes. The simulation results showed that the ancient selection signals can indeed be greatly attenuated in the modern genome.
They then collected and analyzed the genetic information from more than 1,000 ancient human remains, the oldest specimen being about 45,000 years old.
They compared the selection signals in the ancient genome with those in the modern genome. Older data contained much more complex survey signals than recent samples. Recent scans were particularly vulnerable to erasure due to being rare or missing from at least one of the mixed groups.
The findings confirm that complex scans were indeed part of the human genetic adaptation repertoire. This suggests that we may not be completely different from other animal species.
The genetic basis of adaptation
There is growing genetic evidence for historical mixing events between different populations. This occurs not only in humans, but also in other species, suggesting that such an admixture may be quite common in nature.
And if these mixed events are widespread, our research suggests that complex sweeps may also be more common than we currently think. In general, we may have a preconceived notion of how species are genetically adapted to environmental stresses.
To fully understand how adaptation works at the genetic level, we will need to develop new statistical methods to separate signals from complex scans and other selection events.
Source: Science Alert.