Ancient DNA reveals a hidden history of humanadaptation

Humans may be just as vulnerable to environmental change as other animals, according to our new research analysing genetic data from more than a thousand people who lived across Europe and Asia over 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 striking sweep occurred among early Anatolian farming people, in a genetic region associated with the immune system called MHC-III.

Hard sweeps have often been seen in other species, but until now there has been little sign of them in humans. The traces of the hard sweeps had been hidden by frequent mixing between populations over the past 8,000 years.

Our results show humans’ famed ability to adapt our behaviour and develop new tools and techniques has not always been enough to survive when times have grown tough.

How natural selection works


Modern humans live in a huge variety of natural environments, from the frozen Arctic to sweltering tropical rainforest.

Unlike most animals, humans can draw on cultural innovations – such as fire and clothing – to overcome the challenges these environments present.

However, these innovations may not always have been enough to cope with new environmental conditions. This is when genetic variability among individuals comes into play.

Individuals with genetic variations that make them better equipped to deal with the 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 nearly 200 years ago.

How humans adapt


Using statistical tools to search for evidence of hard sweeps, researchers have found ample evidence for past adaptive events in many animals and plants, but little in human genomes. More specifically, hard sweeps are conspicuously rare in humans.

As a result, that genetic adaptation in humans is rare, perhaps because cultural innovations have made it largely unnecessary. Others have suggested selection has occurred across many moderately beneficial genetic variants, leading to subtle and hard-to-detect signals.

Hidden signals


Almost 40 years ago, new technologies to extract tiny amounts of DNA from archaeological skeletal remains were developed. This has made it possible to , and changed our view of how ancient human groups and civilisations are related to each other.

Ancient DNA research revealed that over the past 10,000 years in Eurasia, intermixing between genetically divergent populations has been particularly frequent.

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.

Around 10,000 years ago, after the end of the last ice age, there was among the the hunter-gatherers living in Europe than there is among the humans living there today.

In fact, the genetic differences between groups of ancient European hunter-gatherers were as large as the differences now observed between contemporary populations in western Europe and east Asia.

This extreme genetic differentiation collapsed over the past 8,000 years due to several migrations and mixing events, making modern Europeans much more genetically homogeneous.

‘Hard sweeps’ in human history


In our new research, , we revisited this question by scanning more than a thousand ancient human genomes sourced from across Eurasia.

We wondered: could these relatively recent mixing events have masked historical selective sweeps, so they were invisible in modern human genomes?

To test this idea, we first carried out some computer simulations based on estimates of genetic mixing from studies of ancient Eurasian genomes. The simulation results suggested ancient selecti