Red hair and thinning hair: Study of ancient DNA shows human evolution has accelerated over the past 10,000 years
Something happened around 4,000 years ago in West Eurasia that made red-haired people more common.
But not just redheads. New research shows genetic variants linked to celiac disease, schizophrenia, light skin, a lower chance of male pattern baldness, and B blood type all arose and caught on quickly among prehistoric humans because they gave some sort of evolutionary advantage. Genes relating to body fat and cognitive performance, and resistance to various diseases such as leprosy also saw major spikes in frequency around this time, and made those who had them more likely to pass their genes on to descendants.
Other genetic variants relating to tuberculosis, arthritis and multiple sclerosis saw rapid decreases in frequency.
It seems to have had something to do with the development of agriculture in this geographical area, roughly Europe and the Middle East, and with the changing evolutionary pressures that farming brought about, including changing diet and exposure to new diseases.
The new research, published Wednesday, shows for the first time that hundreds of these rapid evolutionary adaptations in humans have occurred in just the past 10,000 years, far more than previously thought, and much more recently. Fewer than two dozen had previously been identified, the best known being a tolerance for lactose after infancy that evolved in European populations.
The new study, by lead author Ali Akbari, a computational geneticist in the Harvard University lab run by co-author David Reich, is the largest of its kind in the newish field of ancient human genetics.
It shows changes that happened for the first time among those West Eurasian human populations have major relevance to the health of modern day human populations all over the world, and it suggests human genetic evolution is not fixed in time but is in fact accelerating, the authors say.
The majority of the newly discovered adaptations are related to disease risk, although it remains uncertain why each gene gave people an evolutionary advantage in the past.
In a statement, Reich said this research “allows us to assign place and time to forces that shaped us.”
DNA sequencing technology has in recent years allowed scientists to see more deeply into the finds of archeologists, and to track single genes as their prevalence rises and falls in various prehistorical populations whose members today are physically reduced to a few shards of teeth or bone.
So, instead of looking in present-day human DNA for clues about how it evolved, or what Akbari calls “the scars of natural selection” that are visible on the modern human genome, this research shows they can instead track that selection over time.
“Rather than being trapped in the present and studying the scars left by selection on the genomes of descendants, ancient DNA makes it possible to test directly whether frequencies of variants shifted more than could be expected by chance,” reads the paper in the journal Nature .
What they are looking for is known as directional selection, when a certain gene gives such a strong evolutionary fitness advantage that its frequency spikes in a population.
In the paper, geneticists call these “classic hard sweeps driving advantageous mutations to fixation.”
They have so far been thought to be rare over the full history of human evolution, going back more than 200,000 years to precursor populations in Africa.
The classic example of this is lactose tolerance in adulthood, which is thought to have evolved to be common among Europeans in response to stresses such as famine in just the past few millennia. This new work suggests these strong “sweeps” of directional selection are not so rare, but actually pretty common.
A major conclusion of the paper is that there have in fact been almost 500 of them in just the past 10,000 years.
Other things can change the frequency of genes and thereby masquerade as directional selection in this kind of study, however, such as migration or a change in population structure.
And some traits, such as red hair, might also piggyback on other traits and get carried along into descendant populations without actually being the effective cause of increased evolutionary fitness.
Some of the genes that underwent directional selection are related to body fat levels or cognitive performance, or even susceptibility to tobacco use, but these effects were measured in industrialized societies, not in prehistoric populations. So, Akbari writes, it remains unclear how these effects relate to traits that gave humans an evolutionary advantage in prehistoric West Eurasia.
In order to better identify actual directional selection, Akbari and colleagues compiled a collection of 16,000 samples of ancient human DNA, 10,000 of which were newly sequenced genetically, and the rest had previously been published.
They then devised a method to analyze these samples and to identify alleles — genetic variants — that are statistical outliers, appearing with greater or lesser frequency in populations at different prehistorical times.
This involved computational strategies known as “data cleaning.” This strategy also allowed them to calculate an overall rate at which selection for these variant genes occurred, and thus to conclude human evolution accelerated after the introduction of agriculture.
National Post
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