Humans
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When our distant ancestors first traded nomadic life for farming, villages, and permanent homes, you might assume that the beastly forces of natural selection lost their ability to shape our species, but this isn't the case at all. In fact, it appears that natural selection may have actually accelerated in recent human evolutionary history.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.In a new study, scientists found that natural selection has shaped hundreds of genes in the population of West Eurasia over the last 10,000 years.
A team led by researchers from Harvard University studied nearly 16,000 ancient genomes taken from skeletons found across Europe and parts of the Middle East. This “unprecedented amount of ancient genomic data” was combined with over 6,400 modern genomes fed into a computer algorithm that tracked how different gene variants spread and declined in the human population of Western Eurasia.
Importantly, this method was able to highlight which variants had emerged through directional selection, while weeding out those caused by other changes not related to adaptive evolution, such as human migration, population mixing, and random genetic fluctuations that occur in small populations.
This identified at least 479 gene variants, aka alleles, that were strongly selected for or against in West Eurasian genomes in the past 10,000 years. Before this latest study, scientists had identified only a few dozen variants that had been recently selected for.
What's more, the rate of selection appears to have accelerated after people transitioned from hunting and gathering to farming just after the last ice age thawed.
“With these new techniques and large amount of ancient genomic data, we can now watch how selection shaped biology in real time,” Ali Akbari, first author of the study and senior staff scientist in the lab of Harvard geneticist David Reich, said in a statement. “Instead of searching for the scars natural selection leaves in present-day genomes using simple models and assumptions, we can let the data speak for itself.”
The majority of those relatively recent gene variants (over 60 percent) still have clear links with traits we see in the present-day human population, like:
- Light skin tone
- Red hair
- Risk of celiac disease and Crohn’s disease
- Immunity to HIV infection and resistance to leprosy
- Lower chance of male-pattern baldness
- Lower risk of rheumatoid arthritis and alcoholism
- Having type B blood proteins
- Reduced risk of bipolar disorder and schizophrenia
- Lower body fat percentage, waist-to-hip ratio, and body mass index
- Less susceptibility to tobacco smoking
In other words, these traits didn't arise by chance, but were actively shaped by natural selection in certain populations living in Western Eurasia.
All of this happened relatively recently, within the last 10,000 years, after the advent of agriculture and the emergence of early settled societies. While this social shift meant we spent less time fleeing predators, starving between hunts, and dying of exposure to the elements, natural selection didn't simply disappear. Our new diets, crowded settlements, and unfamiliar diseases kept the evolutionary pressure firmly on, causing human populations to keep adapting to an entirely new world.
Many of these changes are complex, however. For instance, people in Eurasia wouldn’t have had access to tobacco until the 16th century CE since the plant is native to the Americas. As such, the genetics linked to tobacco susceptibility must be related to something else entirely.
Other links may seem counterintuitive. For example, why would people develop a celiac disease just after wheat had become domesticated and farmed en masse? These puzzling cases are a reminder that while a gene variant may shape a particular trait today, that doesn't necessarily mean the trait itself was what made it important or valuable in the past; genetics is rarely straightforward, and evolution isn't linear.
“This paper shows how complex selection can be and provides an opportunity to consider the richness of variation in human populations,” explained David Reich, senior study author and professor of human evolutionary biology in the Harvard University Faculty of Arts and Sciences.
This latest study used a novel computational method not previously applied to human genomics on this scale. To push it further, the team now wants to examine other populations around the world.
“To what extent will we see similar patterns in East Asia or East Africa or Native Americans in Mesoamerica and the central Andes? If we can’t use ancient DNA to study the most important period in human evolution 1 million to 2 million years ago, then at least we can study selective pressure on human genomes during more recent periods of change and learn broader principles,” said Reich.
The study is published in the journal Nature.
