Article URL: https://www.harvardmagazine.com/research/harvard-human-evolution-genes-selective-pressure Comments URL: https://news.ycombinator.com/item?id=49054307 Points: 31 # Comm…

During the last 50,000 years, human beings have transformed from nomadic hunter-gatherers roaming savannas to a species that traveled to the moon, invented the internet, and created an artificial form of intelligence. Yet many scientists believed that, during the same time frame, human DNA hadn’t changed much. Most research during the past two decades hinted that human evolution has largely slowed or changed so subtly that existing tools couldn’t detect it; evidence seemed to suggest that civilization put humans beyond the reach of natural selection. “It’s like we had in our heads that a few hundred thousand years ago we had reached some kind of evolutionary optimization,” says professor of genetics and human evolutionary biology David Reich. That changed with Reich’s latest work. Using the largest collection of ancient human DNA samples ever compiled, paired with new statistical methods, Reich and his colleagues discovered that the human genome has actually evolved rapidly during the past 10,000 years. The study, published in Nature in April, showed hundreds of genetic shifts in 16,000 individuals who lived in modern-day Europe across an 18,000-year period. The longer timespan provided a baseline that allowed the researchers to see how genes evolved during the most recent 10,000 years. What’s more, the findings demonstrate that the pace of evolution is accelerating, with more intense natural selection occurring in the past 5,000 years than in the 5,000 years that came before. “Natural selection has not slowed down,” says Ali Akbari, a senior scientist at the Broad Institute and the paper’s first author. “We were just missing the signal.” By collaborating with roughly 250 archeologists from around the world, Reich and his team amassed a collection of bones and teeth from more than 5,836 ancient humans. They extracted and analyzed ancient DNA from the samples and built a Genetic Relationship Matrix—a chart showing how genetically similar each person is to each of the other people studied. A common challenge in population genetics is the difficulty of filtering out “noise.” People migrating, mixing, and having children can produce large shifts in gene frequencies that can obscure small changes in their DNA that result from natural selection alone. For instance, certain traits, such as brown eyes, may rise in a population, but it’s hard to know whether that is because a wave of brown-eyed families migrated in or because having brown eyes confers a survival edge. Using the matrix allows the researchers to cancel out this background noise so they can see the signal of evolution clearly. If a gene’s frequency trended upward consistently over diverse places and times (even just a little bit), Reich and Akbari could be confident it was due to the forces of natural selection. Breaking with the pattern followed in many previous studies, the researchers did not focus on the genetics within individual demographic groups, such as “early European farmers” or “nomadic hunter-gatherers.” Instead, they looked for genetic variants that occurred within multiple populations at multiple time points, including in modern populations, letting the data tell the story of evolution without forcing it into categories. The end result was the identification of 479 genes that showed evidence of natural selection. The sheer number of these genetic variants—which represent small changes in the DNA sequence of the gene—was staggering, even to the study’s authors. “It was a crazy result,” Reich says.