By analyzing the genetic material of thousands of ancient humans, researchers have mapped how natural selection has influenced hundreds of physical and behavioral traits across Western Eurasia over the past 10,000 years. The findings reveal that evolution continually makes certain genetic variations more or less common, influencing everything from blood type to disease risk. The study was published in the journal Nature.
Evolution is driven by multiple forces, but one of the most prominent is directional selection. This occurs when a particular genetic mutation has a survival or reproductive advantage, so that the mutation becomes increasingly common in a population. Conversely, unfavorable traits are driven out of the population with each generation.
Tracing this process in humans has proven extremely difficult for geneticists. As humans migrated, conquered, and mixed over thousands of years, the frequency of certain genes naturally changed. This natural, random variation is known as genetic drift. It is difficult for researchers to distinguish between random genetic drift and true directional selection. For a long time, scientists could not easily determine whether a gene became common because it was highly advantageous or simply because a group of migrating people happened to have it.
A research team led by Ali Akbari, a geneticist at Harvard Medical School, set out to solve this problem. They developed new statistical methods aimed at tracking genetic changes over thousands of years and separating the effects of human migration from true evolutionary pressures.
Researchers collected a vast dataset of genetic material from 15,836 ancient humans who lived in Western Eurasia. This geographic area includes neighboring regions of Europe and the Near East. The human bones spanned 18,000 years. The research team sequenced more than 10,000 of these genomes for the first time, significantly expanding the data available on ancient human genetics.
The group used a new statistical approach to examine nearly 10 million different points within the human genome. They looked for consistent trends in which certain genes become more or less steadily common across different populations and time periods. The large sample size allowed them to discover subtle evolutionary movements that earlier studies lacked the statistical power to detect conclusively.
They found that directional selection was incredibly common in Western Eurasia over the past 10,000 years. The research team identified hundreds of specific genetic variations that were favored or actively eliminated by the forces of evolution. Many of these genes are related to the human immune system and the body’s response to changes in diet.
Among the specific biological findings, the study tracked an increase in genes associated with high risk of celiac disease. This gene has become more common over the past 4,000 years. This suggests that this gene provides a powerful but unknown defense against certain pathogens, and that this ancient defense outweighed the negative health effects of gluten sensitivity.
Researchers also noticed that evolutionary preferences are changing within the human blood group system. Over the past 6,000 years, blood type B has become more common at the direct expense of blood type A. Because different blood types have different levels of resistance to different pathogens, this change likely reflects a change in the ancient infectious disease landscape.
Another discovery involved a genetic mutation that confers complete resistance to HIV infection. Previous theories suggested that this mutation became common in medieval Europe as a protection against the bubonic plague. A new study places the mutation’s occurrence much older, between 2,000 and 6,000 years ago, raising new questions about which ancient pathogen actually caused its spread.
The data also showed that evolutionary pressures can completely reverse direction. Genes associated with high risk of tuberculosis were positively selected starting about 9,000 years ago, but experienced strong negative selection starting 3,000 years ago. This suggests that the diseases that plagued early humans changed dramatically as humans changed their living environments.
Our physical appearance has also been deeply shaped by these evolutionary pressures. The research team found strong evidence for the selection of lighter skin tones. They also noted that genes associated with straight hair and male pattern baldness have steadily declined over the past 7,000 years.
Several previous theories about human disease have been challenged by new data. For example, some scientists had hypothesized that the gene that causes cystic fibrosis remained in the population because it conferred resistance to ancient cholera epidemics. Akbari et al. found no evidence of directional selection for cystic fibrosis genes during the historical period when cholera was endemic in the region.
The researchers also evaluated polygenic traits. Most physical and behavioral traits are not controlled by a single mutation, but are influenced by hundreds or thousands of different genes that work together. Scientists can now combine these genetic variations to predict the likelihood of traits in living humans. By tracking how the combination of these genes has changed in the past, the researchers found striking patterns.
The combination of genes that now predict increased body fat, increased waist circumference, and increased risk of type 2 diabetes were no longer actively selected as ancient peoples transitioned to an agricultural lifestyle. This challenges the common thrifty gene hypothesis that early humans were adapted to store fat to survive periods of famine. Rather, the genomic record suggests that excess body weight has been an evolutionary disadvantage in recent human history. Gene combinations known today to increase the risk of schizophrenia and bipolar disorder also decreased in frequency.
The most controversial results involve behavioral and lifestyle characteristics. The researchers uncovered selection for a combination of genes associated with modern smoking behavior and poorer overall health. Conversely, they observed positive selection for a combination of genes that predict today’s faster modern walking speeds, higher intelligence test scores, and more years of schooling.
Interpreting these complex behavioral traits requires great care. Genetic predictors of traits such as intelligence test scores and years of schooling were developed using data from modern industrialized societies. How these same genetic variations emerged in ancient illiterate societies remains completely unclear. Genetic sequences associated with success in the modern classroom may have promoted very different advantageous behaviors in ancient farming communities.
The researchers also note that evolutionary pressures have not been constant over time. Genes that confer resistance to disease in one millennium may become useless or harmful in the next millennium when entirely new pathogens emerge. This study assumes a constant selection rate just to enable statistical analysis, but this masks the reality of a fluctuating environment.
Future research could apply these methods to other regions of the world to see if similar patterns exist around the world. By examining longer timelines and different geographic areas, scientists hope to build a more complete picture of how human biology has adapted to a rapidly changing world. Detailed genetic maps can be extracted from ancient bones, allowing researchers to observe human evolution in real time.
“Ancient DNA The study, “Revealing directional selection widespread across Western Eurasia,” was conducted by Ali Akbari, Annabelle Perry, Alison R. Burton, Mohammadreza Kariminejad, Stephen Ghazal, Zheng Li, and Yeting. Zeng, Alyssa Mitnik, Nick Patterson, Matthew Ma, Xiang Zhou, Arquez L. Price, Eric S. Lander, Ron Pignasi, Nadine Rowland, Swapan Mallick, and David Reich.
