Infections have imposed strong selection pressures throughout human evolution, making the study of natural selection's effects on immunity genes highly complementary to disease-focused research. This review discusses how ancient DNA studies, which have revolutionized evolutionary genetics, increase our understanding of the evolution of human immunity. These studies have shown that interbreeding between modern humans and Neanderthals or Denisovans has influenced present-day immune responses, particularly to viruses. Additionally, ancient genomics enables the tracking of how human immunity has evolved across cultural transitions, highlighting strong selection since the Bronze Age in Europe (<4,500 years) and potential genetic adaptations to epidemics raging during the Middle Ages and the European colonization of the Americas. Furthermore, ancient genomic studies suggest that the genetic risk for noninfectious immune disorders has gradually increased over millennia because alleles associated with increased risk for autoimmunity and inflammation once conferred resistance to infections. The challenge now is to extend these findings to diverse, non-European populations and to provide a more global understanding of the evolution of human immunity.

Ancient DNA has transformed our understanding of population history¹, but its potential to reveal as much about human evolutionary biology has not been realized because of limited sample sizes and the difficulty of distinguishing sustained rises in allele frequency increasing fitness-directional selection-from shifts due to migrations, population structure, or non-adaptive purifying or stabilizing selection^2-7. Here we present a method for detecting directional selection in ancient DNA time-series data that tests for consistent trends in allele frequency change over time, and apply it to 15,836 West Eurasians (10,016 with new data). Previous work has shown that classic hard sweeps driving advantageous mutations to fixation have been rare over the broad span of human evolution^8,9. By contrast, in the past ten millennia, we find that many hundreds of alleles have been affected by strong directional selection. We also document one-standard-deviation changes on the scale of modern variation in combinations of alleles that today predict complex traits. This includes decreases in predicted body fat and schizophrenia, and increases in measures of cognitive performance. These effects were measured in industrialized societies, and it remains unclear how these relate to phenotypes that were adaptive in the past. We estimate selection coefficients at 9.7 million variants, enabling study of how Darwinian forces couple to allelic effects and shape the genetic architecture of complex traits.

The prediction of phenotypes from ancient humans has gained interest due to its potential to investigate the evolution of complex traits. These predictions are commonly performed using polygenic scores computed with DNA information from ancient humans along with genome-wide association study (GWAS) data from present-day humans. However, numerous evolutionary processes could impact these phenotypic predictions. In this work, we investigate how natural selection shapes the temporal dynamics of variants with an effect on the trait and how these changes impact phenotypic predictions for ancient individuals using polygenic scores. We find that stabilizing selection accelerates the loss of large-effect alleles contributing to trait variation. Conversely, directional selection accelerates the loss of small- and large-effect alleles that drive individuals farther away from the optimal phenotypic value. These phenomena result in specific shared genetic variation patterns between ancient and modern populations that hamper the accuracy of polygenic scores to predict phenotypes. Our results assume perfectly estimated effect sizes at the causal loci of complex traits segregating in a GWAS performed in the present and, therefore, provide a putatively loose upper bound on the polygenic score portability to predict traits in the past. Furthermore, we show how natural selection could impact the predictive accuracy of ancient polygenic scores for two widely studied traits: height and body mass index. Our results emphasize the importance of considering decreases on the reliability of polygenic scores to perform phenotypic predictions in ancient individuals due to allele frequency changes driving the loss of alleles via natural selection.