Scientists from the Institut Pasteur set out to understand how the demographic changes associated with the Neolithic transition also influenced the efficacy of natural selection. By comparing the genome diversity of more than 300 individuals from groups of forest hunter-gatherers (pygmies) and farmers (Bantu-speaking peoples), from western and eastern Central Africa, they discovered that the reason pygmies did not suffer from excessive deleterious mutations was because of their genetic diversity and their admixture with the Bantu peoples.Research into the diversity of the human genome and any rare or frequent mutations is vital in identifying mutations that increase susceptibility to "complex" diseases, such as infectious or autoimmune disorders. "The elimination of these mutations that are harmful for human health is largely conditioned by the evolutionary history of populations, especially their demographic fluctuations," explains Lluis Quintana, Head of the Human Evolutionary Genetics Unit and author of the study. Throughout their history, human populations have undergone significant and extremely varied changes in terms of numbers of individuals, giving rise to differences in the number and severity of mutations carried by these populations. The accumulation of disease-causing genetic variants, known as the "burden of deleterious mutations", varies from one population to another depending on its past. "The aim of our project is to understand how demographic events such as the rise and fall in numbers of individuals, as well as genetic admixtures, have had an impact on the efficacy of some evolutionary mechanisms such as purifying selection."