The role of infectious disease epidemics in the human evolution investigated by means of ancient DNA analyses
Friday seminar by Barbara Bramanti
Since the geographical dispersal of modern humans 100 000-150 000 years ago, the human genome has been shaped by evolutionary and demographic forces. According to the demic diffusion model, gene frequency clines in Europe have been interpreted to coincide with the spread of agriculture throughout the continent in Neolithic times. However, genetic clines of some alleles might also have been caused by natural selection. Some polymorphisms are in fact proposed to confer resistance against infectious diseases that spread epidemically in Europe in the past. Epidemics that killed large parts of the populations in early centuries might have positively selected individuals carrying protective alleles. For instance, pathophysiological evidence indicates that cystic fibrosis mutations may confer high resistance to cholera and other chloride-secreting diarrheas, whereas plague is suspected to have caused the high incidence of the hemochromatosis alleles and of the CCR5-Delta32 mutation, which give nowadays immunity against AIDS. These evolutionary relationships remained to the most part speculative, since there was no way to test them other than in animal models. We investigated the mentioned interactions by means of ancient DNA analysis directly on skeletal human remains from cholera and plague mass graves. In addition, we dated the origin of such mutations in prehistorical skeletal remains.
In the history, the term ‘plague’ was however attributed to several infectious diseases producing epidemics; thus, we could not test the previous hypotheses concerning resistance against Yersinia pestis without proving that this bacterium was the actual causative agent of the medieval pestilences, the first pandemic (5th- 7th c.) and the second one (14th-18th c.). Here, I describe the retrieval of Y. pestis ancient DNA from victims of the past pandemics and the phylogenetic characterization of the ancient strains. Despite these revolutionary results several questions about the origin, the dispersal and the modalities of dissemination in medieval times remain opened and need further investigation. I shortly present a new project which proposes to answer these open questions by combining genetics with climate-ecological information.
Institute of Anthropology, Johannes Gutenberg-University Mainz, Germany