Summary

Ancient DNA (aDNA) studies have significantly changed anthropological perceptions of human evolution. The caves where many of the Eurasian archaic hominin remains have been found are particularly well suited for study as DNA preserves best at low temperature and humidity. This research has provided important information about our close relatives, the Neanderthals, and the Denisovans, a group previously unknown in the fossil record. Scientists have found traces of these, and perhaps other archaic hominin groups, in modern human genomes. A small but significant admixture or genetic exchange occurred among these groups with Neanderthal alleles being present in modern European and Asian people, while Denisovan alleles are confined to Asia and Oceania. No one knows exactly what happened to these archaic groups, but hybrid incompatibility, pathogen resistance, and population dynamics may have contributed to their disappearance. The ancient genetic sequences found in living people are not distributed uniformly throughout their genomes. Negative selection against archaic alleles may explain “archaic deserts” where these alleles are rare. In areas where archaic alleles are more common, both positive selection and genetic drift could explain these higher frequencies and why they are often difficult to distinguish from each other. Current work indicates that archaic alleles that confer resistance to disease and strengthen immune function are likely candidates for positive selection. Archaic alleles involved in phenotypic skin color variation are likely relatively neutral and more likely provide examples of genetic drift. Medically focused research has just begun to reveal the positive and negative effects that archaic alleles may have on human health. Human adaptation is ongoing, so genetic information obtained from archaeological contexts should provide otherwise unobtainable information about health and disease. The aDNA field will continue to grow as the technologies improve permitting access to genetic sequences from ever older samples and expanding the preservation conditions that can yield DNA.