Human genetic diversity and its history

The evolutionary and historical processes that shaped present-day human diversity can be inferred from patterns of genetic variation in time and place. The main questions currently being addressed include the evolutionary relationships among contemporary humans and the different human forms documented in the fossil record, the extent and causes of the genetic differences among modern populations, and the implications of such differences for applied research in fields such as medical genetics, pharmacogenomics and forensic science. In this chapter we outline the main current models of human evolution and review the available ancient and modern DNA evidence in the light of these models. We suggest that most data are easier to reconcile with a model in which the ancestors of modern populations dispersed recently from Africa and essentially or completely replaced previously settled human forms. However, it has become increasingly evident that models assuming the expansion of a small African group are probably oversimplified, and more complex scenarios need be envisaged, including the possibility of multiple expansions and of genetic substructuring in the expanding population. The available studies show that the differences among modern populations tend to be patterned in the are geographical space, but that variation tends to be continuous and clear-cut genetic boundaries are difficult or impossible to objectively define, suggesting a major role of isolation by distance in shaping human diversity. As a consequence, assuming that humans come in well-distinct genetic clusters does not seem a promising approach for applied research. Throughout the chapter we emhasize the large amount of uncertainty surrounding the interpretation of genetic data, when they are used both to make inferences on our origins and to predict disease risk