The paper by Lohmueller et al. 2008 shows that there is proportionally more deleterious genetic variation in European than in African populations. This revelation enables conclusions to be made on the group that emigrated from Africa several thousand years ago.
Literature: Kirk E. Lohmueller, Amit R. Indap, Steffen Schmidt, Adam R. Boyko, Ryan D. Hernandez, Melissa J. Hubisz, John J. Sninsky, Thomas J. White, Shamil R. Sunyaev, Rasmus Nielsen, Andrew G. Clark & Carlos D. Bustamante: Proportionally more deleterious genetic variation in European than in African populations. Nature Vol 451, 21 February 2008.