It seems Homo sapiens have recently been through a period of very small total population, which has caused low genetic variability among the species now. From The Genetic Archaeology of Race: "...sometime in the period 100,000 to 200,000 years ago our ancestors went through a severe genetic bottleneck. Perhaps an environmental change drove ancient people to the brink of extinction. A more likely scenario, however, is that a relatively small group, numbering fewer than 20,000 at times and probably living in eastern Africa, was isolated for many thousands of years from the many groups of archaic human beings scattered throughout Africa, Europe, and Asia. The people who emerged from this genetic bottleneck had traits never before seen in human beings. They had lighter builds, new ways of interacting among themselves, and perhaps a greater facility with language".
There are a number of possibilities of how the Homo sapiens bottleneck occurred, with a number of expansion models. (And what is a human, anyway?)