According to a recent study reported by The Australian, the evolutionary divergence of humans and chimps may have taken place over a period of 4 million years, during which time the two proto-species interbred on a fairly regular basis. Commenting on the study, anthropologist Daniel Lieberman politely notes the ick-factor involved: "My problem is imagining what it would be like to have a bipedal hominid and a chimpanzee viewing each other as appropriate mates, not to put it too crudely."
But we should set our grossed-outness aside long enough to realize how cool this is. It's easy to think of speciation as having occurred at a moment in time, when the last crucial mutation needed to produce a new species took place. The reality is messier. Delineating one species from another is not easy to begin with, and the features that distinguish them must accumulate over time (as Eugene Volokh once implied, the difficulty of picking a dividing line is what makes the "chicken-or-the-egg" problem so problematic). We shouldn't be surprised, then, to find a transition period in which the genetic changes were great enough to create substantial differences between two kinds of animal, but not great enough to stop them from procreating. According to the article, hybridization is known to be common in plants (whose interbreeding somehow doesn't produce the same "yuck" reaction), but the new study provides the first strong evidence of the same phenomenon in humans. As geneticist David Reich notes, "That such evolutionary events have not been seen more often in animal species may simply be due to the fact that we have not been looking for them."