Humans and chimps did not undergo a speciation event. Some pair of species (one an ancestor of humans, the other of chimps) speciated. It was thought that this event occurred approximately 6.5-7.4 million years ago, based on fossil evidence. A new paper coming out in the week’s issue of Nature, however, suggests that the speciation event was sloppy. The authors argue, based on differing amounts of divergence between humans and chimps in different parts of the genome, that there was some hybridization and the speciation event extended until 5.4 million years ago. You can read about the study here, here, here, here, and here. The Nature article does not appear to be available yet, so I can’t really comment on the methods.