Fast evolution, in fact, probably drove the initial Neanderthal/human divergence, which likely began as genetic drift -- random changes in DNA. As the two groups parted ways, their changing environments likely drove more substantial changes in body shape and size, in response to differing needs.
Weaver and colleagues Charles Roseman and Chris Stringer created a model to determine how long it would have taken genetic drift to create the cranial differences observed between Neanderthal and modern human skeletons.
The model used prior information on how microsatellites, aka "junk DNA," can change, or drift, over time in a species. Over time, those changes can accumulate enough for an entirely new species to evolve.
They're determining the most recent common ancestor.
UPDATE: See also this.