According to research conducted by Ben Sacks from the University of California at Davis and his colleagues, the Southeast Asian dogs prospered because, after they were brought south of the Yangtze River some 6,000 years ago, the dogs were isolated from their wolf forebears. Without that proximity, the Southeast Asian dogs could no longer interbreed with wolves, and thus followed their own evolutionary path. In contrast, northern Asian and European dogs still had contact with, and interbred with, the native wolf populations. Put more clearly, if dogs and wolves interbreed, as they did in Europe, they ended up in an evolutionary cul-de-sac. Isolated from one another, traits that benefited the newly emergent dog lineage flourished.