Mark Derr, in his book “How the Dog Became the Dog – From Wolves to Our Best Friends”, presents an interesting theory about the pathways that led to that sleeping bundle of fur at our feet.
He suggests that wolves who followed human hunting parties to feed on the leftovers became socialized to people and thus became dependent on them. These camp followers split off from the wilder dogs to eventually become a different species.
Derr also looks at how dogs became so differentiated into the various breeds. You can read more about this book and listed to an interview with the author at: http://www.npr.org/2011/11/08/142100653/how-dogs-evolved-into-our-best-friends?ps=cprs .