All of the fossils shown on this page come from North America.  The majority of domesticated dog (Canis familiaris) breeds once kept by the aboriginal Americans have long disappeared through interbreeding with the large stock of European dogs that were introduced to the continent several centuries ago.  However, some ancient Mexican breeds, such as the chihuahua and Mexican hairless, still survive today.  Estimates of when humans first began to domesticate dogs vary from 12,000 to 50,000 years ago.  It is also theorized that domestication may have occurred not once, but a number of times, and in several parts of the world.  There is a good possibility that multiple subspecies of wolves contributed to the ancestry of modern dogs.