All breeds of domestic dogs, the wild dogs, and related species belong to the family Canidae. Despite the various breeds of domestic dogs that are known, the scientific name for all is Canis familiaris. The origin of domestic dogs is obscure, but they seem to be most closely related to the wolf. The dog has been associated with humans since Neolithic times, at least 10,000 years ago, and there are indications that as early as 4000 years ago several varieties of dogs had been developed by humans. In many respects the dog is structurally primitive and shows a genetic plasticity which accounts for the many varieties. There are more than 100 breeds of domestic dogs, and their classification is based principally on their uses.