Miacis, is described as a weasel sized small carnivore that lived 40 million years back in the Eocene period. This animal is believed to be the oldest ancestor of present day dogs. They were forest dwellers, smaller in size with long tail and short limbs.
Cynidictus was the next to evolve and the evolution of Hesperocyan is thought to be next stage. There are two views on the development of Miocene period was followed by Tomarectus of Pliocene period and then the present day canids. But there is another view that Hesperocyan and then Leptocyan may be the ancestors of dogs or even Tomarectus. The immediate wild ancestors of the dogs are more or less accepted as wolves (Canis lupus). But there are other views like the possibility of mixing of jackal and wolves, which lacks fossil evidence. The production of fertile offspring from the mating of wolf and dogs supports wolfian ancestry for dogs. Moreover the dog breed Alsatian has lot of similarities with wolf. Dogs belong to the class Mammalia, sub class Metatheria, Order Carnivora, Family Canidae, Genus Canis and species familaris.