Dogs are members of the order Carnivora, a group of mammals with origins in the Tertiary era, about fifty-five million years ago. The earliest fossils of ancestors of this order, the Miacoides, predate that distant time.
These animals varied in size, from about the length of a weasel to that of a wolf, and had the characteristic teeth of present-day carnivores.
The story of the Canidae family begins in the Northern Hemisphere during the late Eocene and early Oligocene epochs. On each side of what is now the Atlantic Ocean there lived two closely related animals: Cynodictis in Europe and Pseudocynodictis in North America.
The European animal died out in the Oligocene epoch (35 million years ago) through competition with the more highly evolved Ursidae or "Bear Dog". The North American species survived, later giving rise to what we now recognize as the dog. Pseudocynodyctis was a low-slung and very long animal with a long muzzle. Evolution led to the lengthening of its legs.
Following Pseudocynodictis in North America came Mesocyon, with a skeleton similar to that of a present-day wolf. Its descendant was Cynodesmus (early Miocene epoch, 25 million years ago) which thereafter gave rise to Tomarctus, known to have existed in the middle and late Miocene epoch.
Shortly before the beginning of the Pliocene epoch (10 million years ago) the genus Canis appeared and migrated into what is now Eurasia. Its earliest representative in the Old World, Canis cipio, was discovered in the deposits in Concud, near Teruel, in Spain. It lived about six million years ago. The genus evolved through different species up to the Canis lupus, or wolf, which dates back approximately 300,000 years.