Dogs, ungulates (hoofed mammals), cats, bears, weasels, raccoons, civets and hyenas all share a common ancestor. Thus, the wolf and the animals it preys upon (the ungulates) evolved from a common ancestor from which they both inherited their intelligence and their ability to run swiftly for long periods of time. This common ancestor lived about a hundred million years ago. Most mammalian orders then differentiated during the Paleocene epoch (65 to 55 million years ago) and can be recognized from Eocene (55 - 33 million year old) fossils.
The Creodonts, primitive meat-eaters which originated in the Northern Hemisphere, evolved during the Paleocene, and most were extinct by the Pliocene (5.2 million years ago). The family Carnivora, which includes dogs, cats, bears and weasels, descended from the Creodonts, which included forms similar to the cats, dogs, hyenas and weasels.
Roughly 55 million years ago, a mammal with carnassials (teeth used for tearing flesh) evolved from the creodonts. For the next ten million years, a number of these animals evolved and flourished. Most of these small animals resembled weasels or civets. It was during this time that the Miacis of the family Miacidae, from which the dogs, cats, bears, weasels, raccoons, civets and hyenas diverged from, appeared. The Miacidae included forms that varied greatly in size - some were as small as squirrels, while others were dog sized.
About 30 to 40 million years ago, Miacis (a tree-climbing animal with retractable claws) diverged into two types of mammals that gave rise to the dog and bear lineages. The Cynodictis line eventually gave rise to the dog family. It is debateable as to where the dog family originated. Some authorities believe that it originated in North America, and then spread to South America and Eurasia. Others believe that the dog family evolved in North America, then migrated to Asia and then migrated back to North America. Other paleontologists believe that the dog family originated in Asia, and then spread to North America. The Cynodictis was smaller than today's wolf and its long body resembled that of a weasel's. However, it had the same number of teeth as a wolf. During the next 15 million years, the raccoon family also diverged from the Cynodictis.
15 to 30 million years ago, the Cynodictis began to become more wolf-like as it evolved into the Cynodesmus and the Tomarctus. These ancestors of the wolf had longer legs, longer and more compact feet, and shorter tails than the Cynodictis. Also, the inner digit on the hind foot became vestigial in these animals (meaning the animal no longer used it) and the inner digit of the front foot became reduced. The dew claws that can be found in wolves and domestic dogs are evidence of this process.
The wolf and fox lineages both diverged from Tomarctus. The lineage which gave rise to the fox did not change significantly in size, whereas the lineage that gave rise to the wolf grew continually larger. The dire wolf (Canis dirus) branched from this lineage, and appeared in Eurasia during the early Pleistocene period roughly one million years ago. Dire wolves began to migrate to North America about 750 000 years ago. Dire wolves were heavier and more powerfully built than modern gray wolves, and the two co-existed together in North America for roughly 400 000 years. Dire wolves did have shorter legs than modern gray wolves, and larger teeth and jaws. It likely used its powerful jaws to crush bone, as fossil dire wolf teeth often show large amounts of wear on them. It is quite likely that the two species were capable of interbreeding, although both seem to have arisen from separate lineages within the genus Canis. Coyotes, which also existed at the same time as dire wolves, also likely came from a different lineage within the genus. Dire wolves, along with saber-toothed cats, giant bison, giant beavers, ground sloths and mammoths all went extinct during the Pleistocene (the epoch that began 1.8 million years ago). It is unknown as to what caused these large mammals to disappear, although it is interesting to note that the extinction of much of the Pleistocene megafauna coincided with the arrival of humans to North America from the northwest. It is possible that the dire wolf was hunted to extinction by humans. However, it is also possible that a major climatic change resulted in the extinction of the dire wolf and many other North American ice-age mammals, as their extinction also occurred with such a change. The heavy build and massive jaws of the dire wolf made it well adapted to preying upon the giant herbivores that lived in North America at the time and scavenging upon remains of them. Dire wolves seem to have disappeared completely from North America about 8 000 years ago, although some fossils found in Arkansas suggest that they may have lived in the Ozark mountains as recently as 4 000 years ago. The wolf that exists today is much like it was about one to two million years ago.
It is often assumed that coyotes descended from wolves although that is not necessarily the case. Fossilized coyote bones that have been found in Maryland, New Mexico and New Brunswick predate any gray wolf fossils by tens of thousands of years.
