The story of dog evolution is very similar to the evolution lines of other mammals. They are a small; ancestral species that rises over the course of millions of years, to what we know and have today. The evolution of carnivores is a meandering affair involving, not only dogs, but prehistoric hyenas, bears, cats, etc. According to paleontologists the first carnivorous mammals evolved during the late Cretaceous period, about 75 million years ago. It is also believed that carnivorous animals alive today can trace its ancestry back to Miacis; a large weasel-like creature that lived about 55 million years ago. Miacis ate insects, eggs, and small animals.
The next significant link comes with Hesperocyon, the direct ancestral link to all later canids. This "western dog" was about the size of a small fox; the importance is that the inner-ear structure was characteristic of later dogs. Hesperocyon was one of the most common mammals of prehistoric North America. Another group of early canids were the borophagines, or "bone-crushing dogs." These animals had powerful jaws and teeth which were suitable for scavenging carcasses. There's some evidence that these animals hunted or scavenged in packs, like modern hyenas. Not long after the appearance of Hesperocyon, about 40 million years ago, Leptocyon arrived. Leptocyon was the first true canine and mainly lived in Eurasia and South America. In North America, about six million years ago, populations of Eucyon evolved into the first members of the modern dog genus Canis, which spread to these other continents. The first large wolves evolved elsewhere and came to North America shortly before the Pleistocene period. The Dire Wolf is the most famous of these large wolves. The Dire Wolf competed with the saber-toothed tiger for prey. The end of the Pleistocene period came the rise of human civilization. The first domestic Gray Wolf occurred somewhere in Europe or Asia from 30,000 to 15,000 years ago.