The modern buffalo descended from Bison latifrons, an enormous, shaggy, European herbivore that crossed the land bridge that once connected Asia and North America. The crossing took place between 200,000 and 800,000 years ago and not long thereafter the land bridge submerged and North American Bison latifrons was isolated from the old world forever.
The ancestor of the buffalo that we know was left to evolve on the North American continent during a time of climate warming. About 120,000 years ago that species died out and was replaced by two new forms. Bison antiquus was closely associated with early man in North America and became extinct about 10,000 years ago. But the second descendant of the first buffalo, Bison occidentalis survives to this day. Though still a huge herbivore, it is smaller than its predecessors and its horns angle back rather than protrude outward like the ancient buffalo's. As the years passed, Bison occidentalis learned to utilize the main ecosystems of North America and, in doing so, evolved into two distinct races: the plains buffalo, Bison bison and the mountain or wood buffalo, Bison athabascae. These two races still exist and make up what is now known as the North American buffalo or bison.