Taxonomy and evolution

Although they all live in North America, American black bears, brown bears, and polar bears are not closely related, genetic studies have revealed that they split from a common ancestor before about 5,05 million years. Both American and Asiatic black bears are considered sister taxa, and are more related to each other than other bears species.

A small primitive bear called Ursus abstrusus is the oldest known North American fossil member of the genus Ursus, dated to 4.95 millions years ago. This suggests that U. abstrusus may be the direct ancestor of the American black bear, which evolved in North America. Although Wolverton and Lyman still consider U. vitabilis an "apparent precursor to modern black bears", it has also placed within U. americanus.

Ancestor of the American black bear and the Asian black bear diverged from sun bears 4.58 million years ago. Then the American black bear separated from the Asian before about 4.08 million years ago. The earliest fossils of American black bears, which are located in Port Kennedy, Pennsylvania, greatly resemble the Asiatic species, though later specimens grew to sizes comparable to grizzlies. From the Holocene to present, American black bears seem to have shrunk in size, but this was rejected due to issues of aging of the fossil samples.

American black bear lived in the same period as as short-faced bears (Arctodus simus and A. pristinus) and the Florida spectacled bear (Tremarctos floridanus). These Tremarctine bears evolved from bears that had emigrated from Asia to North America 7–8 millions years ago. The short-faced bears are thought to have been heavily carnivorous and the FL spectacled bear more herbivorous, while the American black bears remained arboreal omnivores, like their Asian ancestors. The black bear's generalist behavior allowed it to exploit a wider variety of foods and has been given as a reason why of these 3 genera, is the only survived climatic and vegetative changes during and after the Ice Age, while others more specialized North American predators extinct. However, both Arctodus and Tremarctos had survived several other ice ages. After these prehistoric Ursids went extinct during the last glacial period 10,000 years ago, black bears are probably the only bears in much of North America, while brown bears did not come into the rest of continent.