The bear family, Ursidae, is thought to have split from other carnivorans about 38 million years ago.
The subfamily Ursinae originated approximately 4.2 million years ago.
The oldest known polar bear fossil is a 130,000 to 110,000-year-old jaw bone, found on Prince Charles Foreland in 2004.
Fossils show that between 10,000 and 20,000 years ago, the polar bear's molar teeth changed significantly from those of the brown bear.
Polar bears are thought to have diverged from a population of brown bears that became isolated during a period of glaciation in the Pleistocene from the eastern part of Siberia (from Kamchatka and the Kolym Peninsula).

The evidence from DNA analysis is more complex.
The mitochondrial DNA (mtDNA) of the polar bear diverged from the brown bear, Ursus arctos, roughly 150,000 years ago.
Further, some clades of brown bear, as assessed by their mtDNA, were thought to be more closely related to polar bears than to other brown bears, meaning that the brown bear might not be considered a species under some species concepts, but paraphyletic.
The mtDNA of extinct Irish brown bears is particularly close to polar bears.
A comparison of the nuclear genome of polar bears with that of brown bears revealed a different pattern, the two forming genetically distinct clades that diverged approximately 603,000 years ago, although the latest research is based on analysis of the complete genomes (rather than just the mitochondria or partial nuclear genomes) of polar and brown bears, and establishes the divergence of polar and brown bears at 400,000 years ago.

However, the two species have mated intermittently for all that time, most likely coming into contact with each other during warming periods, when polar bears were driven onto land and brown bears migrated northward.
Most brown bears have about 2 percent genetic material from polar bears, but one population, the ABC Islands bears, has between 5 percent and 10 percent polar bear genes, indicating more frequent and recent mating.
Polar bears can breed with brown bears to produce fertile grizzly–polar bear hybrids; rather than indicating that they have only recently diverged, the new evidence suggests more frequent mating has continued over a longer period of time, and thus the two bears remain genetically similar.
However, because neither species can survive long in the other's ecological niche, and because they have different morphology, metabolism, social and feeding behaviours, and other phenotypic characteristics, the two bears are generally classified as separate species.

When the polar bear was originally documented, two subspecies were identified: the American polar bear (Ursus maritimus maritimus) by Constantine J. Phipps in 1774, and the Siberian polar bear (Ursus maritimus marinus) by Peter Simon Pallas in 1776.
This distinction has since been invalidated.
One alleged fossil subspecies has been identified: Ursus maritimus tyrannus, which became extinct during the Pleistocene.
U.m.
tyrannus was significantly larger than the living subspecies.
However, recent reanalysis of the fossil suggests that it was actually a brown bear.
The bear family, Ursidae, is thought to have split from other carnivorans about 38 million years ago. The subfamily Ursinae originated approximately 4.2 million years ago. The oldest known polar bear fossil is a 130,000 to 110,000-year-old jaw bone, found on Prince Charles Foreland in 2004. Fossils show that between 10,000 and 20,000 years ago, the polar bear's molar teeth changed significantly from those of the brown bear. Polar bears are thought to have diverged from a population of brown bears that became isolated during a period of glaciation in the Pleistocene from the eastern part of Siberia (from Kamchatka and the Kolym Peninsula).
The evidence from DNA analysis is more complex. The mitochondrial DNA (mtDNA) of the polar bear diverged from the brown bear, Ursus arctos, roughly 150,000 years ago. Further, some clades of brown bear, as assessed by their mtDNA, were thought to be more closely related to polar bears than to other brown bears, meaning that the brown bear might not be considered a species under some species concepts, but paraphyletic. The mtDNA of extinct Irish brown bears is particularly close to polar bears. A comparison of the nuclear genome of polar bears with that of brown bears revealed a different pattern, the two forming genetically distinct clades that diverged approximately 603,000 years ago, although the latest research is based on analysis of the complete genomes (rather than just the mitochondria or partial nuclear genomes) of polar and brown bears, and establishes the divergence of polar and brown bears at 400,000 years ago.
However, the two species have mated intermittently for all that time, most likely coming into contact with each other during warming periods, when polar bears were driven onto land and brown bears migrated northward. Most brown bears have about 2 percent genetic material from polar bears, but one population, the ABC Islands bears, has between 5 percent and 10 percent polar bear genes, indicating more frequent and recent mating. Polar bears can breed with brown bears to produce fertile grizzly–polar bear hybrids; rather than indicating that they have only recently diverged, the new evidence suggests more frequent mating has continued over a longer period of time, and thus the two bears remain genetically similar. However, because neither species can survive long in the other's ecological niche, and because they have different morphology, metabolism, social and feeding behaviours, and other phenotypic characteristics, the two bears are generally classified as separate species.
When the polar bear was originally documented, two subspecies were identified: the American polar bear (Ursus maritimus maritimus) by Constantine J. Phipps in 1774, and the Siberian polar bear (Ursus maritimus marinus) by Peter Simon Pallas in 1776. This distinction has since been invalidated. One alleged fossil subspecies has been identified: Ursus maritimus tyrannus, which became extinct during the Pleistocene. U.m. tyrannus was significantly larger than the living subspecies. However, recent reanalysis of the fossil suggests that it was actually a brown bear.
Polar bear