Asian black bear
Biologically and morphologically, Asiatic black bears represent the beginning of the arboreal specializations attained by sloth bears and sun bears. Asiatic black bears have karyotypes nearly identical to those of the five other ursine bears, and, as is typical in the genus, they have 74 chromosomes. From an evolutionary perspective, Asiatic black bears are the least changed of Old World bears, with certain scientists arguing that it is likely that all other lineages of ursine bear stem from this species. Scientists have proposed that Asiatic black bears are either a surviving, albeit modified, form of "Ursus etruscus", specifically the early, small variety of the Middle Villafranchian (Upper Pliocene to Lower Pleistocene) or a larger form of "Ursus minimus", an extinct species that arose 4,000,000 years ago. With the exception of the age of the bones, it is often difficult to distinguish the remains of "Ursus minimus" with those of modern Asiatic black bears.
Polar bear
The polar bear ("Ursus maritimus") is a hypercarnivorous bear whose native range lies largely within the Arctic Circle, encompassing the Arctic Ocean, its surrounding seas and surrounding land masses. It is a large bear, approximately the same size as the omnivorous Kodiak bear ("Ursus arctos middendorffi"). A boar (adult male) weighs around , while a sow (adult female) is about half that size. Although it is the sister species of the brown bear, it has evolved to occupy a narrower ecological niche, with many body characteristics adapted for cold temperatures, for moving across snow, ice and open water, and for hunting seals, which make up most of its diet. Although most polar bears are born on land, they spend most of their time on the sea ice. Their scientific name means "maritime bear" and derives from this fact. Polar bears hunt their preferred food of seals from the edge of sea ice, often living off fat reserves when no sea ice is present. Because of their dependence on the sea ice, polar bears are classified as marine mammals.