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.
Brown bear
Brown bears seldom attack humans on sight and usually avoid people. In Russia, it is estimated that one in 1,000 on-foot encounters with brown bears results in an attack. They are, however, unpredictable in temperament, and may attack if they are surprised or feel threatened. Despite the low rate of attack, they appear to be the most dangerous northern carnivoran to humans, attacking more people on average annually than American black bears or northern populations of Asian black bears and Siberian tigers, cougars or gray wolves. Several large carnivorans from tropical regions of Africa and Asia may be more dangerous to humans than brown bears, including other ursids such as the sloth bear and Indian populations of the Asian black bear. Each of the latter species of bear may kill up to 12 people annually in certain Indian districts, about as many in a relatively small area as all of the world's brown bears combined. The high levels of human-directed aggression in these ursine bears is reportedly their evolution with once-large numbers of predatory Bengal tigers (arguably itself the world's most dangerous carnivoran to humans even with its extreme decline in modern times, being statistically much more likely to attack than their northern Siberian cousins) and a very large human population, increasing the risk of surprising and angering a defensive Asian black or sloth bear, the latter species often charging aggressively rather than fleeing if a surprise encounter occurs, as do most other bear species.