The bovine tribe (Bovini) split about 5 to 10 million years ago into the buffalos (
Bubalus and
Syncerus) and a group leading to bison and
taurine cattle. Genetic evidence from
nuclear DNA indicates that the closest living relatives of bison are
yaks, with bison being nested within the genus
Bos, rendering
Bos without including bison
paraphyletic. While nuclear DNA indicates that both extant bison species are each other's closest living relatives, the
mitochondrial DNA of European bison is more closely related to that of domestic cattle and
aurochs (while the mitochondrial DNA of American bison is closely related to that of yaks). This discrepancy is either suggested to be the result of
incomplete lineage sorting or ancient
introgression. Bison are widely believed to have evolved from a lineage belonging to the extinct genus
Leptobos during the
Late Pliocene to
Early Pleistocene in Asia. The earliest members of the bison lineage, known from the Late Pliocene to Early Pleistocene of the Indian Subcontinent (
Bison sivalensis) and China (
Bison palaeosinensis), approximately 3.4-2.6 million years ago (Ma) are placed in the subgenus
Bison (
Eobison)
. The oldest remains of
Eobison in Europe are those
Bison georgicus found in
Dmanisi, Georgia, dated to around 1.76 Ma. More
derived members of the genus are placed in the subgenus
Bison (
Bison), which first appeared towards the end of the Early Pleistocene, around 1.2 Ma, with early members of the subgenus including the widespread
Bison schoetensacki. The
steppe bison (
Bison priscus) first appeared during the mid-
Middle Pleistocene in eastern Eurasia, and subsequently became widely distributed across Eurasia. Modern American bison are thought to have evolved from
B. antiquus during the Late Pleistocene-Holocene transition via the intermediate form
Bison occidentalis. The European bison,
Bison bonasus, first appeared in Europe during the late Middle Pleistocene, where it existed in
sympatry with the steppe bison. Its relationship with other extinct bison species is unclear, though it appears to be only distantly related to the steppe and American bisons, with possibly some interbreeding between the two lineages during the Middle Pleistocene. Prior to the late 19th century, the population of American bison likely numbered in the tens of millions, perhaps as many as 60 million. During the population bottleneck caused by the great slaughter of American bison during the 19th century, the number of bison remaining alive in North America declined to as low as 541. During that period, a handful of ranchers gathered remnants of the existing herds to save the species from extinction. These ranchers bred some of the bison with cattle in an effort to produce "cattleo" Herds of importance are found in
Yellowstone National Park,
Wind Cave National Park in
South Dakota,
Blue Mounds State Park in
Minnesota,
Elk Island National Park in
Alberta, and
Grasslands National Park in
Saskatchewan. In 2015, a purebred herd of 350 individuals was identified on public lands in the
Henry Mountains of southern
Utah via
genetic testing of mitochondrial and nuclear DNA. This study, published in 2015, also showed the
Henry Mountains bison herd to be free of
brucellosis, a bacterial disease that was imported with non-native domestic cattle to North America. In 2021, the
American Society of Mammalogists considered
Bison to be a subgenus, and placed both bison species back into
Bos. Relationships of bovines based on nuclear DNA, after Sinding, et al. 2021. }} }}}}}}}} ==Behavior==