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Evolution

Calibrations using fossils of 16 Bovidae species indicate that the Bovini tribe evolved about 11.7 million years ago. The Bos and Bison genetic lineages are estimated to have genetically diverged from the Bovini about 2.5 to 1.65 million years ago. The following cladogram shows the phylogenetic relationships of the aurochs based on analysis of nuclear and mitochondrial genomes in the Bovini tribe:

The cold Pliocene climate caused an extension of open grassland, which enabled the evolution of large grazers. Bos acutifrons is a possible ancestor of the aurochs, of which a fossil skull was excavated in the Sivalik Hills in India that dates to the Early Pleistocene about 2 million years ago.

An aurochs skull excavated in Tunisia's Kef Governorate from early Middle Pleistocene strata dating about 0.78 million years ago is the oldest-known fossil specimen to date, indicating that the genus Bos might have evolved in Africa and migrated to Eurasia during the Middle Pleistocene. Middle Pleistocene aurochs fossils were also excavated in a Saharan erg in the Hoggar Mountains.

The earliest aurochs fossils excavated in Europe date to the Holstein interglacial 230,000 years Before Present (BP). A mitochondrial DNA analysis showed that hybridisation between the aurochs and the steppe bison (Bos priscus) occurred about 120,000 years ago; the European bison (Bos bonasus) contains up to 10% aurochs ancestry.

Fossils of the Indian subspecies (Bos primigenius namadicus) were excavated in alluvial deposits in South India dating to the Middle Pleistocene. This lead Van Vuure to conclude that it possibly migrated west into the Middle East during the Middle or Late Pleistocene.

Late Pleistocene aurochs fossils were found in Affad 23 in Sudan dating to 50,000 years ago when the climate in this region was more humid than during the African humid period. Two aurochs bones found in the Romito Cave in Italy were radiocarbon dated to 20,210 and 19,351 years BP. Aurochs bones found in a cave near San Teodoro, Sicily date to the Late Epigravettian 14,785–14,781 years BP.

Fossils found at various locations in Denmark date to the Holocene 9,925–2,865 years BP. Mesowear analysis of aurochs premolar teeth indicates that it changed from an abrasion -dominated grazer in the Danish Preboreal to a mixed feeder in the Boreal, Atlantic and Subboreal periods of the Holocene.