Initially, cattle were identified as three separate species: Bos taurus (European cattle), Bos indicus (zebu), and the extinct Bos primigenius (aurochs). Over time, scientists experienced differences of opinion. They consider Bos taurus and Bos indicus (both of which are currently domestic cattle) to be subspecies of the same species. Some scientists later argued that aurochs and domestic cattle were still one species. The Integrated Taxonomic Information System (ITIS) classifies cattle in the Bos taurus species consisting of three subspecies: Bos taurus primigenius, Bos taurus Taurus, and Bos taurus indicus.