The European bison (Bison bonasus) or the European wood bison, also known as the wisent, or the zubr, is a European species of bison. It is one of two extant species of bison, alongside the American bison. The European bison is the heaviest wild land animal in Europe and individuals in the past may have been even larger than modern animals. During late antiquity and the Middle Ages, bison became extinct in much of Europe and Asia, surviving into the 20th century only in northern-central Europe and the northern Caucasus Mountains. During the early years of the 20th century bison were hunted to extinction in the wild. The species — now numbering several thousand and returned to the wild by captive breeding programmes — is no longer in immediate danger of extinction, but remains absent from most of its historical range. It is not to be confused with the aurochs (Bos primigenius), the extinct ancestor of domestic cattle, with which it once co-existed.