Bufo bufo is part of a species complex, a group of closely related species which cannot be clearly demarcated. Several modern species are believed to form an ancient group of related taxa from preglacial times. These are the spiny toad (Bufo spinosus), the Caucasian toad (Bufo verrucosissimus) and the Japanese common toad (Bufo japonicus). The European common toad (Bufo bufo) seems to have arisen more recently. It is believed that the range of the ancestral form extended into Asia but that isolation between an eastern and western type occurred as a result of greater aridity and desertification in the Middle East during the Middle Miocene.The exact taxonomic relationships between these species remains unclear.