More than a dozen tobamoviruses are known. In nature, each species probably survives by moving between several closely related host species. Each infected plant contains a population of variants, but in most host populations the tobamovirus population is stable. The phylogenetic relationships of tobamovirus species broadly correlate with those of their angiosperm hosts. The simplest explanation for this correlation is that they have coevolved with the angiosperms, and hence, like them, are about 120-140 million years old. Gene sequence differences between species also indicate that the tobamoviruses are an ancient genus. Their gene sequences, and the protein motifs they encode, link them to tobraviruses, hordeiviruses and soil-borne wheat mosaic virus, more distantly to the tricornaviruses, and even to hepatitis virus E and other furoviruses, rubiviruses and alphaviruses. Their progenitors may have been associated with charophycean algae, and perhaps also plasmodiophoromycete fungi.