In 1987 (I believe it was) it was determined that rainbow trout (including steelhead) as well as cutthroat, had more physical characteristics in common with the genus Oncorhynchus, which includes the Pacific salmons, than with other members of the genus Salmo in which they were formerly included (the genus Salmo includes the Atlantic salmon, S. salar, and the brown trout, S. trutta). The rainbow (formerly S. gairdneri, the change of the specific name from gairdneri to mykiss having to do with priority of identification) became O. mykiss, and the cutthroat (formerly S. clarki) became O. clarki. It is presumed that the two genera (Oncorhynchus and Salmo) began to evolve from a common ancestor over 3 to 5 million years ago.