In mammals the structure of pituitary GH is

generally strongly conserved, indicating a slow basal

rate of molecular evolution. However, on two

occasions, during the evolution of primates and of

artiodactyls, the rate of evolution has increased

dramatically (25- to 50-fold) so that the sequences

of human and ruminant GHs diVer markedly from

those of other mammalian GHs.