The steroid hormones and their cellular receptors responsible for the sexual development and activity ofh umans and other vertebrates evolved in the late Precambrian—Cambrian and have remained unchanged since that time. Growth factors, which are small molecular weight proteins responsible for growth, sexual maturation and also for the cell cycle whereby cells replicate themselves evolved at around the same time. The molecular structures of hormones and growth factors in different vertebrates are essentially the same, so that a chemical capable of causing harmful effects in one species is likely to cause ill effects in other, even taxonomically distinct, species. The particular effects can vary because individual hormones and growth factors are associated with different functions in different species.