There are clear similarities in the control mechanisms for cell survival and multiplication in the two eukaryotes, the ciliate Tetrahymena thermophila and the yeast, Saccharomyces cerevisiae. Cell multiplication in both organisms is activated by the same compounds (phorbol esters, diacylglycerol, tetrapyrroles, etc.). These compounds also affect cell multiplication and other activities in mammalian cell systems. This homology in control mechanisms in two distinct groups of unicellular eukaryotes on the one hand, and in cells from multicellular animals on the other, leads us to propose that these cytoplasmic control mechanisms for cell survival and multiplication originated in the unicellular eukaryotes.