Until the early 1980s, it was thought that all enzymes were proteins. Indeed, that statement was regarded as a fundamental truth of cellular biology and was found in every textbook. Cell biologists became convinced that all enzymes were proteins because every enzyme isolated in the 55 years following Sumner’s purification of urease in 1926 turned out to be a protein. But biology is full of surprises, and this statement has been revised to include RNA catalysts called ribozymes. In fact, many scientists now believe that the earliest enzymes were molecules of catalytic, self-replicating RNA and that these molecules were present in primitive cells even before the existence of DNA.