Hammerhead ribozymes with self-cleaving properties have been found in a range of organisms, including plants, newts, schistosomes and cave crickets. In the November 1 Nature, Salahi-Ashtiani and Szostak of the Howard Hughes Medical Institute and Massachusetts General Hospital describe an in vitro system to address the origins of hammerhead ribozymes (Nature 2001, 413:82-84). They used a DNA collection encoding large random-sequence RNAs to select self-cleaving RNAs. Repeated rounds of selection lead to the artificial evolution of self-cleaving molecules. Of these, several have sequence features of hammerhead ribozymes. The authors suggest that the hammerhead ribozyme is the simplest RNA motif with self-cleaving abilities, which explains why it has been selected repeatedly during evolution.