Ribozymes are certain types of naturally-occurring RNA molecules that can behave as enzymes, catalyzing their own assembly. Before their discovery it was assumed that all enzymes were proteins.
The discovery of ribozymes, by Thomas Cech at the University of Colorado and Sydney Altman at Yale University, provided an empirical basis for the concept of the RNA world. At first it seemed that ribozymes were limited in their enzymatic behavior to the catalysis of only the RNA sugar-phosphate backbone. However, in April 1995, Jack W. Szostak and Charles Wilson of the University of California announced1 that they had manufactured ribozymes capable of a broad class of catalytic reactions, including ones which promoted the formation of peptide bonds (needed to form proteins). Szostak's work was criticized on the basis that it would be unlikely for nature to select, from a primordial pool of trillions of different sequences of RNA, just those ones that gave rise to catalytically versatile ribozymes. However, the Colorado team countered by arguing that the ease with which these ribozymes "evolved" in the laboratory suggested that they were almost certainly part of a larger class of similar molecules that nature was capable of creating spontaneously given the right conditions.
1. Szostak, J. W., and Wilson, C. "In vitro Evolution of Self-Alkylating Ribozyme," Nature, 374, 777 (1995).