As the name suggests, these entities are complexes of RNA and protein. This might sound a little bizarre but such hybrid catalysts have proven to be essential for the cell. Ribosomes, telomerase, SRP, splicing - all fundamental and core components of the system.

The distribution of RNA among the protein machines has been interpreted as the surviving relics or fossils of an ancient RNA world. That is, a pre-protein era of life where RNA fullfilled the function catalysis and heredity. That the fundamental process of translation is almost entirely RNA based ('the ribosome is a ribozyme') is very suggestive.

That is not to say, however, that evolution might not reuse RNA for its older, catalytic, abilities. After all, mutation changes genes all the time. This necessarily means that new mRNAs (messenger RNAs) are also produced. Perhaps conditions in a cell are no longer condusive to ribozyme evolution, and this is why almost all catalytic RNA is now wedded to a protein partner.