The selfish ribosome.

The ribosome is responsible for protein synthesis in all cells, and is the cell's largest energy consumer. We propose that the ribosome originated as a mutualistic symbiont of an RNA-dependent RNA polymerase ribozyme, supplying peptides that enhanced replication. As life transitioned from the RNA to the RNA-protein world, autonomous replicators became irreversibly addicted to the ribosome for producing replication proteins. Subsequent evolution is construed as a ribosomal takeover, whereby the ribosome evolved to consume most of the cell's resources, while other cellular componentry ensured the propagation of the ribosome, while being fully dependent on it. Under this perspective, the ribosome is a complex symbiont of the cell with pronounced selfish properties.