The problem is as follows: the ribosome is a specialized macromolecular complex with the specific function of translating messenger RNA into proteins. So how would it have evolved prior to proteins in an RNA world (or even a metabolite world)?
Selection pressure is lacking. There are no proteins that need to be made.
So the thought is, it must have been doing something else. What might that else be? And can we glean that from its current form?
My introduction to this fascinating question came from my reading of Eric Smith and Harold Morowitz’s 2013 book, The Origin of Life on Earth: The Emergence of the Fourth Geosphere. My copy of which, is currently at my university office–probably the one book I am really missing here at home.