The whole DNA to mRNA to protein system is extremely complex. Going on the assumption that it didn't just spring forth on Earth fully completed (from aliens!), what sort of precursors would there be? DNA is very expensive to maintain in terms of energy, ribosomes need a lot of specialty proteins and RNA protein combos to work. Does anyone here know of any good examples of primitive forms of DNA use. Obviously viruses, and prokaryotes just have their dna floating around in the cytoplasm, but I'm more wondering about how the ribosome evolved.