Flaws in the RNA world

The hypothesis of an ‘RNA world’ as the font of all life on Earth has been with us now for more than 30 years, the term having been coined by the biologist Wally Gilbert in 1986. You could be forgiven for thinking that it pretty much solves the conundrum of how the replication of DNA could have avoided a chicken-and-egg impasse: DNA replication requires protein enzymes, but proteins must be encoded in DNA. The intermediary RNA breaks that cycle of dependence because it can both encode genetic information and act catalytically like enzymes. Catalytic RNAs, known as ribozymes, play several roles in cells.

It’s an alluring picture – catalytic RNAs appear by chance on the early Earth as molecular replicators that gradually evolve into complex molecules capable of encoding proteins, metabolic systems and ultimately DNA. But it’s almost certainly wrong. For even an RNA-based replication process needs energy: it can’t shelve metabolism until later. And although relatively simple self-copying ribozymes have been made, they typically work only if provided with just the right oligonucleotide components to work on. What’s more, sustained cycles of replication and proliferation require special conditions to ensure that RNA templates can be separated from copies made on them.