RNA has long been thought to be a key molecule in the primordial soup that was Earth a few billion years ago, because it can not only store genetic information but also act as an enzyme—two key functions needed for the development of life as we know it. But whether RNA-based life really existed, what it looked like, and how it evolved into the DNA-, RNA-, and protein-based organisms of today have remained open questions.
Now, a study published yesterday (May 11) in Nature points to the possibility that RNAs may have played a role in building early proteins by simply linking amino acids together. Thomas Carell, an organic chemist at Ludwig Maximilian University of Munich in Germany, and his colleagues created synthetic RNA molecules that could produce peptides up to 15 amino acids long.
The discovery “opens up vast and fundamentally new avenues of pursuit for early chemical evolution,” Bill Martin, who studies molecular evolution at Heinrich Heine University Düsseldorf in Germany but was not involved in the study, tells Nature. Georgia Tech biophysical chemist Loren Williams, who also did not participate in the research, adds: “If the origins of RNA and the origins of protein are linked, and their emergence is not independent, then the math shifts radically in favour of an RNA–protein world and away from an RNA world.”
See “RNA World 2.0”
The researchers engineered the RNA molecules to include two modified nucleosides that are found in ribosomes, enzymes made up of RNAs and proteins that synthesize proteins from RNA transcripts. According to Nature, scientists have considered these unusual, or noncanonical, bases to be traces of the components of the ancient primordial broth from which life emerged. The fact that RNAs containing a couple of these nucleosides were able to bind amino acids and string them together to build peptides “is a very exciting finding,” Martin tells Nature, “not only because it maps out a new route to RNA-based peptide formation, but because it also uncovers new evolutionary significance to the naturally occurring modified bases of RNA.”
The idea that RNAs and peptides could have interacted in the early days of life on Earth provides one way that life could have initially evolved without ribosomes. That’s useful because ribosomes are complex structures whose origin is largely unclear, Claudia Bonfio, an origin of life chemist at the University of Strasbourg who was not involved in the work, tells Science. “It’s an important advance.”
Still, many questions remain, including when and how ancient RNA molecules may have started to encode the peptides they help form. Carell tells Nature that it’s possible certain RNAs fold in such a way that they bind particular amino acids in a rudimentary form of genetic coding. Plus, he adds, if these RNA–peptide hybrid molecules could replicate themselves, they could have evolved to become more efficient. “If the molecule can replicate, you have something like a mini organism.”