Peptide-RNA Theory
A new theory says peptides also, and not just RNA, were responsible for catalyzing the first single-cell organisms on Earth. An illustration of primordial chemicals by Max Englund.

Move over, RNA. Proteins too were involved in the origin of life on Earth, a new theory on the subject claims, challenging the dominant “RNA-world” hypothesis which says only nucleic acids were the primary molecules responsible for kick-starting the process.

Two new papers by scientists from the University of North Carolina at Chapel Hill and the University of Auckland, New Zealand, present a “peptide-RNA” hypothesis which says small proteins and nucleic acids worked closely together for the first living cells on Earth to be born over four billion years ago, creating the first signs of biology.

“Until now, it has been thought to be impossible to conduct experiments to penetrate the origins of genetics. But we have now shown that experimental results mesh beautifully with the ‘peptide-RNA’ theory, and so these experiments provide quite compelling answers to what happened at the beginning of life on Earth,” the studies’ coauthor Charles Carter from UNC said in a statement Wednesday.

The experiments he referred to were studies of two enzyme superfamilies which found that “special attributes of the ancestral versions of these enzyme superfamilies” combined with a “self-reinforcing feedback system they would have formed with the first genes and proteins.” The earliest biology on Earth would have started as a result, and life-forms would eventually move toward increasing complexity and diversity.

To understand the theory, some terms must first be explained. Peptides are small protein molecules formed by the coming together of a small number of amino acids which are the building blocks of proteins. Proteins themselves are responsible for chemical reactions inside cells. RNA, like DNA, is a type of nucleic acid with an important role in coding, regulating, and expressing genes. Nucleotides are the building blocks of nucleic acids.

Soon after its formation when Earth was still a lifeless planet, there were only simple chemicals that are commonly found in space, and those can be thought of as a primordial chemical soup. According to the RNA-world theory, RNA somehow managed to raise itself out of that soup first which eventually led to the first peptides and then to the earliest cellular life-forms.

The two enzyme superfamilies, with 10 amino acids each, disrupt that theory, according to the researchers. These 20 amino acids are ancient and their remnants are present in all living cells even today, underscoring their importance. Called aminoacyl-tRNA synthetases (aaRSs), they link amino acids to RNA strings, playing an important role in the essential life process of converting genes into proteins.

“The enforcement of the relationship between genes and amino acids depends on aaRSs, which are themselves encoded by genes and made of amino acids. The aaRSs, in turn, depend on that same relationship. There is a basic reflexivity at work here. Theorist Douglas Hofstadter called it a ‘strange loop.’ We propose that this, too, played a crucial role in the self-organization of biology when life began on Earth. Hofstadter argued that reflexivity furnishes the force driving the growth of complexity,” Peter Wills, the other author of the studies, said in the statement.

Wills expressed hope their research will prompt others to consider their theory as a viable one to explain the origin of life, despite the prevalence of the RNA-world hypothesis: “Compared to the RNA-world hypothesis, what we’ve outlined is simply a much more probable scenario for the origin of life. We hope our data and the theory we’ve outlined in these papers will stimulate discussion and further research on questions relevant to the origins of life.”

“That theory is so alluring and expedient that most people just don’t think there’s an alternative. But we are very confident there is,” Carter added.

The papers titled “Interdependence, Reflexivity, Fidelity, Impedance Matching, and the Evolution of Genetic Coding” and “Insuperable problems of the genetic code initially emerging in an RNA world,” appeared in the journals Molecular Biology and Evolution and Biosystems, respectively.