Life From an RNA World: The Ancestor Within
- Michael Yarus
The RNA-world hypothesis proposes that today's DNA-based life forms evolved from earlier ones that were based on much simpler RNA molecules. Although no such RNA-based organism, or ribocyte, has yet been found, biochemist Michael Yarus marshals the theoretical considerations and lab experiments that lend support to this notion for the origin of life.
Life From an RNA World is an unconventional book about RNA. Rather than opening with the central dogma and attendant teachings on molecular biology, Yarus uses evolution as a gateway. He then takes us on a journey through evolutionary time, concentrating on the roles of the various forms of RNA. Although the book reads like a collection of philosophical essays, its author is a proficient guide.
Yarus first describes the basis of Darwinian evolution, in which the “secret is selection”. He explains how differences in the sequence of the RNA found in an organism's ribosomes — part of the protein-making machinery of the cell — are used to classify it into one of the three kingdoms of life. He introduces LUCA, the Last Universal Common Ancestor, “behind or before which lies the RNA world”.
Various hypotheses about the origin of life are related in the book. One such is an idea that was independently reached by British geneticist J. B. S. Haldane and Russian biochemist Alexander Oparin. They suggested that the chemically reducing environment of the primitive Earth plus energy from ultraviolet light, lightning or other sources could generate several organic molecules. Chemists Stanley Miller and Harold Urey tested this at the University of Chicago in 1952 in their 'primordial soup experiments' that created amino acids from methane, ammonia, hydrogen and water when the mixture was exposed to an electrical discharge.
The writing picks up when Yarus lays out the talents of RNA. As a molecule that is stable, single-stranded and able to form complex structures, it can assume many roles. Ribosomal and transfer RNAs provide the scaffolding to help make proteins and translate the information held in messenger RNA, which itself transcribes the information coded in DNA. The ribonucleotide building blocks that make up RNA are essential for enzyme function, and there are RNAs that have their own catalytic activity, known as ribozymes. Discovered by US molecular biologist Thomas Cech, these can cut themselves out from larger molecules. Other recent discoveries have revealed the existence of small RNAs, including microRNAs, that are intimately involved in controlling gene expression and translating messenger RNA. But the fact that RNA can adopt this vast catalogue of forms is insufficient evidence for a precursor RNA world.
More compelling is the ability of RNA to evolve under selection pressure, as demonstrated in the elegant SELEX experiments done in Larry Gold's lab at the University of Colorado, Boulder. This evolutionary adaptability may be why it is the nucleic acid of choice for the genome of some of the most difficult and changeable pathogens, such as the influenza viruses. It is a key part of the argument that it was RNA that generated subsequent life forms, and that RNAs were a primitive system for making short chains of amino acids — a system that evolved to produce the protein-based structural and metabolic machinery found in organisms today.
Further proof for the primitive RNA world could come from next-generation sequencing platforms that allow deep sampling of nucleic-acid populations from microorganisms in exotic locations, such as in deep-sea volcanic vents. But at present, the RNA world remains conjecture, based on powerful observations that this book captures.
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Frontiers in Genetics (2019)