The Origins of Life ~ Pure RNA world?

Summary

Most scientists believe that ribonucleic acid played a key role in the origin of life on Earth, but the versatile molecule isn’t the whole story.

The ubiquity and diverse functionality of ribonucleic acid (RNA) in today’s world suggest that the information polymer could well have been the leading player early on in the establishment of life on Earth, and, in theory, it’s a logical basis for primitive life. One can readily imagine that RNA, as a catalytic molecule capable of serving as a template for its own replication, might have reproduced itself and grown exponentially in the primordial environment. Perhaps such an RNA-based proto–life-form even replicated with an appropriate level of fidelity to allow natural selection to begin directing its evolution.

But there’s a snag: The odds of suddenly having a self-replicating RNA pop out of a prebiotic soup are vanishingly low.”

For decades, researchers from diverse fields have theorized—and argued—about how early life might have begun, and about what sparked the 3.5 billion years of evolution that led to the plethora of cell-based life that occupies almost every nook and cranny of modern Earth. Different camps emerged. So-called “metabolism first” researchers focus on understanding chemical cycles that may have materialized in a prebiotic environment and could have led to the synthesis of nucleotides and other organic molecules. Those subscribing to the theory of “genetics first” want to identify the first information molecule and understand how it arose, replicated, and evolved.

The RNA world, first posited by Francis Crick and others in the late 1960s, remains an attractive hypothesis. Many of the chemical hurdles that once challenged the laboratory synthesis of the molecule under presumed primordial conditions are being overcome, and in vitro evolution experiments are yielding RNA molecules that perform numerous functions, including copying themselves or other RNAs. “I don’t think there can be much doubt that RNA was a major central player as both a catalyst and an early replicator,” says Nick Lane, a biochemist at the University College London whose research falls under the “metabolism first” label. “So the RNA world is absolutely correct, as far as I’m concerned, in that.”

But the notion that RNA, on its own, spontaneously assembled and evolved on early Earth has fallen out of favor. More likely, whatever conditions spawned compounds as complex as nucleotides also generated other organics, perhaps early forms of modern amino acids and fatty acids, the constituent parts of proteins and membranes. “I’m not sure how many people anymore believe in a pure RNA world. I certainly don’t,” says Lane. “I think the field has drifted away from that, and there’s now an acknowledgment it had to be ‘dirty.’ ”

“I think most people would argue that there’s . . . more than just RNA,” agrees Matthew Powner, a “genetics first” origins-of-life researcher, also at University College London. (Suggested further reading “Matthew Powner: Origin Solver,” The Scientist, March 2014.) “People have relaxed their opinions of the RNA world . . . from its original inception where RNA was fundamental to all parts of biology in the earliest form of life.”

References

F.H.C. Crick, “The origin of the genetic code,” J Mol Biol, 38:367-79, 1968.

M.W. Powner et al., “Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions,” Nature, 459:239-42, 2009.

#originsoflife #pathway #protein #RNAworld