Evidence for the RNA world is looking a bit muddier (Image: Frans Lanting/Getty)
WITHIN every living cell lurks a relic from the earliest stages of life on Earth, like a flint spear point in a modern military arsenal. Its discovery is overturning our understanding of how we came to be – and could signal the end of the favoured explanation of the origin of life, “RNA world”.
Life has a chicken-and-egg problem: enzymes are needed to make nucleic acids – the genetic material – but to build them you need the genetic information contained in nucleic acids. So most researchers assume that the earliest life, long before the evolution of cells, consisted of RNA molecules. These contain genetic information but can also fold into complex shapes, so could serve as enzymes to help make more RNA in their own image – enabling Darwinian evolution on a molecular level.
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At some point, the idea goes, this RNA world ended when life outsourced enzymatic functions to proteins, which are more versatile. The key step in this switch was the evolution of the ribosome, a structure that builds protein molecules from genetic blueprints held in RNA.
But such a transition would require abandoning the enzymatic functions of RNA and reinventing them in proteins. “That is not a simple model,” says , a biochemist at the Georgia Institute of Technology in Atlanta.
Williams has further reason to question the RNA world. His detailed study of the ribosome shows that its most ancient part, which is identical in every living thing, acts as an enzyme to link amino acids in a growing protein chain. But this ribosomal core works pretty badly, Williams told the in Chicago last week, and so is unlikely to be the product of a long period of evolution by natural selection.
It has none of the precision of protein enzymes, which have precise three-dimensional shapes and positive and negative charges positioned just right for stabilising the intermediate stage of a specific chemical reaction.
Instead, the ribosomal core is essentially a box in which any right-sized ingredients can be held and shaken until something happens, says Williams. Rather than natural selection, he thinks it results from more primitive chemical evolution, in which some compounds become more common, but there is no heredity so the next generation doesn’t look like the one before, and the most successful solutions aren’t necessarily the ones that survive.
How would this have worked? , also at Georgia Tech, has an idea. Modern proteins and RNA don’t assemble on their own, but Hud has found relatives of each that do, spontaneously forming protein-like molecules up to 14 amino acids long as well as gene-like rows of proto-RNA, he reported at the same conference.
In Williams’s scenario, these crude proto-proteins and proto-RNA would have happened to form a ribosomal core, which helped accelerate the random assembly of more protein and RNA molecules (see diagram). “This is the origin of life. I think this came before replication,” says Williams.
Ancient biology
Any particular proteins or RNA molecules that happened to stabilise one another would accumulate, making the molecular mix increasingly predictable over time. At some point, the proto-RNA and proto-proteins could have started cooperating as crude enzymes to help replicate the RNA sequence, sparking the transition to true Darwinian evolution.
If Williams is right, RNA has cooperated with proteins from the very beginning. “This marriage between RNA and protein is the most ancient and fundamental part of biology,” he says. If so, those who are trying to build self-replicating RNA molecules in the lab are looking for something that never existed.
“This is the origin of life. The marriage between RNA and protein is the oldest part of biology”
Not everyone agrees with this idea, however. “There are other reasons to expect a complex RNA world,” says at the Foundation for Applied Molecular Evolution in Florida. Many modern protein enzymes, for example, have RNA associated with them that serves no functional role. This implies that they must be relics from a time when RNA was the only thing around, he says.
Williams is now synthesising what he thinks is the most ancient ribosomal core, free of all its later add-ons, to test whether it really assembles RNA and protein molecules.
The testability that Williams and Hud bring to these questions is what sets their work apart, says of the University of Canterbury, New Zealand. “We might have thought about it, but we didn’t have a way to test it. Now they’ve done that,” he says.
This article appeared in print under the headline “RNA world may never have existed”
