Right now, the leading explanation for how life rose up out of Earth’s primordial soup some 3.8 billion years ago is the ‘RNA world’ hypothesis, which proposes that RNA came first, and eventually created DNA, which went on to form complex life as we know it. But now a team of chemists from the Scripps Research Institute in California has found evidence that RNA wouldn’t have been able to sustainably give rise to DNA, leading them to suggest that the two molecules might have actually formed at the same time. “Even if you believe in a RNA-only world, you have to believe in something that existed with RNA to help it move forward,” said lead researcher Ramanarayanan Krishnamurthy. “Why not think of RNA and DNA rising together, rather than trying to convert RNA to DNA by means of some fantastic chemistry at a prebiotic stage?” If you need a bit of a refresher on the RNA world hypothesis, RNA (or ribonucleic acid) is widely known as the “older molecular cousin” of DNA (deoxyribonucleic acid). While they share a pretty similar structure – RNA looks like one side of DNA’s ladder – RNA is more brittle and less flexible than DNA, which is most likely why DNA ended up forming our genes. But it’s widely believed that RNA, despite its faults, came first, with many scientists suggesting that it was the first self-replicating molecule on Earth. The RNA world hypothesis states that RNA self-assembled from ancient Earth’s bubbling hot stew of particles, and went on to turn amino acids into proteins and enzymes. Eventually, those enzymes helped RNA to produce DNA, which led to complex life. That’s the short story anyway. Many researchers think that there would have also been cases where RNA nucleotides – the little building blocks of RNA – would have mixed with the backbone that forms DNA, creating mixed ‘chimera’ strands. Those chimeras would have been a crucial step in the transition from RNA to DNA, and it’s this step that the chemists behind the latest study have an issue with. In their research, they tested whether RNA and DNA could realistically share the same backbone, and showed that, when the two molecules are blended, they were highly unstable. “We were surprised to see a very deep drop in what we would call the ‘thermal stability’,” said Krishnamurthy. That means these chimeras in the RNA world would have most likely died off in favour of more stable RNA molecules, or failed to self-replicate, he explains. This isn’t the first time this has been demonstrated – Nobel laureate Jack Szostak from Harvard University has also shown a loss of function when RNA mixed with DNA. Even in today’s cells, if RNA nucleotides accidentally join a DNA strand, enzymes rush in to fix the mistake – and 3.8 billion years ago, RNA wouldn’t have had that self-correct mechanism. “The transition from RNA to DNA would not have been easy without mechanisms to keep them separate,” said Krishnamurthy. That evidence supports the idea that RNA and DNA actually arose at the same time, potentially from similar ingredients in Earth’s primordial soup, the researchers conclude in Angewandte Chemie. They’re not the first team to put forward this idea, but their findings provide new evidence to support this alternate hypothesis for the origins of life. If their findings are confirmed, it doesn’t necessarily mean that RNA didn’t give rise to DNA – but it’s likely that DNA evolved, at least in some primitive state, earlier than we’d predicted. Unfortunately, without a time machine, it’s unlikely we’ll ever really know what went on back at the dawn of life on Earth. But by trying to figure it out, we might have a better shot of predicting where we might find life elsewhere in the Universe.
Science Alert, 29 September 2016 ;http://www.sciencealert.com.au ;