When life emerged, it did so quickly. Fossils suggest microbes were present 3.7 billion years ago, just a few hundred million years after the 4.5-billion-year-old planet had cooled enough to support biochemistry, and many researchers think the hereditary material for these first organisms was RNA. Although not as complex as DNA, RNA would still be difficult to forge into the long strands needed to convey genetic information, raising the question of how it could have spontaneously formed.
Now, researchers may have an answer. In lab experiments, they show how rocks called basaltic glasses help individual RNA letters, known as nucleoside triphosphates, link into strands up to 200 letters long. The glasses would have been abundant in the fire and brimstone of early Earth; they are created when lava is quenched in air or water or when the melted rock created in asteroid strikes cools off rapidly.
The result has divided top origin-of-life researchers. “This seems to be a wonderful story that finally explains how the nucleoside triphosphates react with each other to give RNA strands,” says Thomas Carell, a chemist at Ludwig Maximilians University of Munich. But Jack Szostak, an RNA expert at Harvard University, says he won’t believe the result until the research team better characterizes the RNA strands.
Origin-of-life researchers are fond of a primordial “RNA world” because the molecule can carry out two distinct processes vital for life. Like DNA, it’s made up of four chemical letters that can carry genetic information. And like proteins, RNA can also catalyze chemical reactions needed for life.
But RNA also brings headaches. No one has found a set of plausible prebiotic conditions that would cause hundreds of RNA letters—each of them complex molecules—to link into strands long enough to support the complex chemistry needed to ignite evolution.
Stephen Mojzsis, a geologist at the University of Colorado, Boulder, wondered whether basaltic glasses played a role. They are rich in metals such as magnesium and iron that promote many chemical reactions. And, he says, “Basaltic glass was everywhere on Earth at the time.”
He sent samples of five different basalt glasses to the Foundation for Applied Molecular Evolution. There, Elisa Biondi, a molecular biologist, and her colleagues ground each sample into a fine powder, sterilized it, and mixed it with a solution of nucleoside triphosphates. Without a glass powder present, the RNA letters failed to link up. But when mixed with the glass powders, the molecules joined into long strands, some hundreds of letters long, the researchers report this week in Astrobiology. No heat or light was needed. “All we had to do was wait,” Biondi says. Small RNA strands formed after just a day, but strands kept growing for months. “The beauty of this model is its simplicity,” says Jan Špaček, a molecular biologist at Firebird Biomolecular Sciences. “Mix the ingredients, wait for a few days, and detect the RNA.”
Still, the results raise many questions. One is how the nucleoside triphosphates could have arisen in the first place. Biondi’s colleague Steven Benner says recent research shows how the same basaltic glasses could have promoted the formation and stabilization of the individual RNA letters.
A bigger issue, Szostak says, is the shape of the long RNA strands. In modern cells, enzymes ensure most RNAs grow into long linear chains. But RNA letters can also bind in complex branching patterns. Szostak wants the researchers to report the type of RNA the basaltic glasses created. “I find it very frustrating that the authors have made an interesting initial finding but then decided to go with the hype rather than the science,” Szostak says.
Biondi admits her team’s experiment almost certainly produces a small amount of RNA branching. However, she notes that some branched RNAs exist in organisms today, and related structures may have been present at life’s dawn. She also says other tests the group performed confirm the presence of long strands with connections that most likely mean they are linear. “It’s a healthy debate,” says Dieter Braun, an origin-of-life chemist at Ludwig Maximilian. “It will trigger the next round of experiments.”