When life emerged, it did so very 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 cooled enough to support biochemistry, and many researchers believe that the hereditary material of these early organisms was RNA. Although not as complex as DNA, RNA would still be difficult to forge into the long strands needed to transmit genetic information, raising the question of how it could have formed spontaneously.
Now researchers may have an answer. In lab experiments, they show how rocks called basalt glasses help individual RNA letters, known as nucleoside triphosphates, bind together into strands up to 200 letters long. Glasses would have been abundant in the fire and brimstone of the early Earth; they are created when lava is quenched in air or water or when molten rock created during asteroid strikes cools rapidly.
The result has divided top researchers on the origin of life. “It seems like a wonderful story that finally explains how nucleoside triphosphates react with each other to form strands of RNA,” says Thomas Carell, a chemist at Ludwig Maximilians University in 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 perform two distinct processes vital to life. Like DNA, it is made up of four chemical letters that can carry genetic information. And like proteins, RNA can also catalyze the chemical reactions necessary for life.
But RNA also brings headaches. No one has found a set of plausible prebiotic conditions that would cause hundreds of letters of RNA – each of them complex molecules – to bind together into strands long enough to support the complex chemistry needed to kick-start evolution.
Stephen Mojzsis, a geologist at the University of Colorado at Boulder, wondered if basalt glasses played a role. They are rich in metals like magnesium and iron which promote many chemical reactions. And, he says, “basalt glass was everywhere on Earth back then.”
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 the presence of glass powder, the RNA letters failed to bind. But when mixed with the glass powders, the molecules come together in long strands, a few hundred letters long, researchers report this week in Astrobiology. No heat or light was needed. “All we had to do was wait,” says Biondi. Small strands of RNA formed after just one day, but the strands continued to grow for months. “The beauty of this model is its simplicity,” says Jan Špaček, a molecular biologist at Firebird Biomolecular Sciences. “Mix the ingredients, wait a few days and detect the RNA.”
However, the results raise many questions. One is how the nucleoside triphosphates could have appeared in the first place. Biondi’s colleague, Steven Benner, says recent research shows how the same basalt glasses could have promoted the formation and stabilization of individual RNA letters.
A bigger problem, Szostak says, is the shape of the long strands of RNA. In modern cells, enzymes ensure the growth of most RNAs in long linear chains. But RNA letters can also link in complex branching patterns. Szostak wants researchers to report the type of RNA created by basalt glasses. “I find it very frustrating that the authors made an interesting initial discovery, but then decided to follow the hype rather than the science,” says Szostak.
Biondi admits his team’s experiment almost certainly produces a small amount of RNA branching. However, she notes that some branched RNAs exist in organisms today and that related structures may have been present at the dawn of life. She also says that other tests performed by the group 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. “That will trigger the next set of experiments.”