Where life began? (Image: Oyvind Martinsen/Alamy)

IF YOU thought life evolved in bubbling hot springs, think again. Pieces of RNA have been made that can copy RNA strands longer than themselves, supporting the idea that the first life was based on self-replicating RNA, not DNA. What’s more, they work best in the cold, hinting that life began on ice.

RNA is a jack-of-all trades. Like DNA it can store genetic material, but it can also catalyse chemical reactions. For this reason, many believe it was the basis of the first life. If this was the case then those early organisms must have had an enzyme created out of RNA to copy their RNA genomes. But no known RNA enzyme can copy a stretch of RNA as long as itself, without which RNA organisms couldn’t have survived for long.

To find such an enzyme, Philipp Holliger of the MRC Laboratory of Molecular Biology in Cambridge, UK, has been creating libraries of RNA sequences and screening them for the ability to copy other RNA. In 2011, his team created an RNA enzyme that could copy RNA sequences up to 96 nucleotides long. They also found that such enzymes work better in the cold.


Their latest creation goes a step further. “It makes RNA big enough to encode itself,” says Holliger. “There’s no reason why self-replication couldn’t occur.” The RNA enzyme is 202 nucleotides long and makes RNA 206 nucleotides long, even at -17 °C (Nature Chemistry, doi.org/pcs).

Crucially, the enzyme does not yet copy itself. The main barrier seems to be the folded structure that allows it to copy other RNA. Enzymes that copy DNA have a similar issue: DNA is folded up, so they use tools to unzip it. Holliger hopes to add this function.

The RNA enzyme’s effectiveness at cold temperatures suggests ice was crucial to the first life. When a mix of RNA and metal ions freezes, growing ice crystals suck up the water, leaving tiny pockets of RNA and concentrated salt. RNA replication can happen in these pockets. “They’re a little bit like artificial cells,” says Holliger, and could be where evolution started.

“It certainly makes a cold RNA world something to think about,” says RNA expert Adrian Ferré-D’Amaré of the National Heart, Lung and Blood Institute in Bethesda, Maryland.

However, the theory has some weaknesses. At cold temperatures, RNA strands often stick together, making it tricky to separate them after the RNA has been copied. Primitive life would need to warm up to separate the strands, says Jack Szostak of Harvard Medical School. “It couldn’t just live at continuously cold temperatures.”

True, says Holliger, but there’s a fix. “Ice freezes and melts all the time, so you can easily see how an RNA replicator could be enclosed and then released in a cyclical way and allowed to spread.”

Szostak also points out that the enzyme only occasionally makes long strands of RNA. “I’m afraid we still have a long way to go to get a self-replicating ribozyme.”

This article appeared in print under the headline “A frozen cradle for Earth’s first life”