Illustration of early Earth. Credit: Harvard University.

Billions of years ago, from the fiery depths of a prehistoric Earth constantly bombarded by asteroids and engulfed in a haze of noxious gases, life found a way. Exactly how and under what conditions are some of the questions scientists have been trying to answer for the past century. Making life out of inanimate matter is not exactly like cooking lasagna, but with every attempt, researchers are coming closer to the winning recipe. For instance, a recent study found a new candidate ingredient for primordial life.

…and then just add water

Researchers who have made it their life’s work to unravel the origin of life tend to focus on RNA, a molecule which stores and transmits genetic information in addition to synthesizing protein. Many scientists regard RNA as the cornerstone of all life on Earth, which probably carried the first information required for creating life. To make RNA, however, you need the right nucleotides — the building blocks for both RNA and DNA.

RNA is made out of four nucleotides: adenine, guanine, cytosine, and uracil (A, G, C, and U). Previously, scientists were able to find precursors to C and U, but molecules that combine into A and G at conditions thought to occur more than four billion years ago have proven to be more elusive,

Researchers at Harvard University led by Jack W. Szostak, a professor of chemistry and chemical biology, have been following a different lead. Their research suggests that its possible that RNA could have started with a different set of nucleotide bases, using inosine instead of guanine. “Our study suggests that the earliest forms of life (with A, U, C, and I) may have arisen from a different set of nucleobases than those found in modern life (A, U, C, and G),” said Seohyun Kim, a graduate student at Szostak’s lab.

Like everybody else, initially, the researchers also tried to craft A and G in the lab. However, the purine-based nucleotides proved to be unstable. They then settled for two slightly modified versions of adenosine and inosine: 8-oxo-adenosine and 8-oxo-inosine.

The problem was that when the researchers assembled the nucleotides into RNA, the result wasn’t satisfying. The 8-oxo-based RNA still looked and behaved like RNA, but not nearly at the speed and accuracy needed to copy itself efficiently. But this ‘failure’ actually led to a happy accident.

When the researchers compared 8-oxo-inosine against a control, inosine, the latter enabled RNA to replicate with high speed and few errors. Writing in the Proceedings of the National Academy of Sciences, the authors note that inosine “turns out to exhibit reasonable rates and fidelities in RNA copying reactions. We propose that inosine could have served as a surrogate for guanosine in the early emergence of life.”

The findings are important for research focused on the origin of life based on RNA. In time, this work might help confirm the hypothesis or help scientists on other paths. Understanding how life assembled out of basic building blocks is important not only for our own history but also for our search of life on other planets.