If Thomas Carell is right, around 4 billion years ago, much of Earth might have been blanketed with a greyish-brown kind of mineral. This was no ordinary rock, however: it consisted of crystals of the organic molecules that scientists now call A, U, C and G. And some of these, the theory goes, would later serve as the building blocks of RNA, the evolutionary engine of the first living organisms, before DNA existed.
Carell, an organic chemist, and his collaborators have now demonstrated a chemical pathway that—in principle—could have made A, U, C and G (adenine, uracil, cytosine and guanine, respectively) from basic ingredients such as water and nitrogen under conditions that would have been plausible on the early Earth. The reactions produce so much of these nucleobases that, millennium after millennium, they could have accumulated in thick crusts, Carell says. His team describes the results in Science on October 3.
The results add credence to the ‘RNA world’ hypothesis, says Carell, who is at the Ludwig Maximilian University of Munich in Germany. This idea suggests that life arose from self-replicating, RNA-based genes—and that only later did organisms develop the ability to store genetic information in the molecule’s close relative, DNA. The chemistry is also a “strong indication” that the appearance of RNA-based life was not an exceedingly lucky event, but one that is likely to happen on many other planets, he adds.