Life on Earth is built on Deoxyribonucleic Acid, more commonly known as DNA. But most scientists believe that Ribonucleic Acid, or RNA, actually came first. These days, RNA translates the instructions in DNA to produce the corresponding proteins that make life possible. But according to the so-called RNA World Hypothesis, it also provided the scaffolding that DNA itself would one day evolve from. Some kind of simple form of life may have used RNA alone to support and replicate itself until DNA took over.
But there's one big problem with that theory: The ingredients present on prebiotic Earth – Earth before life – don't mesh together to make RNA.
In a study published Monday in Nature Communications, researchers take a big step in proving this theory of the origin of everything. They've found convincing candidates for the nucleotides that may have formed the "N" in a pre-RNA polymer – something that looks and acts very much like RNA, but is built from materials readily available in our planet's prebiotic days.
And these molecules came together so easily, the researchers say, that they can imagine this first burst of life occurring spontaneously in a puddle.
The researchers found that two molecules known as barbituric acid and melamine readily combined with the sugar ribose (the "R" in RNA) and with each other when left alone in lukewarm water. They even bonded into structures like the ladder-rung pattern seen in RNA and DNA, forming long strands of polymers that turned their watery incubators into a jelly-like substance. The barbituric acid and melamine filled in for adenine and uracil, the molecules that combine with ribose to form RNA as we know it.
"It looks like these molecules could have been formed on early Earth, near each other," study co-author Nicholas Hud, a professor in the School of Chemistry and Biochemistry at the Georgia Institute of Technology, told The Washington Post. "These two bases that seem to be working really well have shown that they can form in the same prebiotic chemical reaction."
Portland University's Niles Lehman, who wasn't involved in the study, agreed that the ease with which the molecules meshed was worth celebrating.
"When you're trying to do prebiotic chemistry, you have to show that things are easy, because there were no chemists around to do it in those days," Lehman told The Post. "Twenty-four hours at 20 degrees celsius. . . that's pretty easy chemistry. It's pretty remarkable."
"The implications are that even if things weren’t perfect on earth, there were probably little pockets where this could have happened," Lehman explained.
The molecules seem to show that a pre-RNA could have arisen spontaneously in Earth's early days. But the study doesn't go so far as to prove that the molecules actually did come together in this way – or that they share an ancestral link with RNA, and by extension DNA.
"Here we have a conundrum," University of Missouri's Francis J. Schmidt, who wasn't involved in the Nature study, told The Post in an email. "If it’s easier to make these compounds from a prebiotic gemisch, what is the driving force for replacing them with the compounds, like RNA , that we have currently? In general, life is like the QWERTY keyboard: inefficient, but it works, and replacing such a system with a more logical one would cause more trouble than it’s worth (it’s easier to invent a spell-checker)."
In other words, we still don't have a convincing path from pre-RNA to RNA. We just know that this pre-RNA could have existed.
When it comes to 4-billion-year-old chemistry, we can't look back to the fossil record to confirm our theories – so scientists may never settle on a perfect timeline for the evolution of life.
But they could get a little closer.
"We could get to chemistry in the lab that allowed us to do a kind of artificial evolution of molecules," Hud said. "If we can get something that’s a good model system, it could show us how life could have gotten started. Maybe not exactly how it did, but at least how it could have."
In the meantime, these chemical processes could help produce new kinds of molecules for industrial applications. They could even help us find alien life by giving us new, non-DNA structures to keep an eye out for.
"These sorts of molecules have properties that we will want to look at when we search exoplanets for signs of life," Schmidt said. "We like to think that we are the center of the universe, and that if we find something out there it will be like us, sort of like the Klingons in 'Star Trek', or the menagerie in the taverns of 'Star Wars' movies, which wouldn’t present much of a challenge to a good zoologist. These molecules could be the basis for a different form of life, because the Laws of Chemistry hold everywhere, unlike Biology, which carries its history along with it."