How life on Earth first started is a mystery mankind has grappled with for thousands of years. Scientists generally agree that all life must have evolved from single-celled organisms that first emerged around 3.8 billion years ago. But how this first form of life emerged is subject of much debate.
In a study published in Nature Communications Friday, scientists from the U.S. and the U.K. have proposed a new theory about how a group of molecules essential to all life on Earth could have formed on our primordial planet.
The researchers focus on a group of molecules called nucleotides. These form the building blocks of DNA and RNA (a nucleic acid that carries instructions from DNA), which are essential to all life on Earth.
In the paper, the scientists look at purines and pyrimidines—the two nitrogenous bases of nucleotides. Previously, it was thought that purines and pyrimidines must have emerged as a result of separate processes. This is because they can only form in completely incompatible conditions—recent research “uniformly fails to account for simultaneous access to both pyrimidine and purine,” the scientists write. “Remarkably, all proposed prebiotic synthesis of pyrimidine and purine nucleotides have yielded either pyrimidines or purines separately, but never (yet) both by the same strategy.”
However, the researchers have now shown how these two nucleotides could have formed by the same process. "We provide a new perspective on how the original RNA molecules were made and suggest a simple chemical solution for delivering both purine and pyrimidine nucleotides at the origins of life," study author Matthew Powner said in a statement.
The scientists show how purines and pyrimidines can form from a common precursor molecule that could have existed before life on Earth began. They find 8-oxo-purine shares underlying similarities with pyrimidine nucleotides. These two nucleoids can form under the same chemical conditions on a sugar scaffold, then bind together through molecular interactions—eventually resulting in the molecules that go on to become RNA.
The process identified “may have played a key role in in primordial nucleic acids prior to the emergence of canonical nucleotides of biology [the base of DNA],” they conclude.
Powner said: "RNA is the cornerstone of all life on Earth and probably carried the first information at the outset of life, but making RNA requires both purine and pyrimidine nucleotides to be simultaneously available. A solution to this problem has remained elusive for more than 50 years.”
Shaun Stairs, first author of the study, adds: "The mechanism we've reported gives both classes of molecule the same stereochemistry that is universally found in the sugar scaffold of biological nucleic acids, suggesting that 8-oxo-purine ribonucleotides may have played a key role in primordial nucleic acids.”