They think they might have discovered a way:
“Years ago, the naive idea that pools of pure concentrated ribonucleotides might be present on the primitive Earth was mocked by Leslie Orgel as ‘the Molecular Biologist’s Dream,'” said Jack Szostak, a Nobel Prize Laureate, professor of chemistry and chemical biology and genetics at Harvard University, and an investigator at the Howard Hughes Medical Institute. “But how relatively modern homogeneous RNA could emerge from a heterogeneous mixture of different starting materials was unknown.”
In a paper published in the Journal of the American Chemical Society, Szostak and colleagues present a new model for how RNA could have emerged. Instead of a clean path, he and his team propose a Frankenstein-like beginning, with RNA growing out of a mixture of nucleotides with similar chemical structures: arabino- deoxy- and ribonucleotides (ANA, DNA, and RNA).
In the Earth’s chemical melting pot, it’s unlikely that a perfect version of RNA formed automatically. It’s far more likely that many versions of nucleotides merged to form patchwork molecules with bits of both modern RNA and DNA, as well as largely defunct genetic molecules, such as ANA. These chimeras, like the monstrous hybrid lion, eagle and serpent creatures of Greek mythology, may have been the first steps toward today’s RNA and DNA.
Caitlin McDermott-Murphy/Harvard University, “First building blocks of life on Earth may have been messier than previously thought” at ScienceDaily
The paper is: Seohyun Chris Kim, Lijun Zhou, Wen Zhang, Derek K. O’Flaherty, Valeria Rondo-Brovetto, Jack W. Szostak. A Model for the Emergence of RNA from a Prebiotically Plausible Mixture of Ribonucleotides, Arabinonucleotides, and 2 -Deoxynucleotides. Journal of the American Chemical Society, 2020; DOI: 10.1021/jacs.9b11239 (paywall)
From the paywalled paper, via a friend:
Multiple distinct processes, in addition to primer extension and ligation, are likely to have contributed to the transition from heterogeneous primordial nucleic acids to the relatively homogeneous RNA genomes of the first cells. For example, photochemical reactions preferentially degrade the noncanonical nucleobases (and the corresponding nucleosides and nucleotides) and also preferentially degrade the alphaanomeric byproducts of nucleotide synthesis. Steric constraints and variations in chemical reactivity may have influenced the composition of the first oligonucleotides formed through nontemplated polymerization; for example, nucleotides with acyclic sugars rapidly cyclize to unreactive products following phosphate activation. The synthesis of standard 3 −5 phosphodiester bonds when copying templates with 2 −5 linkages or 3 −5 pyrophosphate linkages may also have contributed to the gradual elimination of the variability in nucleic acid structure that is the inevitable consequence of nonenzymatic polymerization. It has also been shown that while mixed RNA/DNA oligonucleotides can exhibit functional properties such as molecular recognition, the homogeneous systems exhibit better function and, thus, the emergence of homogeneous genetic polymers could be the result of selective pressures for superior function. Taken together these and other mechanisms may explain the transition from a heterogeneous mixture of prebiotically synthesized nucleotides and oligonucleotides to a relatively homogeneous set of RNAs. Considerable additional experimental work must be done to extend this model, as only a fraction of the likely prebiotic variability in nucleotide and nucleic acid structure has been explored to date.
Friends doubt that the random polymerizing of nucleotides is going to explain the origin of information needed for “RNA genomes” to come into existence.
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See also: RNA more flexible than thought but also more error-prone
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