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An Extraterrestrial Spin on the RNA World

RNA World

As Evolution News has previously noted, a recent article in the Proceedings of the National Academy of Sciences presents an extraterrestrial spin on the standard RNA World origin-of-life proposal. The authors argue that RNA molecules appeared in warm little ponds (WLPs) more than 4.17 billion years ago, transported by “meteorites and interplanetary dust particles…to warm little ponds whose wet–dry cycles promoted rapid polymerization.”

The Abstract states:

Before the origin of simple cellular life, the building blocks of RNA (nucleotides) had to form and polymerize in favorable environments on early Earth. At this time, meteorites and interplanetary dust particles delivered organics such as nucleobases (the characteristic molecules of nucleotides) to warm little ponds whose wet–dry cycles promoted rapid polymerization. We build a comprehensive numerical model for the evolution of nucleobases in warm little ponds leading to the emergence of the first nucleotides and RNA. We couple Earth’s early evolution with complex prebiotic chemistry in these environments. We find that RNA polymers must have emerged very quickly after the deposition of meteorites (less than a few years). Their constituent nucleobases were primarily meteoritic in origin and not from interplanetary dust particles. Ponds appeared as continents rose out of the early global ocean, but this increasing availability of “targets” for meteorites was offset by declining meteorite bombardment rates. Moreover, the rapid losses of nucleobases to pond seepage during wet periods, and to UV photodissociation during dry periods, mean that the synthesis of nucleotides and their polymerization into RNA occurred in just one to a few wet–dry cycles. Under these conditions, RNA polymers likely appeared before 4.17 billion years ago.

RNA World advocates envision a process of pre-biotic natural selection beginning once a primitive RNA replicator — an RNA molecule capable of copying itself — arose on the early Earth. RNA World scenarios also favor the idea that the chemical evolution started with RNA molecules because RNA is capable of storing genetic information (like DNA) and catalyzing some important biochemical reactions (like proteins). The new PNAS model advocates this same basic approach but envisions the RNA molecules forming much earlier than other RNA World models do, in warm little ponds during the period of heavy meteorite bombardment ove 4 billion years ago.

Unfortunately, the PNAS model lacks credibility for most of the same reasons that other RNA World models do. In Signature in the Cell, I describe those several problems in detail. One that leaps to mind is the problem of the instability of RNA molecules and their constituent subunits (especially their nucleobases and sugars) — a fact the authors effectively acknowledge by insisting that these chemical subunits of RNA “must have” polymerized extremely rapidly to avoid dissolution. However, the new model seems even less plausible than other RNA models as an origin-of-life scenario because the frequent impact of meteorites in such an early epoch would have sterilized the surface of the Earth and vaporized the oceans.

Even if whole RNA molecules could polymerize under these conditions, the PNAS model does nothing to explain how the precise sequencing of bases — the genetic information — in the RNA molecule could have arisen. Yet, as I show in Signature in the Cell, we now know that precise RNA nucleotide base sequencing would be a precondition of any self-replicating RNA molecule. I note there that ribozyme engineering experiments have succeeded in producing an RNA molecule capable of copying a small portion of itself but only after the intelligent chemist or the “ribozyme engineer” arranges the RNA bases in very specific sequences — i.e., only after chemists provide the information necessary to achieve even that limited replicase function. Thus, RNA self-replication doesn’t explain the origin of the information necessary to getting natural selection going (let alone life). Instead, RNA self-replication depends upon preexisting unexplained sources of information.

In any case, the PNAS model fails to provide a plausible solution to an even more basic problem: the origin of the constituent subunits of the RNA molecules and the synthesis of the whole RNA molecules under realistic pre-biotic conditions.

The authors acknowledge that the nucleobases (adenine, cytosine, guanine, uracil, and thymine) essential to RNA and DNA could not have been easily produced on the early Earth. Therefore, they speculate that these organic molecules must have originated in outer space and then were transported to Earth via dust particles and meteorites. They explain that, “as to the sources of nucleobases, early Earth’s atmosphere was likely dominated by CO2, N2, SO2, and H2O. In such a weakly reducing atmosphere, Miller–Urey-type reactions are not very efficient at producing organics. One solution is that the nucleobases were delivered by interplanetary dust particles (IDPs) and meteorites.” They further speculate that small amounts of nucleobases (.25 to 515 parts per billion) from these meteorites would have dissolved into the warm little ponds. At the same time, ribose purportedly formed through the formose reaction and quickly combined with the nucleobases and phosphorous to form nucleotides. They then envision nucleotides combining into RNA chains through cycles of the ponds evaporating and then refilling with water. Their rationale: building blocks can only be produced in water, but the nucleotides can only form into long chains through cycles of dehydration. They acknowledge that the entire process had to take place within a few years — a geological instant — otherwise everything would have been eliminated by such forces as UV radiation, hydrolysis, and seepage.

In Signature in the Cell, I describe several factors that argue strongly against the formation of RNA in any realistic pre-biotic environment given the entire history of the Earth, let alone a few years.

First, nucleobases would have been highly unstable in the Earth’s early environment (even if trace amounts of these RNA subunits were transported from space on meteorites). As I note:

[T]he bases of RNA are unstable at temperatures required by currently popular high-temperature origin-of-life scenarios. The bases are subject to a chemical process known as “deamination,” in which they lose their essential amine groups (NH2). At 100 degrees C, adenine and guanine have chemical half-lives of only about one year; uracil has a half-life of twelve years; and cytosine a half-life of just nineteen days. (p. 302)

Second, the formation of ribose would have been next to impossible, particularly in the presence of the nucleobases:

The presence of the nitrogen-rich chemicals necessary for the production of nucleotide bases prevents the production of ribose sugars. Yet both ribose and the nucleotide bases are needed to build RNA. As Dean Kenyon explains, “The chemical conditions proposed for the prebiotic synthesis of purines and pyrimidines [the bases] are sharply incompatible with those proposed for the synthesis of ribose.” Or as Shapiro concludes: “The evidence that is currently available does not support the availability of ribose on the prebiotic earth, except perhaps for brief periods of time, in low concentration as part of a complex mixture, and under conditions unsuitable for nucleoside synthesis.” (p. 303)

Third, interfering cross reactions would have inhibited the synthesis of RNA molecules and further chemical evolution in a life-friendly direction:

[B]oth the constituent building blocks of RNA and whole RNA molecules would have reacted readily with the other chemicals present in the prebiotic ocean or environment. These “interfering cross-reactions” would have inhibited the assembly of RNA from its constituent monomers and inhibited any movement from RNA molecules toward more complex biochemistry, since the products of these reactions typically produce biologically inert (or irrelevant) substances. (p. 303)

To assess the plausibility of the RNA World scenario, I invite you to read an excerpt here from Chapter 14 of Signature in the Cell. This excerpt addresses five critical problems facing the RNA World hypothesis, including the implausibility of forming the chemical subunits of RNA and getting them to link together on the early Earth whether in warm little ponds or elsewhere. The excerpt addresses even more significant weaknesses of the — oddly — still popular RNA World. After you read them, you might want to go back and reread the Abstract of the PNAS article. Does the scenario it outlines still seem at all plausible?

Image credit: Frantisek_Krejci, via Pixabay.