The spark of life, the authors say, was the creation of RNA polymers: the essential components of nucleotides, delivered by meteorites, reaching sufficient concentrations in pond water and bonding together as water levels fell and rose through cycles of precipitation, evaporation and drainage. The combination of wet and dry conditions was necessary for bonding, the paper says.
In some cases, the researchers believe, favorable conditions saw some of those chains fold over and spontaneously replicate themselves by drawing other nucleotides from their environment, fulfilling one condition for the definition of life. Those polymers were imperfect, capable of improving through Darwinian evolution, fulfilling the other condition.
“That’s the Holy Grail of experimental origins-of-life chemistry,” says Pearce.
That rudimentary form of life would give rise to the eventual development of DNA, the genetic blueprint of higher forms of life, which would evolve much later. The world would have been inhabited only by RNA-based life until DNA evolved.
“DNA is too complex to have been the first aspect of life to emerge,” Pudritz says. “It had to start with something else, and that is RNA.” Paper. (paywall) – Ben K. D. Pearce, Ralph E. Pudritz, Dmitry A. Semenov, Thomas K. Henning. Origin of the RNA world: The fate of nucleobases in warm little ponds. Proceedings of the National Academy of Sciences, 2017; 201710339 DOI: 10.1073/pnas.1710339114 More.
From Stephen C. Meyer at Evolution News & Views :
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. More.
But the model does what’s needed. It gets the origin of life researchers through the publishing cycle.
May I (O’Leary for News) offer an analogy for these origin of life claims?
Suppose I tell you that a local electrician fell down and banged his head hard on the ice. And, after he came to, he took up writing and became a great novelist.
You object that there is a lot more to being a great novelist than can be achieved by banging one’s head on the ice, even highly favourable ice.
But I reply, Nonsense! Everyone knows that people who have sustained brain injuries can undergo marked changes in their behaviour. Anyway, this is the best model we have, and a great many Top People believe it. Your own belief system prevents you from accepting the facts!
Of course, the really big fact here is the gap between the simplicity of the proposed origin and the specified complexity of the result.
That’s much easier to talk around than to get around. Elegant essays loom.
See also: Before RNA world: Motivated soup world
Welcome to “RNA world,” the five-star hotel of origin-of-life theories
and Origin of life: Could it all have come together in one very special place? (hydrothermal vents)