Tuesday, May 19, 2009

Origin of life: Latest scenario gives RNA world a boost

In "Ribonucleotides and the revival of the "warm little pond" scenario" (ARN Reports, 05/19/09), David Tyler dissects another media meltdown over the origin of life:
According to one commentator, the newly published research provides "one of the great advances in prebiotic chemistry". The media have captured the excitement with headlines like "Chemist Shows How RNA Can Be the Starting Point for Life" (New York Times), "Molecule of life emerges from laboratory slime" (New Scientist) and "How RNA got started" (Science News). These are strong statements and they deserve closer attention. What is going on in the field of OOL research?
What indeed? Given the many scenarios offered for the origin of life, I would guess that maybe one in one hundred might provide useful information.

The biggest single problem, one that researchers gloss over and that pop science media don't reckon, is that evidence for any one theory subtracts from evidence for any other. So it is not clear that an advance in information has occurred.

An argument for RNA world is an argument against clay world or silicon world or prebiotic pizza or prebiotic soup, or whatever other flavour is on offer.

"RNA World" has been around for a while, to be sure. Tyler comments,
The researchers have synthesised both pyrimidine ribonucleotides (but not the purine ribonucleotides). As Van Noorden described it, they have "shown that it is possible to build one part of RNA from small molecules". They have not formed RNA molecules; they have not addressed the chirality problem, they have not generated any biological information and they have not made RNA do anything of biological significance, let alone become clothed with a membrane and undergo replication. Nevertheless, what they have done can be applauded as an elegant example of systems chemistry.
And as Bill Dembski notes,
Excuse me? Doing the chemical reactions in precise sequence and purifying the products at each step hardly seems like recreating realistic prebiotic conditions. In fact, it almost sounds like, dare I say it, intelligent design.
No, wait! It's not design. Those guys aren't taking any credit for it. It all just sort of happened, see?

Here's the abstract, and some links:
Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions Matthew W. Powner, Beatrice Gerland, John D. Sutherland Nature 459, 239-242 (14 May 2009) doi:10.1038/nature08013

At some stage in the origin of life, an informational polymer must have arisen by purely chemical means. According to one version of the 'RNA world' hypothesis this polymer was RNA, but attempts to provide experimental support for this have failed. In particular, although there has been some success demonstrating that 'activated' ribonucleotides can polymerize to form RNA, it is far from obvious how such ribonucleotides could have formed from their constituent parts (ribose and nucleobases). Ribose is difficult to form selectively, and the addition of nucleobases to ribose is inefficient in the case of purines and does not occur at all in the case of the canonical pyrimidines. Here we show that activated pyrimidine ribonucleotides can be formed in a short sequence that bypasses free ribose and the nucleobases, and instead proceeds through arabinose amino-oxazoline and anhydronucleoside intermediates. The starting materials for the synthesis - cyanamide, cyanoacetylene, glycolaldehyde, glyceraldehyde and inorganic phosphate - are plausible prebiotic feedstock molecules, and the conditions of the synthesis are consistent with potential early-Earth geochemical models. Although inorganic phosphate is only incorporated into the nucleotides at a late stage of the sequence, its presence from the start is essential as it controls three reactions in the earlier stages by acting as a general acid/base catalyst, a nucleophilic catalyst, a pH buffer and a chemical buffer. For prebiotic reaction sequences, our results highlight the importance of working with mixed chemical systems in which reactants for a particular reaction step can also control other steps.

See also:

Szostak, J.W., Origins of life: Systems chemistry on early Earth, Nature 459, 171-172 (14 May 2009) doi:10.1038/459171a

Van Noorden, R., RNA world easier to make, Nature News, 13 May 2009 doi:10.1038/news.2009.471

Wade, N., Chemist Shows How RNA Can Be the Starting Point for Life, New York Times, 14 May 2009.