Getting past the RNA world: the initial Darwinian ancestor - PubMed (original) (raw)

Review

Getting past the RNA world: the initial Darwinian ancestor

Michael Yarus. Cold Spring Harb Perspect Biol. 2011.

Abstract

A little-noted result of the confirmation of multiple premises of the RNA-world hypothesis is that we now know something about the dawn organisms that followed the origin of life, perhaps over 4 billion years ago. We are therefore in an improved position to reason about the biota just before RNA times, during the era of the first replicators, the first Darwinian creatures on Earth. An RNA congener still prominent in modern biology is a plausible descendent of these first replicators.

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Figures

Figure 1.

(A) Structure of NADP, the 3′ phosphorylated derivative of nicotinamide adenine diphosphate (NAD). The image is a public domain illustration from Wikipedia. (B) A solution structure for NAD, with the nucleobases at the top, ribose in the middle, and the pyrophosphate at the bottom. Possible base pairings for replication are indicated. The figure is derived from the model of (Smith and Tanner 2000). (C) Replication scheme for an AMP-containing cofactor, like NAD or FAD. For simplicity, the backbone linking chemistry is shown as that for RNA catalysis (Huang and Yarus 1997), but this might vary. The complementary “template” strand and its replication on an NAD-like template are unspecified, because it is unclear what polarity it would have, or if it would be 5′-5′ linked. However, 5′-5′ polarity appears simplest, and is adopted in the text for clarity. R is nicotinamide nucleotide or a congener and N is its hypothetical complement.

Figure 2.

The IDA in context; the origin of life. (A) Activated nucleotides and compatible molecules oligomerize arbitrarily. (B) Replicators necessarily become abundant, by templating with minimal catalysis. (C) 5′-5′ replicators with reactive nucleotides are selected to participate in metabolism. (D) 5′-3′ RNA replicase creates an RNA world, ±5′-5′ cofactor initiation and reactivity. (E) RNAs devise translation; 5′-5′ cofactors are adopted by peptide catalysts (ribbon). The pathway is initiated by its most complex event, geochemical creation of several activated nucleotide-like materials. Thus, while “simple” is a debatable evolutionary characterization, progress might be relatively simple once begun. After panel (B), all crucial transitions depend on somewhat similar selections for enhanced chemical proficiency.

References

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Getting past the RNA world: the initial Darwinian ancestor - PubMed (original) (raw)