Primordial soup or vinaigrette: did the RNA world evolve at acidic pH?

We propose that RNA is well suited for a world evolving at acidic pH. This is supported by the enhanced stability at acidic pH of not only the RNA phosphodiester bond but also of the aminoacyl-(t)RNA and peptide bonds. Examples of in vitro-selected ribozymes with activities at acid pH have recently been documented. The subsequent transition to a DNA genome could have been partly driven by the gradual rise in ocean pH, since DNA has greater stability than RNA at alkaline pH, but not at acidic pH.We have proposed mechanisms for two key RNA world activities that are compatible with an acidic milieu: (i) non-enzymatic RNA replication of a hemi-protonated cytosine-rich oligonucleotide, and (ii) specific aminoacylation of tRNA/hairpins through triple helix interactions between the helical aminoacyl stem and a single-stranded aminoacylating ribozyme.Our hypothesis casts doubt on the hypothesis that RNA evolved in the vicinity of alkaline hydrothermal vents. The ability of RNA to form protonated base pairs and triples at acidic pH suggests that standard base pairing may not have been a dominant requirement of the early RNA world.This article was reviewed by Eugene Koonin, Anthony Poole and Charles Carter (nominated by David Ardell).The concept of an RNA world - an early stage of evolution where RNA functioned as both gene and catalyst - has wide, though certainly not unanimous, support among those who study the origin of life. However, any such evolutionary model needs to be firmly rooted in an understanding of likely primordial physical and chemical Earth conditions [1]. The hypothesis that life may have emerged during the Hadean Eon 4.3-3.8 billion years ago [2] is supported by the discovery of zircon crystals that suggest the presence of liquid water and continental crust on the Earth as early as 4.4 billion years ago, or within 150 million years of the Earth's formation [3]. A projected high level of atmospheric CO2 is proposed to have produced an acidic ocean of the or