Darwin's prescient guess.

Pearce et al. (1) have made a bold and challenging attempt to provide a quantitative estimate for the accumulation of nucleobases, like adenine, on the Hadean Earth some 4 billion y ago, before life began. Why bold? There must have been a source of organic compounds for life to begin, and they chose an extraterrestrial source—carbonaceous meteorites—rather than the geochemical source more commonly assumed in origins of life research. Why challenging? The authors chose to model the origin of life in fresh-water ponds on volcanic land masses emerging from a global ocean, in direct contrast to the current paradigm that life originated in sea water using chemical energy associated with hydrothermal vents. The reason for these choices is that so little is known with certainty about the late Hadean Era, that it was essential to impose constraints to perform a mathematical treatment. This is why Pearce et al. (1) modeled extraterrestrial delivery and lifetimes of a nucleobase in the “warm little ponds” of the article’s title. The use of this phrase, which they abbreviate WLP, alludes to the site first proposed by Darwin in 1871 in a letter to his friend Joseph Hooke. Small, fresh-water ponds are common in volcanic regions of today’s Earth, making it feasible that there were multiple volcanoes and ponds 4 billion y ago. In fact, a recent publication reported evidence that the oldest fossil evidence of life on Earth 3.5 billion y ago are stromatolites produced by microbial mats that thrived in a fresh-water hydrothermal field (2). Pearce et al. (1) incorporated the adenine content of carbonaceous meteorites as one of the parameters and used the lunar cratering record to estimate the rate of deposition from meteorites between 4.5 and 3.7 billion y ago. They also assumed a certain area and depth of a typical WLP in which organic material would accumulate, and took into account the rate at which the ponds would go through wet–dry cycles related to precipitation and evaporation. Such cycles would be necessary to concentrate reactants and provide chemical energy required for polymerization of RNA. Finally, Pearce et al. estimated loss of adenine from ponds by seepage and degradation by UV light. Perhaps most challenging is Pearce et al.’s (1) choice of WLPs rather than hydrothermal vents. The ponds were optimized in terms of an area that permits sufficient collection of organic material, but also of a depth that could undergo periodic cycles of evaporation and filling by precipitation. Fig. 1A shows a “black smoker” emitting a plume of iron-rich mineral particles (3). Another type of vent discovered later are the alkaline vents shown in Fig. 1B (4), and these have been the subject of a series of publications by Russell and colleagues (5–8), who proposed that such vents are plausible sites for life to originate. Fig. 1C shows a hydrothermal field on Mount Lassen, California, with an enlarged view of a small WLP. What Pearce et al. (1) propose is that an impacting carbonaceous meteorite would produce smaller fragments that fall into ponds and release soluble organic compounds, such as adenine. Something like this occurred in 1969 when a boulder-sized bolide exploded in the skies over Murchison, Australia. Hundreds of fragments fell over an area of 5 square miles and ∼100 kg were quickly collected. Carbonaceous meteorites are Fig. 1. Hydrothermal vents and WLP. There are two distinct types of hydrothermal vents. So-called “black smokers” (A) are acidic and driven by heat from underlying magma. Alkaline vents (B) are produced by a chemical reaction called serpentinization, when sea water reacts with certain minerals in olivine. Fresh-water hydrothermal fields (C) are generally acidic due to dissolved sulfur oxides and are analogous to the WLP analyzed by the Pearce et al. (1). Images courtesy of (A) Office of Ocean and Atmospheric Research/National Undersea Research Program/National Oceanic and Atmospheric Administration; and (B) Wikipedia Commons/National Science Foundation (University of Washington/ Woods Hole Oceanographic Institution).