The origin of life as a planetary phenomenon

Abstract

We advocate an integrative approach between laboratory experiments in prebiotic chemistry and geologic, geochemical, and astrophysical observations to help assemble a robust chemical pathway to life that can be reproduced in the laboratory. The cyanosulfidic chemistry scenario described here was developed by such an integrative iterative process. We discuss how it maps onto evolving planetary surface environments on early Earth and Mars and the value of comparative planetary evolution. The results indicate that Mars can offer direct evidence for geochemical conditions similar to prebiotic Earth, whose early record has been erased. The Jezero crater is now the chosen landing site for NASA's Mars 2020 rover, making this an extraordinary opportunity for a breakthrough in understanding life's origins.

Fig. 1. Carbon and nitrogen oxidation states and biology.

Nitrogen in biological molecules is predominantly in the −3 oxidation state, whereas carbon is in a broader range roughly centering on 0. Thus, if methane (CH₄) were to have been used as an initial carbon feedstock, oxidation chemistry would have been required. Similarly, carbon dioxide (CO₂) would have required reduction. In light of just the foregoing, hydrogen cyanide (HCN) can be seen as a near-ideal feedstock, as it provides both nitrogen in approximately the right oxidation state and carbon only in need of some reduction. This partial reduction actually turns out to be necessary for the C─C bond construction chemistry required to link carbon atoms into the linear and branched chains so often found in biological molecules. During cyanosulfidic chemistry, these bonds can be made highly efficiently by the addition of HCN to aldehydic reduction products of itself, or related nitriles, to give cyanohydrins [R₂C(OH)CN], which can be further reduced and homologated (i.e., add a constant unit, often −CH₂⁻) by the addition of more HCN. Hydrogen cyanide, along with derivatives produced through environmental processing, can be converted by cyanosulfidic chemistry into all the compounds shown (for those which are chiral, in racemic form).

Fig. 2. Creating stockpiles of initial compounds for prebiotic chemistry.

Conceptual model for accumulation of strata in shallow subaqueous basins and their interaction with water and the atmosphere is shown. The model is based on early Mars, but we expect it to be applicable to prebiotic Earth. (A) Lake or shallow sea in contact with bedrock and the atmosphere. Water is supplied by surface runoff from precipitation and/or melting ice, groundwater infiltration from adjacent highlands, and thermally buoyant deep basinal waters warmed by the ambient geothermal gradient and flowing upward through permeable fractures in the crust. HCN in the atmosphere (due to impact delivery and processing) interacts with dissolved iron in the water body and below the depth of UV penetration allows ferrocyanide to form. (B) Variations in climate result in episodic drying of the shallow water body, creating a variety of salts including ferrocyanide. Preservation of ferrocyanide is enhanced by burial beneath the reach of UV energy. (C) Thermal pulses created by igneous intrusions, volcanic activity, or large impacts cause contact metamorphism of cyanide salt deposits to a variety of reaction products including CaCN₂, KCN, Mg₃N₂, and NaCN. (D) Exposure of these metamorphic reaction products to neutral pH water yields H₂CN₂, HCN, and NH₃ in solution. In turn, if these species are exposed to SO₂ and surface water is shallow enough to be affected by mid-range UV radiation, cyanosulfidic chemistry can then produce a diverse set of products, which correspond to the nucleotide, amino acid, and lipid precursor molecules of extant biochemistry.

Fig. 3. Schematic timeline for Mars and Earth.

Comparison of the key events in the environmental histories of Earth and Mars. For example, Mars has lost the bulk of its early atmosphere, followed by loss of its hydrosphere, whereas Earth has always retained both. Therefore, the formation of hydrated minerals continues on Earth but ceased early in Mars’ history. On the other hand, Mars can offer evidence for geochemical conditions similar to Hadean and Archaean Earth during the epoch of Earth’s prebiotic chemistry. This evidence has been mostly erased from Earth’s rock record: This is illustrated by the panels marked “Surface age,” referring to crater retention age for Mars, and preserved rock record for Earth.

Fig. 4. Planet core formation and atmospheres.

Rocky planets in a wide range of masses are commonly expected to usually have atmospheres dominated by N₂ and CO₂ when their orbits fall within the broader habitable zone (aka liquid-water belt) of their host stars and after some iron has been removed from the mantle by forming a core—the full transition takes 10⁷ to 10⁸ years and is illustrated by the first and second cutouts for the case of Earth (60, 61). A highly reducing atmosphere (as shown in the first cutout) is photochemically unstable on rocky planets with surface oceans and vapor H₂O in the atmosphere because of the fast escape of H to space after photolysis of CH₄ and NH₃. In general, outgassed carbon would be converted into CO₂, and an N₂-CO₂ atmosphere would be generated on a time scale of about 10⁵ years (–66).