Darwinian evolution. Organic chemistry offers many examples of useful chemical reactivity in nonwater liquids. Macromolecular structures reminiscent of those found in terran biology can be formed with silicon and other elements.
Accordingly, the committee identified high-priority Earth-based laboratory and field studies aimed at doing the following:
Explore the limits of life on Earth, with an emphasis on detection of life in extreme environments that might have chemical structures and metabolisms different from those of terran life that has already been characterized.
Pursue the origin of life, especially on the basis of information from NASA missions, the inventory of organic materials in the cosmos, and interactions between organic materials and minerals set in a planetary context.
Contribute basic research to understand interactions of organic and inorganic species in exotic solvents, including water under extreme conditions (as found on Venus, Mars, Europa, Enceladus, and elsewhere), water-ammonia eutectics at low temperatures (as might be possible on Titan), and liquid cryosolvents (as found on Triton and elsewhere).
Contribute to laboratory synthetic-biology research into molecular systems that are capable of Darwinian evolution but are different from standard DNA and RNA, especially those designed to improve understanding of the chemical possibilities of supporting Darwinian evolution.
The committee offers the following recommendations:
Recommendation 1. The National Aeronautics and Space Administration and the National Science Foundation should support these kinds of laboratory research:
Origin-of-life studies, including prebiotic-chemistry and directed-evolution studies that address physiologies different from those of known organisms;
Further studies of chirality, particularly studies focused on the hypothesis that specific environmental conditions can favor chiral selection, or on an alternative model that life with L-amino acids and D-sugars is better “fit,” from an evolutionary perspective, to evolve into complex organisms; and
Work to understand the environmental characteristics that can affect the ability of organisms to fractionate key elements, including not only carbon but also sulfur, nitrogen, iron, molybdenum, nickel, and tungsten.
Recommendation 2. The National Aeronautics and Space Administration and the National Science Foundation should support these kinds of field research:
A search for remnants of an RNA world in extant extremophiles that are deeply rooted in the phylogenetic tree of life;
A search for organisms with novel metabolic and bioenergetic pathways, particularly pathways involved in carbon dioxide and carbon monoxide reduction and methane oxidation coupled with electron acceptors other than oxygen;
A search for organisms that derive some of their catalytic activity from minerals rather than protein enzymes;
A search for organisms from environments that are limited in key nutrients, including phosphorus and iron, and determination of whether they can substitute other elements, such as arsenic, for phosphorus;
A search for life that can extract essential nutrients—such as phosphorus, iron, and other metals—from rocks, such as pyrites and apatite;
A search for anomalous gene sequences in conserved genes, particularly DNA- and RNA-modifying genes;
Study of the resistance of microorganisms that form biofilms on minerals to the harsh conditions of interplanetary transport; and
A search for life that stores its heredity in chemicals other than nucleic acids.