tory studies, field studies, and space studies. The first two are clearly appropriate for both NASA and the National Science Foundation (NSF), and the third is likely to be solely in the purview of NASA.

8.1
LABORATORY STUDIES

Laboratory studies are needed that will help to elucidate the origin of life, especially studies that use information from NASA missions, the inventory of organic materials in the cosmos, and interactions between organic materials and minerals set in a planetary context. There is a need for 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 possible on Titan), and liquid cryosolvents (as found on Triton and elsewhere). A need also exists for synthetic biology that constructs and studies molecular systems capable of Darwinian evolution that are different from standard DNA and RNA, especially those designed to improve understanding of the chemical possibilities that support Darwinian evolution. Such studies should include the search for self-sustaining energy-driven metabolic cycles. The committee’s recommendations to NASA and NSF for specific kinds of laboratory studies are as follows.

  • Origin-of-life studies, including prebiotic-chemistry and directed-evolution studies that address physiologies different from those of known organisms. Such alternate physiologies can include novel metabolisms and growth in extreme conditions that are not found on Earth but are found on other planets and moons. Some examples are growth in media with low ratios of water to organic solvents, the substitution of arsenic for phosphate, the use of carbon-silicon polymers, and the use of mineral catalysis instead of enzyme catalysis.

  • 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.

  • 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. An understanding of how life fractionates transition elements could provide an essential marker for past and present life and insight into their metabolic potential. Even weird carbon life will use elements for energy, such structural polymers as proteins or protein analogues, and oxidation and reduction reactions.

8.2
FIELD STUDIES

Many of the environments on Earth that have analogous environments on other planets and moons have not been adequately sampled. They include the deep subseafloor crust, the deep oligotrophic ocean, and the upper atmosphere. Therefore, the committee places particular emphasis on the search for organisms that have novel metabolisms that use novel energy sources, including black-body radiation or sources thought to be ancient but ubiquitous on other rocky planets. They include organisms that use hydrogen and sulfur as energy sources and iron (FeIII) as an electron acceptor. The focus should be on microbial ecosystems that do not depend on photosynthesis. There is also an opportunity to look at submarine hydrothermal vent environments as primordial and to design experiments to search for organisms that have relic genes and metabolisms that can reveal something about early life. The committee’s recommendations to NASA and NSF for field studies are as follows:

  • A search for remnants of an RNA world in extant extremophiles that are deeply rooted in the phylogenetic tree of life. They could include RNA genes that, unlike the common retrotransposons found in eukaryotes that are just “selfish genes,” may have some function in the cell. The search should also include viruses from hyperthermophilic archaeans that have already been shown, in the study at Yellowstone, to be unlike anything that has been seen before and that have characteristics of all three domains 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. Regardless of how weird carbon life might be, there will be a “unity of metabolism” in which all



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement