The Search for Life's Origins Progress and Future Directions in Planetary Biology and Chemical Evolution (1990) / Chapter Skim
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Executive Summary
Executive Summary
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Chapter 3 discusses a cardinal problem in the history of biological evolution, elucidating the processes by which materials came together on the primordial Earth to form the first living organisms. The current status of the field of biological evolution and recommended future research in that area are detailed in Chapter 4.
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In addition, there is increasing evidence for the survival of interstellar molecular material within objects present in the solar system today, such as interplanetary dust particles, asteroids, meteorites, and comets. The cosmic history of the biogenic elements and their compounds thus becomes a critical field of study for exobiologists.
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The first goal is to understand the processes responsible for the chemical evolution of organic matter in the outer solar system. Attainment of this goal can be achieved by the following objectives: · Determine the origin and distribution of organic matter and disequilibrium products containing the biogenic elements in the hydrogen-rich atmospheres of the outer planets; · Elucidate the organic chemistry and the origin of carbon oxides on Titan; and · Characterize the organic matter on the dark surfaces of the asteroids, satellites, and planetary rings of the outer solar system.
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Sources of information that may shed light on this question are the record of the early solar system as preserved in comets, asteroids, the Moon, and Mars; the geological record; the paleogeological record of ancient microorganisms and their physiological activity; and the history recorded in nucleotide and amino acid sequences found in living cells (molecular phylogeny)
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This goal can be fulfilled by working toward the following objectives: . A search for simple organic replicating systems template-directed replication using ribonucleotides is the most straightforward general model of replication, but even simpler systems warrant experimental study; · Investigation of the possible role of ribonucleic acid (RNA)
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This goal may be fulfilled by rooting the universal phylogenetic "tree." Studies of molecular phylogeny have brought this tree within reach; besides providing the starting point for studying the course of biological evolution within the three primary kingdoms, this set of phylogenetic relationships provides a framework for investigating life's history prior to the segregation of the three extant lineages. A principal objective of this line of research should be the recognition and evolutionary evaluation of gene families.
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To fulfill the goal of understanding the nature of the distribution of life in the universe, the committee recommends pursuing the following object~ves: · Determination of the frequency and morphology of nearby planetary systems this will require a new generation of instruments capable of detecting low-mass planets and investigating solar-system-scale phenomena within protoplanetary disks in nearby regions of star formation; · Determination of the frequency of occurrence of conditions suitable to the origin of life obtaining information on the surface temperature and atmospheric chemical composition of extrasolar planets requires direct im
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Thus, investigations of Mars can contribute to the elucidation of objectives discussed previously in connection with early planetary environments and the origin of lifeboth on the Earth and, possibly, on Mars as well as with the course of biological evolution on this planet. The committee therefore recommends studies to · Conduct chemical, isotopic, mineralogical, sedimentological, and paleontological studies of Martian surface materials at sites where there is evidence of hydrologic activity in any early clement epoch, through in situ determinations and through analysis of returned samples; of primary interest are sites in the channel networks and outflow plains; highest priority is assigned to sites in which there is evidence suggestive of water-lain sedi
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Furthermore, these bodies are also of interest as projectiles that may have had significant effects on the course of biological evolution by impacting the Earth. The committee therefore recommends that · Measurements be made, by remote spectroscopic observations and in situ, of the elemental and isotopic composition of cometary comae and nuclei and of the principal asteroid types, including determination of the molecular composition of components containing the biogenic elements hydrogen, carbon, nitrogen, oxygen, phosphorus, and sulfur in comets and primitive asteroids; such measurements should be made at various surface locations and depths to determine the degree of homogeneity; and · A cometary sample be obtained for detailed laboratory analysis of atmospheric, surface, and subsurface materials.
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The Interstellar Medium and Cosmic Dust Particles The earliest stages of chemical processing involving the biogenic elements are taking place in molecular clouds and protosolar nebulae. Studies of these objects can therefore answer fundamental questions about the early history of organic chemical evolution.
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in light of recognizable extraterrestrial signals; · The continued search on Earth for igneous and sedimentary rocks formed prior to 3.8 billion years ago; and · The development of robust phylogenies relating living organisms, through the comparison of sequences in informational macromolecules, especially small subunit ribosomal RNAs, and the elucidation of the biochemical and ultrastructural characters of microorganisms in order to relate patterns of phenotypic diversity to phylogeny. Mars-Related Studies Ground-based studies are necessary to understand present environmental conditions in order to plan effective exploratory investigations related to exobiology.
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In relation to these issues, the committee recommends · The maintenance of a vigorous program of research on the chemical, isotopic, mineralogical, and petrographic properties of meteorites and laboratory studies of the molecular and isotopic compositions and yields of organic molecules produced in realistic simulations of those astrophysical environments within which presolar constituents of carbonaceous meteorites may have been produced; and · Theoretical studies on the physics of comet formation, to determine the maximum size of comets accreted in the solar nebula, as well as thermocalculations of the composition of atmospheres produced by large impacts of cometary and various asteroidal-type bodies. Studies Related to Titan and the Giant Outer Planets Theoretical modeling and laboratory studies are required to elucidate the organic chemistry in the atmospheres of Titan and the giant planets, as well as to effectively interpret relevant data obtained from missions to these objects.
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Because both lines of investigation proceed simultaneously, the overall priorities listed below are those suggested naturally by the existing maturity of the requisite instrumentation. For these studies, the committee recommends · Continued support for ground-based and earth-orbital searches for extrasolar planets · Commencement of a systematic ground-based search through the low end of the microwave window for evidence of signals from an extraterrestrial technology; and · Studies leading to the development of future technologies for these investigations, including large-scale optical, infrared, and submillimeter arrays or monoliths in orbit or on lunar farside for imaging extrasolar planets
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The committee also urges NASA to encourage the timely development of instrumentation for potential use in space experiments involving planetary biology and chemical evolution, well in advance of payload selection, by setting aside specific funds for this purpose. Because of the essential role of space technology in many aspects of research in planetary biology and chemical evolution, almost all of the support for this field, and for integration of its various elements, is now borne by a single federal agency, NASA, through its grants and in-house activities.
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Opportunities for such interactions can be facilitated by NASA sponsorship of workshops, symposia, and innovative interdisciplinary research projects. Also, because the subject matter of this field cuts across both the physical and the biological sciences, specific training in this area is not normally available to students as they prepare for their scientific careers, and young people entering into the pool of scientific talent are less apt to seek careers in chemical evolution and planetary biology.
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