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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making C Additional Perspectives on Contamination from Space A substantial body of speculative literature on the origins of life was considered by the Task Group on Sample Return from Small Solar System Bodies but was judged to have too little experimental or theoretical justification to warrant evaluation in the body of this report. The task group also noted earlier reports, which have since been discredited, of evidence for living cells in meteorites. This appendix directs the reader to some of this material. CLAY-BASED LIFE Graham Cairns-Smith, in a series of papers and books, has argued that the earliest form of life on Earth was inorganic, probably based on a self-replicating clay (Cairns-Smith, 1982). The task group saw no way of evaluating this proposal and no reason to take account of it in formulating its recommendations. PANSPERMIA The idea that the first living organisms on Earth came from space has appeared in many guises over a period of more than 100 years. In the 19th century the seeds of life were thought to have arrived driven by the pressure of light from distant stars or encapsulated in meteorites (see, e.g., Oparin, 1957). More recently, deliberate or accidental seeding through the activities of a technologically advanced civilization has also been suggested, although not always seriously (Gold, 1960; Crick and Orgel, 1973). These proposals for the origins of life are in no way irrational, although most students of the subject think it more likely that life originated de novo on the primitive Earth. The possibility of cross-contamination of planetary bodies via meteorites and other small bodies is considered in the appropriate parts of this report. The possibility of the contamination of the planets by microorganisms carried on space probes has been discussed elsewhere. A somewhat different proposal was put forward by Hoyle and Wickramasinghe (1986), who suggested that bacteria and viruses capable of establishing infections in terrestrial organisms are constantly impinging on Earth and suggested comets as a possible source of viral epidemics. The task group did not think it necessary to take account of these suggestions in formulating its recommendations. ARTIFACTS IN METEORITES As stated above, the idea that living organisms arrived on Earth embedded in meteorites was widely discussed in the 19th century (see, e.g., Hahn, 1880). Subsequently, it has been claimed from time to time that organisms
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Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies: Framework for Decision Making have been identified in samples of meteorites collected in the field and/or stored in museums (see, e.g., Claus and Nagy, 1962). Although such claims have attracted considerable attention, subsequent investigations have shown in all cases that the putative microfossils were either terrestrial contaminants or abiotically produced indigenous mineral structures (see, e.g., Rossignol-Strick and Barghoorn, 1971). There is at present no evidence for any organisms of extraterrestrial origin in meteorites from planetary satellites or small solar system bodies. REFERENCES Cairns-Smith A.G. 1982. Genetic Takeover and the Mineral Origins of Life. Cambridge, England: Cambridge University Press. Crick, F.H.C., and L.E. Orgel. 1973. Directed panspermia. Icarus 19:341-346. Claus, G., and B. Nagy. 1962. Microfossils, new science, resembling algae and flagellates, found in meteorites. Pollen Spores 4:339. Gold, T. 1960. Space garbage. Air Force and Space Digest, May, p. 65. Hahn, O. 1880. Die meteorite (chrondrite) und ihre organismen/dargestellt und beschreiben von Otto Hahn, Litchtdruck von Martin Rommel in Stuttgart. Tubingen: H. Laupp'schen Buchhandlung. Hoyle, R., and N.C. Wickramasinghe. 1986. The case for life as a cosmic phenomenon. Nature 322:510. Oparin, A.I. 1957. The Origins of Life on Earth. New York: Academic Press. Rossignol-Strick, M., and E.S. Barghoorn. 1971. Extraterrestrial abiogenic organization of organic matter: The hollow spheres of the Orgueil meteorite. Space Life Sci. 3:89-107.
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