Here we propose that the radioresistance (tolerance to ionizing radiation) observed in several terrestrial bacteria has a martian origin. Multiple inconsistencies with the current view of radioresistance as an accidental side effect of tolerance to desiccation are discussed. Experiments carried out 25 years ago were reproduced to demonstrate that "ordinary" bacteria can develop high radioresistance ability after multiple cycles of exposure to high radiation dosages followed by cycles of recovery of the bacterial population. We argue that "natural" cycles of this kind could have taken place only on the martian surface, and we hypothesize that Mars microorganisms could have developed radioresistance in just several million years' time and, subsequently, have undergone transfer to Earth by way of martian meteorites. Our mechanism implies multiple and frequent exchanges of biota between Mars and Earth.

synopsisA simple quantitative theory of inactivation of transforming DNA has been formulated. It is based on the idea that genetic recombination of the molecule of transforming DNA with the bacterial genome is possible only from the point mutation of the gene up to the nearest hit or point of damage on the corresponding DNA molecule. Our data and the data of other authors on the kinetics of inactivation by HNOz or U.V. light or both are consistent with these theoretical predictions. Theoretical predictions were made concerning the extent of reactivation by fusion and annealing of damaged DNA with a diluting genetically inactive DNA. These estimates have been reasonably confirmed by our experimental results.

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