At the beginning of biological evolution before a protective ozone layer had developed in the atmosphere, high intensities of energy-rich solar ultraviolet (UV) radiation could reach Earth's surface. Today, the full spectrum of solar UV radiation is only experienced in space, where other important space parameters, such as vacuum, cosmic radiation, temperature extremes, and microgravity, influence survival and genetic stability. To reach a better understanding of the processes leading to the origin, evolution, and distribution of life on Earth, several in-space experiments have been performed with microorganisms. The ability of resistant life forms, such as bacterial spores, to survive high doses of extraterrestrial solar UV—alone or in combination with other space parameters, e.g.: vacuum—was investigated. The protective effects of organic as well as inorganic substances, such as artificial or real meteorite material, were determined in satellite experiments, on the Space Shuttle, and on the MIR station. It could be shown that thin layers of inorganic material are able to protect spores against the deleterious effects of the energy-rich UV radiation and that they are able to survive under these conditions for very long periods of time in space. With different cut-off filters, the effect of an increasing atmospheric ozone layer on the solar spectrum was simulated as it had occurred on Earth ˝2 Ga ago, and the resulting changes in the DNA damage inducing potential of solar UV radiation was investigated. Extraterrestrial solar UV radiation was found to have a thousand times higher biological effectiveness than UV radiation filtered by stratospheric ozone concentrations found today on Earth.
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