Before undertaking a discussion on the search for extraterrestrial life, it is just as well to define what we mean by life. Perforce, the criteria are chosen based on the sole example at our disposal, that of life on Earth. Biologists agree on using the following criteria:
M. Ollivier et al., Planetary Systems. Astronomy and Astrophysics Library, DOI 978-3-540-75748-1.9, © Springer-Verlag Berlin Heidelberg 2009
• auto-reproduction (i.e., reproduction of identical organisms)
• evolution by mutations (corresponding to accidental changes in the reproduction mechanism)
• self-regulation with respect to its surroundings (which ensures the growth and survival of the individual life form).
The second criterion allows organisms to adapt to environmental conditions and to their evolution (such as a change in the ambient temperature or the quantity of water available). The species resulting from natural selection are those that have proved most capable of survival.
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