"...The certainty of death was absent at the origin of life. Unlike humans and other mammals, many organisms do not age and die. The process of programmed, inevitable death evolved only after our symbiotic microbial ancestors, some two billion years ago, became sexual individuals...."
http://www.scientificamerican.com/article.cfm?id=sex-death-kefir-lynn-margulis
http://www.scientificamerican.com/article.cfm?id=sex-death-kefir-lynn-margulis

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