Until recently, scientists believed that the chemical elements were synthesized only in stars. The discovery of the Oklo phenomenon in the Republic of Gabon in 1972 has revealed, however, that a nuclear “fire” had existed on the earth and largescale transmutations of the elements were occurring on our planet 1.7·109 years ago. The formation of natural (or Pre-Fermi) reactors is closely related to the appearance of life on our planet earth. The Pre-Fermi reactors were probably never formed until about 2·109 years ago, when oxygen was injected into the earth's atmosphere by a new generation of living organisms carrying out photosynthesis.
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