ABSTRACT
Our planet was born about 4500 million years ago, after the accretion of presolar nebula ended and the Solar System had appeared. From what we know or assume about the presolar nebula, accretion took place very likely in thepresence of abundant radiation and radionuclides. The radionuclides were incorporated into the planet's body and their decay makes possible not only the dating of various events in its past, but also a reconstruction ofsome cosmological events that preceded the birth of the planet. The energy liberated during radioactive decay was an important source of terrestrial heat and an efficient means of promoting evolutionary processes.
The ionizing radiation arose not only from radionuclides dispersed in terrestrial material, but also from localized uranium ore deposits which possessed certain similarities with present-day man-made nuclear reactors. One such fossil reactor site which had remained in obscurity for 2000 million years was discovered in the 1970s during the mining of a uranium deposit at Oklo (Gabon, West Africa). Chemical analyses of thousands of samples taken from the reactor site, together with computer modeling of experimental findings based on the nuclear reactor theory, have provided insight into the working conditions of the six fossil reactor cores. The phenomenon of Oklo is surely not unique in the past history of our planet. From available information it is possible to reconstitute the picture of a natural nuclear reactor on the early Earth. The number of such reactors may be roughly estimated from the amount of uranium in the Earth's crust that was possibly involved in the criticality.
Chemical evolution was a sequence of events on the early Earth which led from simple inorganic molecules to complex organic compounds and eventually to primitive forms of life. It wasfollowed by biochemical (early biological) evolution. Ionizing radiation was one of several types of energy that were available for these evolutionary processes. Radiation-generated free radicals in bodies of water covering the primitive Earth were efficient in promoting important chemical reactions for both abiotic syntheses during chemical evolution and biochemical processes in the primitive forms of life during early biological evolution.
