The discovery of natural radioactivity at the beginning of the twentieth century fundamentally changed our understanding of the physical and biological history of the Earth. Before then, the antiquity of the Earth was a very debatable issue and thus any process that required a long time, such as Darwin’s theory of evolution, was very much in doubt. A number of clever approaches, based on estimates of time required for certain physical processes, were used to constrain the age of the Earth, such as Lord Kelvin’s estimates based on the assumption of a molten Earth cooling to its present state. All of these estimates greatly underestimated the age of the Earth. The discovery of natural radioactivity (Becquerel, 1896) and the fact that the rate at which a particular radioactive nuclide decays is constant opened the way to obtaining absolute dates.
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