Concept of radiometric dating
giving a lower limit for the age of the solar system.It is hypothesised that the accretion of Earth began soon after the formation of the calcium-aluminium-rich inclusions and the meteorites.For biologists, even 100 million years seemed much too short to be plausible.In Darwin's theory of evolution, the process of random heritable variation with cumulative selection requires great durations of time.He did not realize that the Earth mantle was convecting, and this invalidated his estimate.In 1895, John Perry produced an age-of-Earth estimate of 2 to 3 billion years using a model of a convective mantle and thin crust.(According to modern biology, the total evolutionary history from the beginning of life to today has taken place since 3.5 to 3.8 billion years ago, the amount of time which passed since the last universal ancestor of all living organisms as shown by geological dating.) In a lecture in 1869, Darwin's great advocate, Thomas H.
Following the development of radiometric age-dating in the early 20th century, measurements of lead in uranium-rich minerals showed that some were in excess of a billion years old.His value of 56 million years added additional evidence that Thomson was on the right track.By their chemical nature, rock minerals contain certain elements and not others; but in rocks containing radioactive isotopes, the process of radioactive decay generates exotic elements over time.In 1892, Thomson had been made Lord Kelvin in appreciation of his many scientific accomplishments.Kelvin calculated the age of the Earth by using thermal gradients, and he arrived at an estimate of about 100 million years.