The age of the earth.
1779 -
about 75,000 years old
1790s -
96 million years old
1856 -
22 million years old
1862 -
between 20 million and 400 million years old
In
Darwin's theory of
evolution,
the process of random heritable variation with cumulative selection requires great durations of time. (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.)
1869 - Darwin's great advocate,
Thomas H. Huxley, attacked Thomson's calculations, suggesting they appeared precise in themselves but were based on faulty assumptions.
1892 -
18 million years old
In 1895
John Perry challenged Kelvin's figure on the basis of his assumptions on conductivity, and
Oliver Heaviside entered the dialogue, considering it "a vehicle to display the ability of his
operator method to solve problems of astonishing complexity."
Other scientists backed up Thomson's figures.
Charles Darwin's son, the astronomer
George H. Darwin, proposed that Earth and
Moon had broken apart in their early days when they were both molten. He calculated the amount of time it would have taken for
tidal friction to give Earth its current 24-hour day. His value of
56 million years added additional evidence that Thomson was on the right track.
The last estimate Thomson gave, in
1897, was: "that it was
more than 20 and less than 40 million year old, and probably much nearer 20 than 40". In
1899 and 1900,
John Joly calculated the rate at which the oceans should have accumulated
salt from
erosion processes, and determined that the oceans were about
80 to 100 million years old.
In 1892 Kelvin arrived at an estimate of about 100 million years old
In 1895,
John Perry produced an age-of-Earth estimate of 2 to 3 billion years old
Kelvin stuck by his estimate of 100 million years, and later reduced it to about 20 million years old
The discovery of
radioactivity introduced another factor in the calculation.
Invention of radiometric dating
[After some years (1904 - 1960s) of numerous assumptions...]
An age of
4.55 ± 0.07 billion years, very close to today's accepted age, was determined by
Clair Cameron Patterson using uranium-lead isotope dating (specifically
lead-lead dating) on several meteorites including the
Canyon Diablo meteorite and published in 1956.
[Today...]
The age of the Earth is
4.54 ± 0.05 billion years (4.54 × 109 years ± 1%). This age may represent the age of the Earth's
accretion, of core formation, or of the material from which the Earth formed. This dating is based on evidence from
radiometric age-dating of
meteorite material and is consistent with the radiometric ages of the oldest-known terrestrial and
lunar samples.
..........
It is hypothesised that the accretion of Earth began soon after the formation of the calcium-aluminium-rich inclusions and the meteorites. Because the exact amount of time this accretion process took is not yet known, and the predictions from different accretion models range from a few million up to about 100 million years, the exact age of Earth is difficult to determine. It is also difficult to determine the exact age of the
oldest rocks on Earth, exposed at the surface, as they are
aggregates of minerals of possibly different ages.
Does anyone know the History of evolutionary thought?