Carbon-14 has a half-life of 5,730 ± 40 years—, half the amount of the radioisotope present at any given time will undergo spontaneous disintegration during the succeeding 5,730 years.
Because carbon-14 decays at this constant rate, an estimate of the date at which an organism died can be made by measuring the amount of its residual radiocarbon.
And by comparing the ratios of those atoms to atoms from meteorites, they could estimate how long ago it was that the Earth formed along with the rest of the solar system.
In 1956 the American geologist Clair Patterson (left) announced that the Earth was 4.5 billion years old.
Nineteenth century geologists recognized that rocks formed slowly as mountains eroded and sediments settled on the ocean floor.
But they could not say just how long such processes had taken, and thus how old their fossils were.
Learn about half-life and how it is used in different dating methods, such as uranium-lead dating and radiocarbon dating, in this video lesson. As we age, our hair turns gray, our skin wrinkles and our gait slows.
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Some of these methods have helped to pin down the evolution of our hominid ancestors; anatomically modern humans evolved about 100,000 years ago.
While that's nearly 20 times older than the Earth was once thought to be, it's a geological eye blink.
The primordial cloud of dust that came to form the Earth contained unstable atoms, known as radioactive isotopes.