These long time periods are computed by measuring the ratio of daughter to parent substance in a rock and inferring an age based on this ratio.
This age is computed under the assumption that the parent substance (say, uranium) gradually decays to the daughter substance (say, lead), so the higher the ratio of lead to uranium, the older the rock must be.
As soon as a living organism dies, it stops taking in new carbon.
The ratio of carbon-12 to carbon-14 at the moment of death is the same as every other living thing, but the carbon-14 decays and is not replaced.
Carbon dating is used to determine the age of biological artifacts up to 50,000 years old.
Potassium-40 is another radioactive element naturally found in your body and has a half-life of 1.3 billion years.Other useful radioisotopes for radioactive dating include Uranium -235 (half-life = 704 million years), Uranium -238 (half-life = 4.5 billion years), Thorium-232 (half-life = 14 billion years) and Rubidium-87 (half-life = 49 billion years).The use of various radioisotopes allows the dating of biological and geological samples with a high degree of accuracy.Most scientists today believe that life has existed on the earth for billions of years.This belief in long ages for the earth and the existence of life is derived largely from radiometric dating.We could be sure that a mineral containing parentium originally had no daughterium.