Scientists use a process known as [[radiometric dating]] to determine the age of the fossils. Radiometric dating is a technique in which scientists observe the amount of an elemental [[isotope]] within the fossil, then use the rate at which the isotope is presumed to decay to calculate when the organism died. There are a number of different methods, some of which claim to be able to date a specimen that died up to three billion years ago.<ref>Georgia Perimeter College - Historical Geology: [http://www.gpc.edu/~pgore/geology/geo102/radio.htm "Radiometric Dating"]</ref> Since individual radiometric methods are independent they can be cross-check with each other to reduce error. | Scientists use a process known as [[radiometric dating]] to determine the age of the fossils. Radiometric dating is a technique in which scientists observe the amount of an elemental [[isotope]] within the fossil, then use the rate at which the isotope is presumed to decay to calculate when the organism died. There are a number of different methods, some of which claim to be able to date a specimen that died up to three billion years ago.<ref>Georgia Perimeter College - Historical Geology: [http://www.gpc.edu/~pgore/geology/geo102/radio.htm "Radiometric Dating"]</ref> Since individual radiometric methods are independent they can be cross-check with each other to reduce error. |