The numbers of parent and daughter nuclei can be counted using a mass-spectrometer. The rules of radiometric dating in geology. Living organisms, largely made up of carbon and oxygen, maintain this ratio while alive - in living and newly formed cells. This ratio, by virtue of the known half-life, is a direct measure of the time since the organism died. Earth is 4.6 billion years old.
Subdivisions become finer the younger they are.
The farther back we go, the less we know. The first abundant fossil evidence does not appear until the beginning of the Cambrian period. Because Precambrian rocks are old they have been subjected to a great many changes, and many of the clues present in the original rocks have been destroyed. There are difficulties in dating the geologic time scale in general, and sedimentary sequences in particular. Not all rocks can be dated by radiometric techniques.