Eras, the four largest time blocks in the scale, are named to indicate the fossils they contain: Precambrian (before ancient life), Paleozoic (ancient life), Mesozoic (middle life), and Cenozoic (recent life).
The last three eras are then subdivided into 11 periods.
Radioactive decay: The predictable manner in which a population of atoms of a radioactive element spontaneously fall apart.
Because of this stable process, scientists are able to estimate when a particular element was formed by measuring the amount of original and transformed atoms in that element. The age of the whole Earth is deduced from the ages of other materials in the solar system, namely, meteorites.
Meteorites are pieces formed from the cloud of dust and debris left behind after the beginning of the solar system.
Geologists easily identify rocks containing fossils of primitive life-forms as being older than rocks containing fossils that are more evolved or advanced.
However, the method of relative dating is exactly that, relative. In addition, complex lifeforms have existed on Earth for only the last 600 million years.
The last radioactive element in a series of these transformations will decay into a stable element, such as lead.