Radioactive dating rocks and fossils
Biologists actually have at their disposal several independent ways of looking at the history of life - not only from the order of fossils in the rocks, but also through phylogenetic trees. Relative dating is done by observing fossils, as described above, and recording which fossil is younger, which is older.The discovery of means for absolute dating in the early 1900s was a huge advance.
Each chemical element, such as carbon and oxygen, consists of atoms unique to it.
The methods are all based on radioactive decay: The first radiometric dates, generated about 1920, showed that the Earth was hundreds of millions, or billions, of years old.
Since then, geologists have made many tens of thousands of radiometric age determinations, and they have refined the earlier estimates.
A key point is that it is no longer necessary simply to accept one chemical determination of a rock’s age.
Age estimates can be cross-tested by using different isotope pairs.