from human evolution

Before the age of a fossil can be determined, it is first necessary to establish that it is contemporary with the deposit in which it was found and not a more recent, intrusive burial in an older deposit. Often, contemporaneity can be determined by the fossil's association with other fossils or rock strata of known age from the same or other sites. This will establish what is called a fossil site's biostratigraphy. Chemical tests for fluorine and nitrogen in fossilised bone should show equivalent amounts throughout an assemblage (if it is contemporaneous), and the newer technique of X-ray microanalysis may establish an elemental "fingerprint" that is typical of a given location or stratum. An absolute method employing the decay of uranium also is useful for establishing contemporaneity.

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