The molecular clock is a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged.The biomolecular data used for such calculations are usually nucleotide sequences for DNA or amino acid sequences for proteins.If this is correct, the cytochrome c of all mammals should be equally different from the cytochrome c of all birds.Since fish diverges from the main stem of vertebrate evolution earlier than either birds or mammals, the cytochrome c of both mammals and birds should be equally different from the cytochrome c of fish.It took a canal surveyor circa 1800, William Smith in England, who noticed that he could map out great tracts of rocks on the basis of their contained fossils.The sequences he saw in one part of the country could be matched precisely with the sequences in another.The benchmarks for determining the mutation rate are often fossil or archaeological dates.
Radiometric dating is self-checking, because the data (after certain preliminary calculations are made) are fitted to a straight line (an "isochron") by means of standard linear regression methods of statistics.
In a related article on geologic ages (Ages), we presented a chart with the various geologic eras and their ages.
In a separate article (Radiometric dating), we sketched in some technical detail how these dates are calculated using radiometric dating techniques.
So, we have a “clock” which starts ticking the moment something dies.
Obviously, this works only for things which were once living.