The best radioactive element to use to date human fossils is Carbon-14.
There are several reasons why, but the main reasons is that Carbon-14 is a naturally occurring isotope in all forms of life and its half-life is about 5730 years, so we are able to use it to date more "recent" forms of life relative to the Geologic Time Scale.
As we have mentioned before each radioactive isotope has its own decay pattern.
Not only does it decay by giving off energy and matter, but it also decays at a rate that is characteristic to itself.
After one half-life of a given radioisotope, only one half as much of the original number of atoms remains active.
Another way to look at this is that if the radiation intensity is cut in half; the source will have only half as many curies as it originally had.
You would need to have access to scientific instruments at this point that could measure the amount of radioactivity in the sample, so off to the lab we go!
The energy of the radiation for a given isotope is considered to be constant for the life of the isotope.The half-lives of several radioactive isotopes are known and are used often to figure out the age of newly found fossils.Different isotopes have different half-lives and sometimes more than one present isotope can be used to get an even more specific age of a fossil.One way that helps scientists place fossils into the correct era on the Geologic Time Scale is by using radiometric dating.Also called absolute dating, scientists use the decay of radioactive elements within the fossils or the rocks around the fossils to determine the age of the organism that was preserved.