Half life radioactive dating
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.
The fossil record may be incomplete and may never fully completed, but there are still many clues to evolution and how it happens within the fossil record.
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.
The rate at which a radioactive isotope decays is measured in half-life.
The term half-life is defined as the time it takes for one-half of the atoms of a radioactive material to disintegrate.
In other words, half (50%) of the Carbon-14 you started with has decayed into the daughter isotope Nitrogen-14.