Radioactive isotope dating fossils
Levels of carbon-14 become difficult to measure and compare after about 50,000 years (between 8 and 9 half lives; where 1% of the original carbon-14 would remain undecayed).
Half of 50% is 25%, so you would have 25% Carbon-14 and 75% Nitrogen-14.
The age of the carbon in the rock is different from that of the carbon in the air and makes carbon dating data for those organisms inaccurate under the assumptions normally used for carbon dating.
This restriction extends to animals that consume seafood in their diet.
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.
Carbon dating is used to determine the age of biological artifacts up to 50,000 years old.