This discussion is a simplified introduction to radiocarbon dating.
Carbon-14 will have all disappeared by radioactive decay.
When a radiocarbon dating lab doesn’t see any carbon-14 in a fossil, it knows the fossil is more than 50,000 years old.
It also means that if a dead plant has 50% as much carbon-14 in it than in a living plant, the dead plant was alive about 5,700 years ago.
After 50,000 years, a fossil won’t have any radiocarbon left in it.
Therefore all living plants, animals, and human beings have the same amount of carbon-14 in their bodies at the same time.