The radiometric dating
For potassium 40, the half-life is about 1.3 billion years.In general, in one half-life, half of the parent will have decayed.Potassium is about 1/40 of the earth's crust, and about 1/10,000 of the potassium is potassium 40.
Therefore, my main concern is with rocks of the Cambrian periods and later.
In two half-lives, half of the remainder will decay, meaning 3/4 in all will have decayed.
In general, in n half-lives, only 1/(2^n) of the original parent material will be left.
We can assume that the Precambrian rocks already existed when life began, and so the ages of the Precambrian rocks are not necessarily related to the question of how long life has existed on earth.
The Cambrian period is conventionally assumed to have begun about 550 million years ago.
On the surface, radiometric dating methods appear to give powerful support to the statement that life has existed on the earth for hundreds of millions, even billions, of years.