This usually requires what is commonly known as a "dating method".
Then, in 1999, Thorne (not to be outdone) and other scientists from the Australian National University published a new comprehensive study on the age of Mungo Man. Bowler and Magee described this 20,000-year stretch as ‘commendable in intent.’ There was just one small problem.
The radiocarbon dating method is based on the fact that radiocarbon is constantly being created in the atmosphere by the interaction of cosmic rays with atmospheric nitrogen.
The resulting radiocarbon combines with atmospheric oxygen to form radioactive carbon dioxide, which is incorporated into plants by photosynthesis; animals then acquire in a sample from a dead plant or animal such as a piece of wood or a fragment of bone provides information that can be used to calculate when the animal or plant died.
Later, this date was confirmed by two other dating methods (paleomagnetism and fission tracks), and was widely accepted.
Then Richard Leakey found a skull (called KNM-ER 1470) the KBS tuff, a skull that looked far too modern to be 3 million years old.