Radiometric dating is based on the idea that
The move towards agriculture started in the Middle East.
During the Palaeolithic Age humans grouped together in small bands.
Acheulean tools are typically found with Homo erectus remains.
In some regions Acheulean tool-using groups were contemporary with other, less sophisticated industries such as the Clactonian.
Then, later, Acheulean tools occur at the same time as the more sophisticated Mousterian.
He had found them in prehistoric lake deposits along with the bones of extinct animals and concluded that they were made by people "who had not the use of metals" and that they belonged to a "very ancient period indeed, even beyond the present world".
His ideas were ignored by his contemporaries however, who held a pre-Darwinian view of human evolution.