Relative dating helps determine what came first and what followed, but doesn't help determine actual age.Radiometric dating, or numeric dating, determines an actual or approximate age of an object by studying the rate of decay of radioactive isotopes, such as uranium, potassium, rubidium and carbon-14 within that object. This rate provides scientists with an accurate measurement system to determine age.
In the first paragraph of the Introduction in The Origin of Species, Darwin highlighted the centrality of Lyle’s and Herschel’s influence of on his theory: “to throw some light on the origin of species — that mystery of mysteries, as it has been called by one of our greatest philosophers.” Herschel had used this signature phrase, “that mystery of mysteries,” in a letter to Lyle endorsing his Principles of Geology.
A long-age of the Earth is an absolute pre-requisite for the theory of biological evolution.
Use of the radioactive decay of Uranium to Lead was first published in 1907 by radiochemist Bertram Boltwood (1879-1927) to measure the age of rocks.
Boltwood’s first rock measurements estimated an age of the Earth at 400 to 2200 million years old.
Relative dating and radiometric dating are used to determine age of fossils and geologic features, but with different methods.