By 1905, the British physicist Lord Rutherford made the first clear suggestion for using radioactivity as a tool for measuring geologic time directly; shortly thereafter, in 1907, Professor B. Boltwood, radiochemist of Yale Uniyersity, published a list of geologic ages based on radioactivity.
Although Boltwood's ages have since been revised, they did show correctly that the duration of geologic time would be measured in terms of hundreds-to-thousands of millions of years.
In keeping with this view, both humans and apes have been assigned to the same taxonomic Family - Hominidae.
However, most creationists would deny that humans have evolved in this manner, and instead believe that these ancient hominids were either fully ape or fully human.
The next 40 years was a period of expanding research on the nature and behavior of atoms, leading to the development of nuclear fission and fusion as energy sources.
A byproduct of this atomic research has been the development and continuing refinement of the various methods and techniques used to measure the age of Earth materials. A chemical element consists of atoms with a specific number of protons in their nuclei but different atomic weights owing to variations in the number of neutrons.
Absolute dating provides a numerical age for the material tested, while relative dating can only provide a sequence of age.
According to theory, the sample starts out with daughter isotopes present at constant ratios in relation to one another, but with the parent isotope, the ratio is arbitrary.
Loss of uranium moves the point up and to the right, while a loss of lead moves the point down and to the left.
Fission-track dating involves counting the damage tracks left by fragments of the spontaneous fission of uranium-238.
However, research by creationists has revealed a large number of problems with radiometric dating.
In some cases such as Carbon-14 dating, radioactive dating actually gives strong evidence for a young Earth, while other methods such as K-Ar dating and Isochron dating are based on faulty assumptions and are so unreliable as to be useless.