They are engaged in defining the stages of hominid evolution and their artifactual record, and the assignment of a chronology to these stages.
Chronology is the science of measuring time and ordering of the things in time.
The half-life, or amount of time it takes for half of the measured isotope to decay can be measured directly if it’s quite short, but long half lives can’t – we have to compare the percentage of the isotope today with its stable daughter or we have to know how much isotope there was before the decay started.
In other words, an isotope with a very short half-life can’t be used to date very old fossils.
In the past, relative dating methods often were the only ones available to paleoanthropologists.