Half lives used carbon dating
One way that helps scientists place fossils into the correct era on the geologic time scale is by using radiometric dating.
It uses the naturally occurring radioisotope carbon-14 (14C) to estimate the age of carbon-bearing materials up to about 58,000 to 62,000 years old. Carbon-14 has a relatively short half-life of 5,730 years, meaning that the fraction of carbon-14 in a sample is halved over the course of 5,730 years due to radioactive decay to nitrogen-14.
From this science, we are able to approximate the date at which the organism were living on Earth.
Radiocarbon dating is used in many fields to learn information about the past conditions of organisms and the environments present on Earth.
The half-lives of several radioactive isotopes are known and are used often to figure out the age of newly found fossils.
Different isotopes have different half-lives and sometimes more than one present isotope can be used to get an even more specific age of a fossil.