Understanding the ages of related fossil species helps scientists piece together the evolutionary history of a group of organisms.
For example, based on the primate fossil record, scientists know that living primates evolved from fossil primates and that this evolutionary history took tens of millions of years.
But we wouldn’t expect an all-knowing God to make that kind of a mistake. In essence, He gave us a “birth certificate.” For example, using a personal birth certificate, a person can calculate how old he is at any point. Genesis 1 says that the earth was created on the first day of creation ().
From there, we can begin to calculate the age of the earth.
By comparing fossils of different primate species, scientists can examine how features changed and how primates evolved through time.