How do you explain the fact that so much more is known about Cenozoic Earth and life history than about the previous intervals of geologic time?

The Cenozoic era is the most recent era and includes the emergence of mankind and its' ancestors. It includes earth history from 66 million years to almost the present and also is known as the age of mammals which includes mankind. Evolution of the mammals can be traced relatively easily since there are most likely more abundant fossils than back in say the Cambrian or especially the Precambrian.