How do you think we know exactly in years when a time period Begins and end

Hello there.

Scientists decide when an era begins and ends, due to mass extinction. Scientists determine when drastic changes are made by the fossil record, which shows earths past organisms that have become extinct. Scientists also use the geologic time scale to illustrate the order in which events on Earth have happened...They used relative dating to divide Earth's past in several chunks of time when similar organisms were on Earth. Later, scientists used absolute dating to determine the actual number of years ago that events happened.

a. the production of carbon dioxide through the process of photosynthesis