How did scientist use changes in organisms living on the planet to create the geologic time scale

The scientists have used the common historical and functional basis of living organisms to create the geological time period.

Explanation:

All the living things present on earth have gone through the evolutionary process. According to the researches, the oldest fossil found on the earth can be traced 3.5 to 3.7 billion years back, and life today on earth has been evolved from a common ancestor through natural selection or adaptions.

The geological time period shows all the changes in living organisms in different eras eons,  eras, periods, epochs.

b. the carbon cycle ensures living things have a source of hydrogen