How did geologists decide where one division of the geologic time scale ends and the next begins?

the law of superpostion

events in the history of life on earth, like mass extinctions

they divided the time equally between eras

the placement of fossils throughout the surface layers of earth

The placement of fossils throughout the surface layers of Earth made the geologists decide where one division of the geologic time scale ends and the next begins.

Explanation:

Fossils play a major role in identifying or deciding the geologic time scale eras or eons. Fossils are the only evidence in the form of specimen remains like bones, leaves, shells, teeth, or burrows, tracks etc all of which provide information about life on Earth during a particular time.

Fossil placement on earth’s surface creates fossil records. On the earth’s surface like rocks, a younger fossil will be buried on a layer above the older fossil layer. Each layer will represent a unique life form that existed during a time period and determines the division of geologic time scale.

In fact each of the eras like Paleozoic, Mesozoic, or Cenozoic ends with the suffix zoic since the time change of these eras is based on the evolution of animal life during that period .

the sun creates it's own light, also gives light and life to bunch of stuff like plants,animals,humans etc.photosyntesis is the process of making food which needs sun,water,nutrients etc.