Which statements about geologic time are true? Check all that apply.

Scientists developed the geologic time scale as they studied rock layers and index fossils.
The fossil record helps scientists describe life only in the seas and other bodies of water.
Precambrian time is the most recent time in Earth’s history.
Precambrian time makes up 88 percent of Earth’s history.
The first birds appeared during the Mesozoic Era.
The basic units of the geologic time scale are periods, eras, and centuries.
Humans appeared during the Cenozoic era.