Geologic time is divided into hierarchical units including eras, periods, and epochs based on changes in the Earth's surface, climate, and lifeforms. The Precambrian Era spans 88% of Earth's history from 4.6 billion to 542 million years ago during which the atmosphere and life originated. The Paleozoic Era followed and saw the rise of vertebrates and a mass extinction event. During the Mesozoic Era continents drifted apart and dinosaurs dominated until an asteroid caused a mass extinction. The current Cenozoic Era is characterized by decreasing temperatures, the age of mammals, and early humanoid fossils.

1. GEOLOGICTIME

2. Geologic History
■ Determined by major characteristics in the
Earth’s surface, climate, and types of
organisms

3. Era:
■ A unit of geologic time that contains two
or more periods

4. Period:
■ A unit of geologic time that is longer than
an epoch, but shorter than an era

5. Epoch:
■ A unit of geologic time that is longer than
an age but shorter than a period

6. Eras & Periods

7. Eras, Periods, & Epochs

8. Age of the Earth
■ 4.6 BillionYears Old

9. Precambrian Era:
■ 4.6 Billion to 542 MillionYears Ago
■ 88% of the Earth’s History
■ Very little is known because rock layers
have been eroded and deformed by plate
tectonics
■ Period during which the atmosphere and as
a result life originated

10. Paleozoic Era:
■ 542 Million to 251 MillionYears Ago
■ Continents went from separate to Pangea
during this time period
■ Far richer in plant and animal life
■ Vertebrates such as fish, reptiles, and
amphibians developed
■ 90% of marine and 70% of land life died at
end of era

11. Mesozoic Era:
■ 251 million years to 65.5 million years ago
■ Continents begin to drift apart
■ Age of Reptiles, including dinosaurs
■ Mass extinction at end, possibly by an
asteroid
■ Flowering plants begin to develop

12. Cenozoic Era:
■ 65.5 million years ago to Present
■ Temperatures decreased
■ Age of mammals
■ Fossils of first humanoids began to develop