Time Span Geologic Time Period Major Biological Event(s)

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Geologic Time Scale

Time Span

Geologic Time Period

Major Biological Event(s)

5.7-4.5 BYA*

Origin of the Earth


Origin of life on Earth

3.5-1.5 BYA

Archaean Era

“Ancient” Era

About 3 BYA

Origin of photosynthesis—oxygen in the atmosphere—followed by the first and possibly greatest extinction period as anaerobic life forms succumbed to the poisonous effects of molecular oxygen.

About 2 BYA

First eukaryotic organisms

1.5-.57 BYA

Proterozoic Era

“First Animal” Era

About 1 BYA

First multicellular organisms

700-570 MYA**

The Ediacaran Fauna

570-225 MYA

Paleozoic Era

“Ancient Animal” Era

570-500 MYA

Cambrian Period

Cambrian explosion; origin of many kinds of marine invertebrates, including Trilobites. All modern phyla of invertebrates appeared, along with a number of phyla which are no longer found on Earth.

500-425 MYA

Ordovician Period

First vertebrates (all aquatic); First life on land (plants).

425-405 MYA

Silurian Period

Rise of fish; Agnatha (jawless fish) and Placodermi (armored fish)

405-345 MYA

Devonian Period

Often called the Age of Fishes. Modern classes of fish appear (Osteichthes and Chondrichthes), First movement of animals onto land (Arthropods). Insects appear by 400 MYA. First Amphibia appear late in the Devonian.

345-280 MYA

Carboniferous Period

Often called the Age of Amphibia. Amphibia diversify on land; first reptiles appear. Also often called the Age of Ferns. The dominant plant forms were the Lycopods, members of the Fern Allies. These plants formed the Earth’s first forests, and their remains produced major deposits of fossil fuels.

280-225 MYA

Permian Period

Amphibia decline, reptiles increase. Mammal-like reptiles appear. Ancient continents coalesce to form the super-continent Pangaea.

225 MYA

Permian Extinction: During this period of time, 50% of all families of life on Earth became extinct. It is estimated that this meant the extinction of up to 96% of all species.

225-65 MYA

Mesozoic Era

“Middle Animal” Era; The Age of Dinosaurs

225-197 MYA

Triassic Period

First dinosaurs, protomammals give rise to first true mammals.

197-135 MYA

Jurassic Period

Dinosaurs at height. First placental mammals. First birds.

135-65 MYA

Cretaceous Period

Dinosaurs still predominant; mammals abundant, but tiny and nocturnal insectivores. Flowering plants appear, leading to great adaptive radiation among insects.

65 MYA

Cretaceous/Paleocene Discontinuity (K-T Boundary): 70% of all animal species become extinct, including most of the dinosaurs. All kinds of living things were affected by this mass extinction, including plants. A large percentage of the plant species on Earth became extinct, including the dominant group of phytoplankton.

Time Span

Geologic Time Period

Major Biological Event(s)

65-0 MYA

Cenozoic Era

“Recent Animal” Era; The Age of Mammals

65-2 MYA

Tertiary Period

65-54 MYA

Paleocene Epoch

Beginning of dominance of mammals; primitive primates appear. Grasses and early grazers appear

54-35 MYA

Eocene Epoch

All modern mammalian orders exist; early Cetaceans appear.

35-23 MYA

Oligocene Epoch

All modern mammalian families exist; first modern birds and apes appear.

23-6 MYA

Miocene Epoch

Whales appear.

6-2 MYA

Pliocene Epoch

Large extinction of early mammals; probable separation of hominid line from other hominoids.

2-0 MYA

Quaternary Period

Age of Hominids (Man and close ancestors)

2-.01 MYA

Pleistocene Epoch

Early human evolution; appearance of Homo sapiens.

10-0 TYA***

Holocene Epoch

Rise of human civilization/

*BYA = Billion Years Ago

**MYA = Million Years Ago

***TYA = Thousand Years Ago

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