Geologic Time Scale

Geologic Time is the time scale of the history of the earth and its life.

Geologic Time Scale

Time Span	Geologic Time Period	Major Biological Event(s)
5.7-4.5 BYA*		Origin of the Earth
	[Precambrian]	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