Geological Time Scale 

The geologic time scale is a chronological dating system that classifies geological strata in time. This time scale is used by geologists, Paleontologists, and other Earth scientists to describe timing, relationships, and other events related to our planet in geologic history.

  • The geologic time scale is a calendar for events in Earth’s history.
  • An Eon is the largest period of geological time.
  • The subdivisions Eras are formed by the combination of periods, we can say eras are subdivided into periods.
  • Epochs combine together to form periods.
  • The epoch is the smallest unit of time scale.

Table: Geological Time Scale

The geological time scale depicting the evolution of life forms from unicellular bacteria to modern man is given below.

Era Period Epoch Age Life/major events
Cainozoic (From 65 million years ago to present time) Quaternary Holocene



10,000 – 2 Million

Modern Man

Homo Sapiens

Tertiary Pliocene 2-5 Million Early Human Ancestor
Miocene 5-24 Million Ape: Flowering Plants and Trees
Oligocene 24-37 Million Anthropoid Ape
Eocene 37-58 Million Rabbits and Hare
Palaeocene 57-65 Million Small Mammals: Rats-Mice
Mesozoic 65-245 Million Mammals Cretaceous 65-144 Million Extinction of Dinosaurs
Jurassic 144-208 Million Age of Dinosaurs
Triassic 208-245 Million Frog and turtles
Palaeozoic 245-570 Million Permian 245-286 Million Reptile dominate-replace amphibians
Carboniferous 286-360 Million First Reptiles: Vertebrates: Coal beds
Devonian 360-408 Million Amphibians
Silurian 408-438 Million The first trace of life on land: Plant
Ordovician 438-505 Million First Fish
Cambrian 505-570 Million No terrestrial life: Marine Invertebrate
Pre-Cambrian 570 Million -4,800 Million 570-2,500 Million Soft-bodied arthropods
2,500-3,800 Million Blue-green algae: Unicellular bacteria
3,800-4,800 Million Oceans and Continents form-Ocean and Atmosphere are rich in Carbon dioxide
5,000 – 13,700 Million 5,000- Million


12,000 Million

13,700 Million

Origin of the universe


Origin of the Sun stars

Supernova, Big Bang

The image given below shows the geological time scale:

Origin of Life

The last phase in the evolution of the earth relates to the origin and evolution of life. Modern scientists refer to the origin of life as a kind of chemical reaction, which first generated complex organic molecules and assembled them. This assemblage that they could duplicate themselves by converting inanimate matter into living substances.

  • The record of life that existed on this planet in different periods is found in rocks in the form of fossils.
  • The microscopic structures closely related to the present form of blue algae have been found in geological formations much older than some 3,000 million years.
  • It can be assumed that life began to evolve sometime 3,800 million years ago.


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The Origin and Evolution of Earth - PCSSTUDIES Geography · March 26, 2023 at 1:14 pm

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