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Geologic Time Scale
Geologic Time Scale- Divides the history of Earth into a series of non-overlapping time
periods
Absolute age- Any method of measuring the age of an event/object in years
Era- One of the three long units of geologic time between the Precambrian and the present. End of
an era is marked by mass extinction
Three Eras:
Paleozoic- 544-245 million years ago
Mesozoic- 245-66 million years ago
Cenozoic- 66 million years ago- present
Period- Unit of geologic time that divides the eras
Precambrian Time- Geologic time begins with the Precambrian time
(formation of the Earth)
*Starts 4.6 billion years ago (4,600 million years ago)
* Ocean and continents form
*Single celled organisms (example- bacteria) 3.5 bya
* First plants (under water) 2.5 bya
* Ends 544 million years ago
Paleozoic Era- Explosion of life
*Starts 544 million years ago
* All life begins in sea (example- jellyfish & jawless fish)
*Next comes land plants (example-moss)
* Progression of life during this era  notice animals move to land after plants 
Invertebrates
Vertebrates
Fish
Land Plants
Amphibian
reptiles
*Mass Extinction (245 mya) hypothesis is climate change resulting from continental drift
into Pangaea
Mesozoic Era- Age of the reptile
*Starts 245 million years ago
*Pangaea breaks apart
*Progression of life
Reptiles
Dinosaurs
Mammal
Bird
*First flowering plant
*Mass Extinction (66 million years ago)- hypothesis is a meteorite hit the Earth
Cenozoic Era- Age of the mammals (current era)
*Starts 66 million years ago
* The ice age was part of this era
*Mammals continue to diversify and evolve
*Humans appeared within this era
*This is the present era
Mass extinction- When many types of living things become extinct at the same time
Single cell organism- These organisms were the first living things on Earth. They’re an
organism that contains one cell (example- bacteria)
Invertebrates- An animal without a backbone
(examples- jellyfish, sponge or worm)
Vertebrates- Animals that have a backbone
Examples: Birds, Fish, Reptiles, Amphibians and Mammals