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The Earth was formed around 4.6 billion years ago. Scientists have broken this time down into chunks of time with different scales.

In order of scale, we have Eons, Eras, Periods and Epochs. These geological time units are based on the classification of layers of rock (stratigraphy) and are identified by the fossils and/or minerals found in them. Often the boundaries between these epochs are marked by mass extinctions that have led to the end of certain lineages of organisms the diversification of other groups of organisms and the evolution of new species. These are characterised by massive environmental changes caused by geological events such as meteorite hits on the Earth, changes in the atmosphere due to the evolution of life, massive volcanism and climate changes to Hothouse Earth or Snowball Earth.

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Watch this video from PBS (12 min) to understand how these geological timescales have been described and some of the important events that happened in particular epochs.

There have been five mass extinction events and these have been followed by rapid rates of speciation as new opportunities for resource partitioning and new niches become available. Extinction is the complete disappearance of a species and it is forever; once the genetic resource of that species is gone it cannot be replaced naturally. Extinction has always occurred and scientists can calculate the background rate of extinction. This can then be compared to current rates of extinction and this has led scientists to say that we are in a current 6th mass extinction event.

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You are expected to research and summarize each of the mass extinction event. Give brief note about the event, timescale, possible cause, organisms that lived during the event.

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the Ordovician-Silurian extinction. It was theoretically caused by rapid climate change, primarily affecting marine life. It happened 443 MYA, affecting marine life such as trilobites

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Devonian extinction, occurred 375-360 million years ago.

possible causes: climate change, anoxia (lack of oxygen) in oceans, volcanic activity, and possible asteroid impacts.

marine life was primarily affected, such as coral-building life and early fish

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The Great Dying happened around 252 MYA, killing approx. 90-96% of all species.

possible causes: anoxia, volcanic eruptions

pretty much everyone was affected

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Triassic-Jurassic Extinction

Occurred 10 000 years to a few million years

Possible cause: Volcanic eruptions in the Central Atlantic Magmatic Province which leads to global warming

organisms that were present at the time- early dinosaurs, reptiles, amphibians and marine species

Impact: 80% of species went extinct which then allowed for the dinosaurs to dominate

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Cretaceous-Paleogene

66 million years ago

Possible cause: asteroid and volcanic eruptions which leads to climate change

Organisms at the time: Dinosaurs, marine reptiles, ammonites, flowering plants, and early mammals.

Impact: 75% of species went extinct, including non-avian dinosaurs (not of or relating to birds)

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Holocene/Anthropocene Extinction

Ongoing over the past 10 000 years

Possible cause: deforestation, climate change, pollution, hunting, poaching and invasive species

Organisms at the time: mammals, birds, amphibians, marine life

Impact: Huge decline in biodiversity e.g. species like the dodo