The vast majority of population is well aware of the Cretaceous-Tertiary extinction, which gained its recognition due to the disappearance of dinosaurs. It took place around 65 mln years ago and wiped out a huge part of the past flora and fauna. The suggested theory is commonly known too - the asteroid hitting the surface of the planet. The mentioned mass extinction however, was not the only one throughout the Earth's history. Interestingly, there are said to have been at least 5 mass extinctions throughout the planet's life. The other four include:

- End Triassic extinction - around 200 mln years ago, presumed to have been caused by lava eruptions and massive floods, 22% of life in the oceans vanished;

- Permian Triassic extinction - took place around 250 mln years ago, this was probably the most destroying one, it is estimated that 95% of all species existing back then disappeared from the surface of the planet, we need to bear in mind that life back then was not fully evolved and consisted mainly of organisms occupying waters, the causation is said to be a comet or asteroid striking the planet;

- Late Devonian extinction - happened around 370 mln years ago, cause is unknown also more details are hard to define;

- Ordovician - Silurian - around 450 mln years ago, caused by the drop in the sea levels and the forming of glaciers, followed by the melting of these and causing mass decrease in numbers of organisms;

Interestingly, some scientists argue that we are currently stepping onto the sixth mass extinction. Their argument is supported with various facts, such as the present biodiversity crisis. The action bioscience website lists the following reasons for the biological diversity loss:

- "human destruction of ecosystems
- overexploitation of species and natural resources
- human overpopulation
- the spread of agriculture
- pollution"
(http://www.actionbioscience.org/newfrontiers/eldredge2.html)

Let's hope the 6th extinction will be at least slowed down...
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