Mass extinction

An event  when at least half of all species die out in a relatively short time. These events leave Earth ripe for evolutionary changes as new species develop to take the places of those lost.
Scientists have discovered at least five different mass extinctions, referred to as the Big 5, over history when anywhere between 50% and 75% of life were lost.
  • Ordovician – SIlurian (439 million years ago)
  • Late Devonian  (364 million years ago)
  • Permian-triassic  (251 million years ago)
  • Triassic-Jurassic (between 199 million and 214 million years ago)
  • Cretaceous – Paleogene (65 million years ago)
Main identified causes were: glacial cycles on Earth, and corresponding changes in sea level, changes in atmospheric and oceanic chemistry and circulation, decrease in oxygen levels in the ocean, volcanitc activity, asteroid impact, climate change.
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