Source: The Conversation – UK – By Queenie Hoi Shan Chan, Lecturer in Earth Sciences, Royal Holloway
More than 99% of all organisms that have ever lived on Earth are now extinct. Most of these just died out quietly. However, in Earth’s history there have been five major mass extinction events – known as the big five – during which many species became extinct at the same time.
Each of the big five events caused at least a 40% loss of all species on Earth. Yet humans hold a particular grudge against the most recent one, which brought a sudden end to the 160-million-year history of the dinosaurs. This was the Cretaceous-Paleogene extinction and it happened 66 million years ago, wiping out about 75% of all species on Earth at the time. Except sea turtles and crocodiles, no four-legged animals bigger than 25kg survived.