There have been six great extinction events in the Earth's 4.5 billion year history. The most renowned is the extinction of the dinosaurs at 66 million years ago. Scientists think this was the result of a massive asteroid strike and its aftermath. Another occurred more recently at the end of the Pleistocene 13,000 to 8,000 BCE, when the Earth lost most of its megafauna such as mammoths, probably due to climate change and human predation. In terms of the number of organisms lost, the greatest extinction is the Permian-Triassic, or the Great Dying, that affected primarily marine organisms and terrestrial invertibrates, 252 million years ago. During this extinction event ocean surface
temperatures reached 40℃; it was simply too hot for anything to survive. Earth lost some 90% of it's species.
Extinction is not just ancient history. There is another great extinction taking place right now, and this chart from the Center for Biological Diversity shows the grim fact:
The current crisis in the natural world is fundamentally different because this event is almost entirely caused by human activity that is driving habitat loss and global warming. In the past 500 years there have been known extinctions of 1000 species from passenger pigeons to woodland bison. The truth is no one knows exactly how many species have been lost. Many have disappeared before science was able to identify them. In the US alone a leading wildlife expert estimates there are 14 to 35
thousand endangered species.
