I came across one interesting report which I thought of sharing with you all.

The Earth's greatest mass extinction, 250 million years ago, was so severe that it took 10 million years for the planet to recover. Life was nearly wiped out, with only 10 per cent of plants and animals surviving. And the sheer intensity of the crisis, and continuing grim conditions on Earth after the first wave of extinction , meant the bounce-back was very slow.  The end-Permian crisis was triggered by a number of physical environmental shocks - global warming, acid rain, ocean acidification and ocean anoxia - and killed off 90 per cent of living things on land and in the sea.

"It is hard to imagine how so much of life could have been killed, but there is no doubt from some of the fantastic rock sections in China and elsewhere round the world that this was the biggest crisis ever faced by life." says Dr Zhong-Qiang Chen, from the China University of Geosciences in Wuhan.

The grim conditions continued in waves for five or six million years, with carbon and oxygen crises, warming and other ill effects.

"Life seemed to be getting back to normal when another crisis hit and set it back again," says Professor Michael Benton of the University of Bristol.

"The carbon crises were repeated many times, and then finally conditions became normal again after five million years or so."

Once the environmental crises ceased to be so severe, more complex ecosystems emerged. In the sea, new groups, such as ancestral crabs and lobsters, as well as the first marine reptiles, came on the scene, and they formed the basis of future modern-style ecosystems.

The causes of the killing - global warming, acid rain, ocean acidification - sound eerily familiar to us today.  

Perhaps we can learn something from these ancient events and be more active and quick  towards conservation of environment.