Not all all mass extinctions are caused by extraterrestrial events

In fact, for as long as life has existed it has been well able to devastate itself. Charles Darwin envisaged newly evolved life forms entering the world like a wedge, easing into a narrow vacant niche then expanding it gradually. Some do. But others enter like a sledgehammer, smashing away entire branches of the tree of life as they arrive.

SHOOT: In fact it seems that massive planet-wide extinctions - caused by Earth-based systems - were the rule, rather than the exception. Which means, theoretically, man creating a crisis event and then adapting to it, may stand us in good stead for surviving/adapting to a crisis that is not anthropogenic.

clipped from www.newscientist.com Sarah Howell)" /> Earth has repeatedly endured "Medean" events - drastic drops in biodiversity and abundance driven by life itself - and will do so again in the future (see a timeline of Medean events). Around 2.3 billion years ago, for example, Earth endured a gigantic episode of glaciation that lasted 100 million years. It was so intense that the oceans froze completely, creating a "snowball Earth". The cause was life itself. Around 200 million years earlier, evolution had come up with a novel way to make a living: photosynthesis, the process that uses the energy in sunlight to convert inorganic CO2 into sugars. Photosynthetic microbes sucked so much heat-trapping CO2 out of the atmosphere that the planet was plunged into the freezer. A second episode of snowball Earth, brought about by the evolution of the first multicellular plants, happened 700 million years ago.