1st happened around 2.2 billion years BC (in short numerical system, aka 2 200 000 000). It happened because there were no oxygen producing organisms, but there was a plenty of methanic bacterias (they live in swamps) which produced methan, there was no oxygen in the atmosphere so Earth had pink atmosphere. Methan is stronger Greenhouse Gas than CO2 so the temperature was high. But then modern cyanophytes appeared some theories say they are older, but the best fossils begin from 2 billion BC). Anyways oxygen cooled Earth to Snowball Earth (-50°C at average). Live survived in hot springs where happened first endocaryot symbiosis (Eucaryota bacterias have been created).
After that volcanic activity began and helped life to get from $|-|1+$, but it created reverse extreme with Hypercanes, it's in that documentary film, too. Atmosphere contained just 1% of oxygen. Meanwhile happened the 2nd endocaryot symbiosis and algaes were created. But 800 million years BC something went wrong and oxygen went to high levels again (lower vulcanic activity, bad continent setting - they got to the South Pole and other factors), during this snowball era life created the collagen which helps cellules to live. After that, 600 million years BC, the multicellular organisms appeared and oxygen level dropped to lower level (40%?) (BTW, today level is also lower than 8000 years ago, if it drops bellow 15% -16% then we are screwed, giant insects had 40-60%, mammoth hunters had 25% or somehow ).
But truth is that it's not sure if this happened so and that weather disasters will happen more frequently if global warming continues.
Edited by partyzanpaulzy, 01 March 2009 - 15:25.