RE: Comment #472842 by malimbar04
"I remember hearing that the entire world's biosphere was nearly destroyed before photosynthesis evolved - the CO2 levels being astronomically high and poisoning all life."

Life evolved before free oxygen in the atmosphere, so the bacteria that released oxygen into the atmosphere were the first "polluters". Oxygen was actually poisonous to life at that time - at least until organisms evolved to use oxygen to produce energy.


RE: Comment #472913 by ridelo "if the Earth atmosphere once contained a lot of methane and then microbes began producing oxygen in ever greater quantities could it be that on some moment the mixture became self deflagrating producing a worldwide explosion£

Addition of oxygen to the atmosphere would have been a slow, but increasing, and continuous process. Oxygen and methane react with one another in the atmosphere, and so if methane is not continually replenished (usually by lifeforms, hence if we detect an exoplanet with both oxygen and methane in the atmosphere then that is good evidence that it has life) then its levels will drop. So as the early microbes added increasingly greater amounts of oxygen to the atmosphere, the methane levels would have dropped as it isn't replenished as quickly, but since it was a slow continuous process it is unlikely oxygen and methane would both have been in sufficiently high concentrations to have been explosive.