|156131. Tue Mar 13, 2007 10:23 am
|... or 'Explosions (Cambrian), or 'Evolution', possibly.
Q: What poisonous gas nearly wiped out all life on Earth?
The first life on earth was single-celled photosynthesising bacteria, or 'blue-green algae' which used sunlight and Carbon Dioxide to generate its energy. Not very efficiently, but efficiently enough to work.
Their waste product was oxygen, which is very poisonous to them because it's very reactive (oxidation is very common in chemistry, and happens very easily, and it also damages cell walls). Fortunately, there was a lot of iron and sulphur around in the early seas, from the cooling mantle, and the oxygen reacted with that to form iron oxide and sulphate layers that you can see all over the geology of the planet.
However, when these oxidation partners ran out, the oxygen levels began to build up in the sea and the atmosphere, raising from a couple of percent to today's 21%. This had three big effects:
1) it killed off most of the original life, which could handle the poisonous oxygen.
2) it created an ozone layer in the atmosphere (which is what happens when you bombard oxygen with ultraviolet radiation from the sun).
3) it provided a niche for a far more efficient type of metabolism that used oxygen rather than carbon dioxide. And hence the oxygen-respiring cell was born.
Having stacks more energy, multi-cellular animals could really get about, and get pretty large, and even leave the sea altogether. This was helped by the ozone layer, which screens out dangerous ultraviolet light. This marked the Cambrian Explosion, as shown in the fossil record, where hundreds of thousands of new species 'suddenly' came into being, as they flooded into the new niches in the land, and in each other.
And now, ironically, mammalian waste gases, in the form of carbon dioxide and methane, are threatening to change the planet's ecosystem again, particularly as the number of remaining trees that can 'soak up' the carbon dioxide are quickly being cut down. We can expect triffid plagues any day...
The photosynthesisers may take over again, or we may just heat up far too much to support any kind of life, and turn into Venus, with its boiling runaway greenhouse' atmosphere and clouds of acid and ammonia.
Source: 'The Story of Life' by Richard Southwood (Oxford)