A TIMELINE FOR THE PLANET click for Home Page
And it wasn’t plants that made it. It was bacteria.
Many scientists believe that the bacteria that worked
this astounding trick were very similar to their modern descendants. They are still arguing over when
photosynthesis might have evolved. But
it was certainly way back in the early days of the planet.
Like most evolutionary breakthroughs, it was triggered
by stark necessity. Even today,
volcanoes provide ‘free’ supplies of energy-rich food. And they support entire ‘ecosystems’ of
‘anaerobic’ life. Initially the entire
planet was peppered with volcanoes, and life needed no other life support. But as time went on, they became fewer.
Volcanoes were dying all over the planet, and their
ecosystems were dying too. Then, just
once, around a single volcano, a bacterium found the answer – photosynthesis.
It will have been a step-by-step process, as with most
such breakthroughs. But these bacteria
quickly found themselves using the power of sunlight to break down carbon
dioxide from the atmosphere, to get at the carbon. The oxygen was surplus to requirements and
was strong-armed out of the cell before it could do too much damage. (Oxygen is actually a deadly poison.)
This a travesty of how photosynthesis really works,
and the biologists will be after my
blood for it. But we have to avoid
getting bogged down; and it’s the sunlight and the carbon-dioxide bit that’s
important to our story. {Link to proper
description of photosynthesis when I can find one!.}
The bugs that worked the trick were probably very similar to today’s
‘cyanobacteria’ or ‘blue-green algae’.
These are not algae. They are
bacteria, and proud of it. These are the
guys that made the stromatolites, and are
still making them today.
Other bacteria found (after arming themselves against the
poison) that they could use the oxygen to ‘burn’ the carbon-rich dead
remains. So ‘aerobic’ bacteria evolved
and soaked up some of the oxygen. We
animals are aerobic too of course.
They’d have soaked up all the oxygen, if they could
have got hold of all the remains. But
fortunately for us, a great deal of the dead material gets buried out of
reach. Once the cyanos really got going,
huge amounts of oxygen were left over.
Make no mistake about this. Oxygen production is only one factor
controlling the amount available in the environment. As we’ve just seen, given half a chance,
aerobic bacteria will consume the oxygen as fast as it’s produced – and convert
it back to carbon dioxide.
It’s carbon
burial that leaves us with oxygen to breathe. Fortunately a number of geological processes
bury carbon for us. The most famous
episode is of course the great Carboniferous
coal forests, of 360 million years ago.
In fact the oxygen story begins much earlier than
this. We cover it in more detail under ‘oxygen’.
The oxygenation of the planet began. The entire surface of the planet became
flooded with oxygen, from the ocean depths to the highest stratosphere. I’ve read that even the material deep down in
the Earth’s mantle has been oxygenated courtesy of the cyanobacteria.
And virtually all of the carbon dioxide, that formed
the suffocating greenhouse of an early atmosphere, has been extracted. A little of the carbon is inside us. The rest is buried, mainly as ‘carbonate’
rock (chalk and limestone) and as fossil fuel.
This is one of the occasions where it really is
appropriate to say that ‘the world was changed for ever’.
When did it happen?
Opinions vary. But I’m tempted to
follow Professor Andrew Knoll, and go for serious oxygenation starting around 1
kMy ago (see above link).
© C B Pease, December 07