A TIMELINE FOR
THE PLANET
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The first
serious life-forms, bacteria
Bacteria are
good
We are normally told that bacteria are evil and
wicked, and out to get us if we don’t get them first.
The truth is the precise opposite of this. Indeed you could argue that it’s bacteria
that rule our planet, and not us at all.
See what you think, after you have read on a bit.
Dawn of life The
essentials of life
Cyano-bacteria Why can’t we do our own chemistry?
To be sure, there are a few bacteria that can do us
severe 
harm,
and they have to go. But they are a tiny
minority.
Our guts are teeming with bacteria, all doing sterling
work on our behalf. They do the
difficult chemistry that our bodies can’t do for themselves (more). They break down the complex foodstuffs that
we feed to them, into simpler molecules that we can use to keep our bodies
going and growing.
Every teaspoonful of soil is teeming with bacteria,
doing something very similar. And
without them, plants couldn’t live.
All our nutrients come originally from bare rock. And only bacteria have the chemistry to
extract the nutrients directly from rock.
Everything else that plants feed on is recycled. And much of the recycling is done by bacteria
too.
Think about it.
But that’s only the start.
Something
very like modern bacteria seems to have been the first successful life
form. Life emerged on this planet
between 3½ and 4 thousand million years ago.
And the evidence suggests that it hit on a winning formula quite
quickly, and has changed little since.
Even today you could argue that it’s bacteria that really run the place;
and that we are just a sideshow.
For the first couple of thousand million years or so,
they were virtually the only show in town.
That’s nearly half the life of the planet so far! Our planet was still too turbulent for
anything more complex to stand a chance.
As it gradually settled down, more sophisticated organisms did begin to
emerge. And of course we tend to
concentrate on them, and to forget that the bacteria were still there, as
abundant as ever – and in very many ways, still running the place.
The essentials of life
There are two things that we need from the food we
eat. The first is the raw materials that
all life is made of. They are what
William Schopf, in his book Cradle of
Life, calls CHON – Carbon, Hydrogen, Oxygen and Nitrogen. (There are also assorted trace minerals that
life needs. But they are only minor
players in the game.)
The second thing is energy. A cell has to build these simple elements,
stage by stage, into the wide range of different amino acids, enzymes, DNA and
structural components, that go to make up the organism. This takes energy.
Life has invented just two ways of doing this
job.
Plants, and most bacteria, are self-feeders
(autotrophs in the jargon). They take in
simple nutrients and build them up into the chemical building blocks that they
need. They obtain their energy in
various ways, which we’ll come on to.
Animals, and the rest of the bacteria, are life’s
parasites (heterotrophs). We obtain both
our foodstuffs and our energy by preying on others.
Just to complicate matters, both these strategies can
be divided into primitive and advanced versions.
There was no oxygen around on the young planet. So the primitive versions are both
‘anaerobic’. Primitive these versions
may be, but there are plenty of bacteria using them today, where conditions are
suitable.
The very first bacteria will have been ‘anaerobic
heterotrophs’. They got both their
foodstuffs and their energy from chemicals lying around in the primordial
soup. Schopf assures us that the soup
contained plenty of glucose. So, to
‘burn’ this glucose, life quickly invented – wait for it – fermentation. This makes fermentation about the oldest
‘chemical pathway’ on the planet. Indeed
I’ve read that virtually all life’s later pathways are but modified forms of
it.
Next will have come the ‘anaerobic phototrophs’. When the primordial soup began to run out,
life had to find another source of energy.
Presumably by that time the suffocating carbon-dioxide atmosphere will
have thinned out a bit, allowing a certain amount of sunlight to penetrate the
shallower waters. This enabled a strain
of bacteria to invent a primitive form of photosynthesis. They used sunlight to help them break down
carbon dioxide (CO2) and hydrogen sulphide (H2S) to
provide their CH and O. The N was a
separate problem, which we will gloss over.
It’s important to point out that this process only provided just enough
oxygen for the cell’s needs. None was
left over for oxygenating the environment.
It was the sulphur that was leftover, and this is why geysers and such
are yellow with sulphur.
Now we come on to the advanced versions. First came the ‘aerobic phototrophs’. Eventually the supply of hydrogen sulphide
began to run out, and it forced a strain of bacteria to make one of life’s
greatest breakthroughs – true photosynthesis. It was the cyano-bacteria that mastered this
art. They used sunlight to obtain their hydrogen by breaking down water. Periodic table aficionados will know that
oxygen is in the same group as sulphur but one row up. So this is a logical development. But in fact it’s much more difficult. I’ve read that the photosynthesis pathway is
in fact a form of fermentation in reverse.
(Incidentally when plants came along, they didn’t do
anything clever like inventing any new form of photosynthesis. They stole the idea from the cyanos. Indeed they stole the entire cyano. The ‘chloroplasts’ wherein plants carry out
their photosynthesis show every sign of being captured cyanobacteria.)
The cyanos got all they oxygen they needed from the
carbon dioxide that they were also breaking down. The oxygen from the water was surplus to
requirements, and it was strong-armed out of the cell before it could do any
damage.
Damage? Oxygen
is actually a deadly poison. It combines
so readily with life’s other vital components that it can easily get red
hot.
Other bacteria either had to learn to protect
themselves against this deadly poison, or they had to hide away from it. And even today there are strains that do
both.
So it was the cyanos that began the oxygenation of the
planet. For the first thousand million
years or so the wider environment saw little of this oxygen. Other bacteria used up the oxygen as fast as
the cyanos produced it.
The scene was now set for the advanced (aerobic)
heterotrophs to appear. Once the
heterophs had learned to live with this deadly oxygen, they were in a position
to exploit it. They kept the anaerobic
fermentation pathway, and tacked an aerobic step on the end. Think of it this way. Fermentation breaks down sugar to produce
alcohol, and obtains a small amount of energy in the process. That’s as far as you can go without oxygen,
and of course it’s as far as many of us want it to go. But alcohol is actually an excellent
fuel. Once you have oxygen, you can
‘burn’ it and get 18 times more energy.
Cyano-bacteria
Possibly the true masters of creation were the cyano-bacteria.
The name comes from their colour, and they are otherwise known as ‘blue-green
algae’. They are not algae at all. They are bacteria and proud of it.
How long ago was did the cyanos make their
breakthrough? Some authorities think it
was some 2½ thousand million years ago.
Others put it a thousand million years earlier than that (more).
Either way, it was early in the life of our planet.
The cyanos prospered and multiplied, spewing out
oxygen in huge quantities. And slowly,
slowly the entire planet became oxygenated (more).
Today the cyano-bacteria are
regarded mainly as a nuisance. But they
are still giving us oxygen today, although it seems that simple algae, having enslaved
and absorbed individual cyano bacteria, have also stolen their role in
life. These days it’s the algae that
produce about 4/5ths of our oxygen (and you thought it was
plants!). Gratitude does not figure
largely in evolution.
Why can’t we do our own chemistry?
Well we do in fact do a lot of our own chemistry. However I suspect that the principle of “why
keep a dog and bark yourself?” comes into play too. Bacteria have been doing chemistry since the
world was young. They long ago mastered
anything that they or we are likely to need.
Besides every time we change our diet, go on holiday for example, we
need to change our chemistry. Bacteria
can do that. Our gut ‘flora’ contains a
wide range of different types. Many just
eke out a living on our normal diet. But
when we feed them the right raw materials, they can proliferate within a few
days. However that’s not all. Within limits, bacteria can actually alter
their internal workings, if a change of diet is forced on them. Why would we try to compete?
Click for the next phase, the eukaryotic cell.
© C B Pease, November 07