A TIMELINE FOR THE PLANET
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The appearance
of complex creatures
Life started experimenting with complex forms about a
thousand million years ago, during the exciting late Proterozoic. Oxygen was beginning to appear in the
environment. But then came the terrible
Varangarian ice age. When the ice age
was over, totally new forms emerged. But
we start the story a little earlier than that.
Late Proterozoic
Algae Towards animals
Early days
It’s difficult to find out how life developed over
most the planet’s early life. Few if any
fossils have been found, and those which have are disputed. This shouldn’t surprise us. We’ve got used to scientists’ disputes. And what is really amazing is that there’s
any evidence at all of such small organisms so far back.
In particular there has been little to distinguish the
microfossils of ancient bacteria from those of ancient eukaryotes.
Fortunately, the new technique of ‘biomarkers’ have
come to the palaeontologists’ aid. We
discuss elsewhere how the walls of all cells
are made of detergent molecules – lipids in the jargon. Each strain of bacterium has its own
‘design’ of lipid. These lipid molecules
are as ephemeral as any other detergent.
But after the bacterium’s death, some types get converted in the sediment
to chemicals that survive more or less for ever. These are the biomarkers.
The biomarkers of the oxygen-producing cyano-bacteria are particularly distinctive, and they
have been found in some sediments dating back to the late Archaean (2.7
kMy). This was a surprise. Received wisdom had been that the Archaean
landscape had been so cooked, and otherwise mauled about, that no evidence
could possibly survive. This could be
why some scientists cannot believe that Schopf’s
famous fossils of 3½ kMy ago are genuine.
Clearly at least some late Archaean terrain was spared
too much upheaval. And these
particularly rich deposits show that apparently-modern cyanos were already
going strong at that time. Going strong
they may have been, but all the oxygen that they produced was clearly being
grabbed by something, because the environment was still pretty well oxygen-free
for another thousand million years (more).
The same deposits also contain biomarkers for the more
sophisticated eukaryotic cells that we are made
of. These biomarkers have been modified
from the ‘sterols’ that eukaryotic cells use to stiffen their membranes (think cholesterol and don’t eliminate it from your
diet completely).
So, by 2.7 thousand million years ago, the tree of
life had already begun to diverge in a major way. From what I’ve read, I reckon that
eukaryotes probably appeared very much earlier still.
This page is heavily indebted to ‘Life on a Young
Planet’ by Andrew Knoll. This is an
interesting book. It’s mainly about
Professor Knoll’s experiences, collaborating with other workers in field
studies. But Knoll is a leading
authority on the exciting developments during the late Proterozoic. And, buried deep in his book, are priceless
nuggets that I’ve not been able to glean from anywhere else.
The late
Proterozoic
By the late Proterozoic, our planet had
quietened down enough to enable life to get more complex. It was a slow business though. Oxygen was beginning to appear in the
environment (still strictly underwater). But
it was still in very short supply
. This forced everything to
remain tiny. Sizes are quoted in microns
(thousandths of millimetre). Ordinary
bacteria are around one micron, and an organism 100 microns across is huge.
How on earth do the palaeontologists find
these things? Because they now know the
kinds of place where such tiny things might be preserved – basically I think,
places that at the time were seriously muddy.
And they know what to look for.
But it’s still very much a labour of love. Weeks, months, years of searching before you
hit pay dirt.
This ‘outcrop’ lies in the thoroughly
inhospitable
The period also takes in the Great
Varangarian Ice Age that we mentioned earlier.
The
Spitsbergan also contains some of the
biomarkers that we’ve already discussed.
But shales of a similar age in the Grand Canyon and other places have
plenty of them.
Algae
Life’s first pioneers in communal living were the
bacteria. They’ve been stringing
themselves into long filaments right from the earliest days. And occasionally they club together into
larger bodies. I’m not sure why, but it
seems to have to do with running out of food, and forced marches to find
another source.
However bacteria have never got terribly serious about
it. That’s why we’ve just called their
developments ‘communal’ living.
Multicellular
living is a different kettle of fish altogether, and it was the algae that
first got anywhere with that.
Algae range from simple single-celled organisms to
huge forests of kelp and other seaweeds.
But they are simple organisms.
Algae are often classed as plants.
And to be sure they all have the same photosynthetic machinery as plants
– well almost all anyway. As we keep
saying, don’t expect things to be simple in this game.
But Algae don’t have any of the complex structures
that plants have. When the tide comes in
they strain upwards and display their full grandeur. But when it goes out, they just collapse on
to the beach and suffer in the sun.
Simple single-celled algae have probably been around
for a very long time – ever since the first single-celled eukaryote swallowed
its first cyano and kept it alive to do its photosynthesis for it (see cyano
link above).
At some point, the algae proliferated to join the
cyanos as major producers of oxygen.
These days they have taken over the role almost completely, producing
around 4/5ths of the net global production. The cyanos have become nothing
more than bit players.
By about a thousand million years ago they had
developed into large scale structures – the first seaweeds. They had already diverged into two different
forms, ‘red’ and ‘green’. There was not
a lot of difference at first. But 400
million years later (600 million years ago) this divergence had become
dramatic. I’ve not found a lucid
description of the difference between them.
But here are a couple of pictures of modern forms, from Wikipedia.
Here we have a bit of a mystery. Geologists have produced convincing evidence
of the mother and father of all ice ages that occurred between 765 and 600
million years ago (Knoll’s figures).
Some geologists believe that virtually the entire globe iced up, killing
off all life except a few pockets of bacteria living in exceptionally favourable
places.
Other geologists are less sure however, and neither is
Professor Knoll. There seems no doubt
that something pretty terrible happened.
But Knoll’s palaeontological evidence includes a number of species that
had carried on developing right it.
These algae are but one example.
Recently a veritable cornucopia has been found in
China (the Douchantuo formation). It’s
what the scientists call a Lagerstätten.
The most famous of these is the one that encapsulated the Burgess Shale.
What happens is that an underwater mud slide buries an
entire ecosystem. It’s flattened to be
sure, but the details are often preserved in the most exquisite detail. This one quickly buried eukaryotic algae and
seaweeds. Some appear remarkably similar
to modern brown algae, both in shape and in the way they reproduce vegetatively
and disperse their spores. Here’s a
modern picture from Wikipedia.
The ecosystem appears to be only 50-100 My older than the Burgess Shale. Yet they don’t contain any anatomically and
morphological complex animals. Complex
animals appear to have evolved from scratch in the interim.
Towards animals
The ancestors of animals
single-celled are called, in the jargon, protozoans. The name means first 
animals. They are able to move, like animals, and to
engulf microscopic victims. And the
algae made a very good lunch.
No remains of actual creatures have been found as far
as I know. But their spores and other
leavings were made of much sterner stuff.
Spiny microfossils like the one on the left are common in certain areas,
dated to shortly after the end of the ice age.
My friend Will Diver
told me once that you can tell when these simple creatures first started
moving, because they stirred up the mud.
Until about 800 million years ago the detail in the detritus that fell
to the bottom of lakes was extraordinary.
You could chart every single rainstorm and every single gale. This is because each change in the weather
deposited a very fine layer of different rubbish. But as soon as the tiny creatures began to
move, they mixed up the layers and it has never been possible since. From some 700 million years ago, shortly
before the start of the ice age, the creatures began to leave behind trails to
show where they’d been. They were
horizontal trails only though. It took
another million years for the creatures to learn how to climb.
By that time, the creatures were beginning to get
bigger. The picture above, on the
right, is of eggs and embryos of early animals. They seem to be of a similar
age to the spore above. We are not told what kind of animals they might
be. Presumably there’s no way of
telling. Even the eggs are 400-600
microns across. Finally Professor Knoll
gives us pictures of a testate amoeba fossil.
It comes from 750 year old rocks in the Grand Canyon. This is around the start of the ice age. And yet it is almost identical to the modern
testate amoeba also shown.
The next chapter in the story is the Great Cambrian explosion