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2015-07-02 07:45:06
Up until 2.4 billion years ago, there was no oxygen in the air. It took
something big to change that perhaps the biggest evolutionary leap of all
If you could build a time machine and go back to Earth's distant past, you'd
get a nasty surprise. You wouldn't be able to breathe the air. Unless you had
some breathing apparatus, you would asphyxiate within minutes.
For the first half of our planet's history, there was no oxygen in the
atmosphere. This life-giving gas only started to appear about 2.4 billion years
ago.
This "Great Oxidation Event" was one of the most important things to ever
happen on this planet. Without it, there could never have been any animals that
breathe oxygen: no insects, no fish, and certainly no humans.
For decades, scientists have worked to understand how and why the first oxygen
was pumped into the air. They have long suspected that life itself was
responsible for creating the air that we breathe.
But not just any life. If the latest findings are to be believed, life itself
was undergoing a tremendous transformation just before the Great Oxidation
Event. This evolutionary leap forward may be the key to understanding what
happened.
Earth was already 2 billion years old at the time of the Great Oxidation Event,
having formed 4.5 billion years ago. It was inhabited, but only by
single-celled organisms.
They evolved a way to take energy from sunlight
It's not clear exactly when life began, but the oldest known fossils of these
microorganisms date back 3.5 billion years, so it must have been before that.
That means life had been around for at least a billion years before the Great
Oxidation Event.
Those simple life-forms are the prime suspects for the Great Oxidation Event.
One group in particular stands out: cyanobacteria. Today, these microscopic
organisms sometimes form bright blue-green layers on ponds and oceans.
Their ancestors invented a trick that has since spread like wildlife. They
evolved a way to take energy from sunlight, and use it to make sugars out of
water and carbon dioxide.
This is called photosynthesis, and today it's how all green plants get their
food. That tree down your street is pretty much using the same chemical process
that the first cyanobacteria used billions of years ago.
It was the cyanobacteria, pumping out unwanted oxygen, that transformed Earth's
atmosphere
From the bacteria's point of view, photosynthesis has one irritating downside.
It produces oxygen as a waste product. Oxygen is of no use to them, so they
release it into the air.
So there's a simple explanation for the Great Oxidation Event. It was the
cyanobacteria, pumping out unwanted oxygen, that transformed Earth's
atmosphere.
But while this explains how it happened, it doesn't explain why, and it
certainly doesn't explain when it happened.
The problem is that cyanobacteria seem to have been around long before the
Great Oxidation Event. "They're probably among the first organisms we have on
this planet," says Bettina Schirrmeister of the University of Bristol in the
UK.
Maybe the cyanobacteria changed
We can be confident that there were cyanobacteria by 2.9 billion years ago,
because there is evidence of isolated "oxygen oases" at that time. They might
date as far back as 3.5 billion years, but it's hard to tell because the fossil
record is so patchy.
That means the cyanobacteria were busy pumping out oxygen for at least half a
billion years before oxygen started appearing in the air. That doesn't make a
lot of sense.
One explanation is that there were a lot of chemicals around perhaps volcanic
gases that reacted with the oxygen, effectively "mopping it up".
But there's another possibility, says Schirrmeister. Maybe the cyanobacteria
changed. "Some evolutionary innovation in cyanobacteria helped them to become
more successful and more important," she says.
Some modern cyanobacteria have done something that, by bacterial standards, is
remarkable. While the vast majority of bacteria are single cells, they are
multicellular.
Multicellularity could have been a game-changer for Earth's early cyanobacteria
The individual cyanobacterial cells have joined up into stringy filaments, like
the carriages of a train. That in itself is unusual for bacteria, but some have
gone further.
"Many cyanobacteria are able to produce specialised cells that lose their
ability to divide," says Schirrmeister. "This is the first form of
specialisation we see." It's a simple version of the many specialised cells
that animals have, such as muscle, nerve and blood cells.
Schirrmeister thinks multicellularity could have been a game-changer for
Earth's early cyanobacteria. It offers several possible advantages.
On the early Earth, single-celled organisms often lived together in flat layers
of gunk called "mats". Within each mat there would have been many different
species of cyanobacteria, and a host of other things to boot.
The Earth was being bombarded with harmful ultraviolet radiation from the Sun
A multicellular cyanobacterium would have one clear advantage compared to its
single-celled rivals. It would find it easier to spread, because its larger
surface area would mean it was better at attaching itself to slippery rocks.
Such an organism would be "less likely to wash away in the current", says
Schirrmeister.
Many modern multicellular cyanobacteria can move around within their mats.
"They're not extremely fast but they can move," says Schirrmeister. That
suggests the primordial ones could as well.
Moving could have helped them survive. At the time the Earth was being
bombarded with harmful ultraviolet radiation from the Sun, and there was no
ozone layer to keep it out.
"In modern mats, cyanobacteria will turn around and appear vertical instead of
horizontal to protect themselves from excess sunlight," says Schirrmeister.
"You have also movement between layers. It might be these multicellular
cyanobacteria had the ability to position themselves optimally within the mat."
It's a neat idea. But for it to be true, cyanobacteria must have evolved
multicellularity before the Great Oxidation Event.
Schirrmeister has spent the last few years trying to figure out when
cyanobacteria first evolved multicellularity.
The clues lie in their genes. By examining genes that all cyanobacteria share,
and identifying tiny differences between them, Schirrmeister could figure out
how they are all related essentially drawing up a family tree of
cyanobacteria.
With that tree in place, Schirrmeister could then home in on the multicellular
cyanobacteria, and estimate roughly when they first became multicellular.
Her first attempt, published in 2011, suggested that most modern cyanobacteria
are descended from multicellular ancestors. That suggested multicellularity was
ancient, but it was difficult to put a firm date on it.
Her family tree was only based on one gene
Schirrmeister refined her methods for a second paper, published in 2013. This
suggested that multicellularity evolved not long before the Great Oxidation
Event, at a time when cyanobacteria were diversifying rapidly.
But that didn't clinch the argument. Her family tree was only based on one
gene, albeit a gene shared by every single species of cyanobacterium. That
meant the tree was suspect.
So Schirrmeister has now gone one better.
"This time I worked with 756 genes," says Schirrmeister. "The genes I took are
present in all cyanobacteria."
We have multicellularity evolving before the Great Oxidation Event
Her estimate of the origin of multicellularity is still rough, but it seems to
be around 2.5 billion years ago before the Great Oxidation Event.
There are several different ways to calculate these family trees, and they all
gave the same answer. "No matter how we calibrate our phylogeny, it seems more
likely we have multicellularity evolving before the Great Oxidation Event,"
says Schirrmeister.
The results are published in Palaeontology.
This may not be the end of the story. Even if Schirrmeister's results are
confirmed, and cyanobacteria did become multicellular just before the Great
Oxidation Event, there are two big questions.
It is one of the most important things to ever happen on this planet
The first is, did multicellularity really offer them the advantages she thinks
it did? We don't know, but we could find out: by testing how modern
single-celled and multicellular cyanobacteria cope with different situations.
The second question is harder: why did it take so long for cyanobacteria to
become multicellular? If it is so advantageous, why did they not evolve it
sooner, and trigger an earlier Great Oxidation Event?
"The next step is to find out which genes are responsible for multicellularity
in cyanobacteria," says Schirrmeister. "Then I could say why did it take that
long, why didn't it evolve earlier." If lots of new genes were required, it
becomes understandable that it took the cyanobacteria a long time to evolve it.
Whatever caused the Great Oxidation Event, it's clear that it is one of the
most important things to ever happen on this planet.
In the short term, it was probably rather bad news for life.
"Oxygen would have been lethal for many bacteria," says Schirrmeister. "It's
hard to prove, because from the fossil record we don't have a lot of deposits
from that time [but] we can assume we had a lot of bacteria dying at that
point."
Those first multicellular cyanobacteria triggered the evolution of complex life
But in the longer term, it allowed a whole new kind of life to evolve. Oxygen
is a reactive gas that's why it starts fires so when some organisms figured
out how to harness it, they suddenly had access to a major new source of
energy.
By breathing oxygen, organisms could become much more active, and much larger.
Moving beyond the simple multicellularity developed by cyanobacteria, some
organisms became far more intricate. They became plants and animals, from
sponges and worms to fish and, ultimately, humans.
If Schirrmeister is right, those first multicellular cyanobacteria triggered
the evolution of complex life, including us, by producing oxygen on a global
scale. "It made complex life possible," she says.
Not bad for a bunch of tiny blue-green bacteria.