When you think about the basic ingredients for
life to thrive on Earth, no doubt water and oxygen
pop to mind. But there was a time on our planet when
our atmosphere only had one-one thousandth of one
percent of the amount of oxygen it has now, yet
there were plenty of life forms around then too,
although proof of them has been scarce. A recent
discovery of fossilized bacteria dating to about 2.5
billion years ago provides long-sought-for evidence
that the Earth was crawling with life even though it
lacked much oxygen during a phase in our planet's
development known as the
Neoarchean Eon.
"These fossils represent the oldest known
organisms that lived in a very dark, deep-water
environment," says Andrew Czaja, the assistant
professor of geology at the University of Cincinnati
in the US who made the discovery. "These bacteria
existed two billion years before plants and trees,
which evolved about 450 million years ago. We
discovered these microfossils preserved in a layer
of hard silica-rich rock called chert located within
the Kaapvaal craton of South Africa."
The
Kaapvaal craton is located in South Africa's
Northern Cape Province and it's thought that the
bacteria that became fossilized there lived deep
beneath the sea while feeding off sulphur, much in
the same way some
extremophile bacteria still do today.
"These are the oldest reported fossil sulfur
bacteria to date," said Czaja. "And this discovery
is helping us reveal a diversity of life and
ecosystems that existed just prior to the Great
Oxidation Event, a time of major atmospheric
evolution."
The Great Oxidation Event is a period of time
about 2.5 million years ago when blue-green algae
known as cyanobacteria began to proliferate. Those
bacteria began off-gassing oxygen, which triggered a
cooling of the planet along with a massive ice age –
as well as the eventual genesis of the kinds of
lifeforms we now live amongst.
Using radiometric and chemical dating techniques,
Czaja believes that the bacteria became trapped in
seabed sediment that was part of a theoretical
supercontinent known as
Vaalbara, consisting of what is now South Africa
and Western Australia.
In the video below, you can see a 360-degree view
of the collapsed bacteria, which was once round, as
captured by an imaging technique known as
confocal laser scanning microscopy.
Czaja's findings have been published in the
December issue of
Geology, a journal of the Geological
Society of America.
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