When one kind of microbe fed on
another, the result was a rotten-egg stench—and a pathway for complex life.
Early earth had a distinctive aroma. And it wasn't very nice.
That's what scientists have now determined, using advanced imaging techniques
to examine fossils nearly 1.9 billion years old that were collected from rocks
around Lake Superior, Canada.
Their work has revealed spherical and rod-shaped bacteria
dining on the cylindrical outer shells of another, larger bacterium known as
Gunflintia. To digest those Gunflintia sheaths, the feeding bacteria would have
had to use oxygen atoms taken from salts, or "sulfates," in seawater.
In the process, the microbes formed gaseous carbon dioxide, which would have
been released into the atmosphere.
Another byproduct of this biochemical process is hydrogen
sulfide, which produces a stench commonly known as "the rotten egg
smell," explained Martin Brasier, a paleobiologist at Oxford University in
London.
"The whole world didn't smell of rotten eggs," said
Brasier, "but if you had a sensitive nose, it would have been very
widespread indeed."
There is chemical evidence that organisms were eating other
organisms as early as 3.5 billion years ago, but the new images represent the
first visual evidence of this type of feeding, called heterotrophy.
"For the first time in the early fossil record, we see
one kind of creature eating another creature," said Brasier, coauthor of a
new study about the images published in this week's issue of the Proceedings of
the National Academy of Sciences. "This is the first time we've been able
to see it happen in rocks under the microscope under our very eyes."
Gunflintia is thought to have been a type of cyanobacteria or
blue-green algae, a class of photosynthetic microbes that played a crucial role
in pumping early Earth's atmosphere full of oxygen, thus helping pave the way
for complex life.
"This is the group that was producing the oxygen we now
breathe," Brasier said.
An Invisible Feast
Combining a variety of advanced imaging and chemical analysis
techniques, Brasier, David Wacey of the University of Western Australia, and
their team were able to create detailed, three-dimensional images of the
microfossils.
First discovered 60 years ago in 1953, Gunflint
bacteria—named after the Gunflint chert rock where they were found—helped solve
a mystery that dates back to the days of Charles Darwin and that was famously
known as "Darwin's dilemma."
Darwin's theory of evolution predicted that the record of
life on Earth should stretch back billions of years, yet the oldest fossils
known at the time dated to only the Cambrian period, about 542 million years
ago.
That changed with the discovery of the Gunflint fossils.
"That extended the record of life from 500 to 600 million years old to
almost 2 billion years old," said Kenneth Williford, a planetary chemist
at NASA's Jet Propulsion Laboratory in Pasadena, California, who was not
involved in the study.
This new study shows that the Gunflint fossils, discovered
about 60 years ago, can still manage to surprise. The new 3-D images of the
Gunflint microorganisms revealed that some of the smaller microbes were
preferentially feeding on Gunflintia bacteria and ignoring another species
known as Huroniospora.
The team also found other evidence that the Gunflintia were
being consumed: Their sheaths were marred by numerous holes, and in some places
were partially or entirely filled with specks of pyrite. Also known as fool's
gold, this iron sulfide mineral is a waste product of the kinds of
heterotrophic sulfate-reducing bacteria that the scientists suspect fed on
Gunflintia.
As to why the bacteria were such picky eaters, Brasier thinks
Gunflintia was just easier to eat. "We have a hunch that Huroniospora was
made of a different kind of material that was rather waxy," he said.
It would have been like the difference between eating "a
carrot and a piece of wood," Brasier added. Huroniospora "was tougher
and therefore harder to break down."
Vital for Life
Though they are tiny, bacteria play a crucial role in
breaking down dead organic matter and releasing carbon dioxide back into the
atmosphere—processes vital for life on Earth.
Without these bacteria, the carbon dioxide that had been
fixed into the bodies of cyanobacteria through photosynthesis would not have
been released back into the atmosphere. And the loss of all that carbon
dioxide, Brasier said, would have resulted in massive ice ages.
JPL's Williford thinks the technology used to image the
two-billion-year-old Gunflint bacteria could one day be used to analyze
evidence of life on Mars if it were found.
This study "shows the importance of bringing samples
back from Mars," Williford said. "If we really want to understand
whether there was once life on Mars, these are some of the uniquely Earth-based
techniques we would like to apply."
