[meteorite-list] 3 Billion-Year-Old Fossils Show Early Microbes Lived in Cavities

[Ron Baalke]

https://www.newscientist.com/article/dn28678-3-billion-year-old-fossils-show-early-microbes-lived-in-cavities/

3 billion-year-old fossils show early microbes lived in cavities
New Scientist
December 16, 2015

It seems the microbes that formed Earth's first ecosystems looked for 
shade when the sun was strong, just like we do.

Fossils found in South Africa suggest that cavities in tidal sediments 
might have provided refuge from deadly solar rays during the Archaean 
aeon when we think that life emerged on Earth.

At this time, between 4 billion and 2.5 billion years ago, Earth was scorched 
by intense UV radiation, and had no ozone layer to protect it - a bit 
like Mars is today.

So life at the surface would have found survival a challenge.

Some of the oldest fossil cells are around 3.43 billion years old, and 
thought to have lived on sand grains that might have been covered by shallow 
water and overlying grains.

At the Barberton greenstone belt in South Africa, an area where ancient 
volcanic rock has been pushed to the surface, there are thin layers of 
rock thought to be 3.22 billion-year-old microbial mats - sheets of microbes 
that covered tidal areas of the seashore.

Now fossilised bacteria have been discovered underneath the mat in cavities 
covered by a thin layer of sediment. The bacteria are rod-shaped, growing 
end-to-end in long filaments like many bacteria do today.

Like modern microbes

"The shape is quite uniform," says co-author Alessandro Airo, whose colleague 
Martin Homann at the Free University of Berlin, Germany, analysed the 
fossils. "It appears that by that time, they were already able to biochemically 
control diameter and length, and coordinate themselves into a chain. That's 
what modern microbes do all the time."

David Wacey, a palaeobiologist at the University of Bristol, UK, says 
the evidence from the new study looks robust.

'They have studied the geology in detail so we know that the environment 
was habitable for life, and the interpreted setting is closely comparable 
to where we would expect to find such structures today," he says.

"The record of Archaean microfossils is sparse and controversial," says 
Birger Rasmussen at Curtin University, Australia, who previously reported 
the discovery of cavity-dwelling microbes in 2.7 billion-year-old sediments 
in Australia. "This is an exciting find as it extends the record of possible 
life in this habitat a further 500 million years."

The atmosphere and UV radiation during this period of Earth's history 
are thought to have been similar to conditions on Mars. Airo says that 
understanding how life could have survived in this time could give us 
clues about what sort of life might be found on Mars and where to look.

"This study shows that very close to the surface, life was possible back 
then," he says, "so it could well be that microbes thrived even on the 
surface of Mars and not necessarily only in deep water or the subsurface."

Journal reference: Geology, DOI: 10.1130/G37272.1