Oldest microfossils raise hopes for life on Mars

August 21, 2011

The hunt for the most ancient example of life on Earth has a pungent new contender: Sulfur-belching microbes fossilized in 3.4-billion-year-old western Australian sandstone.

The find bodes well for the search for life on Mars, researchers say.

Preserved as tiny spheres and tubes, the newly described “microfossils” hail from 200 million years earlier than the next oldest example of fossil microbes, which were announced last year.

Single-cell life was widespread on Earth by 3.4 billion years ago, researchers generally agree. Until now, though, evidence has been indirect — namely, flecks of organic carbon, the putative remains of ancient microbes embedded in some of the oldest rocks on Earth.

The newly reported microfossils offer more direct proof that life thrived in the hot, harsh conditions that prevailed during Earth’s extended childhood.

“At last we have good solid evidence for life over 3.4 billion years ago,” senior researcher Martin Brasier of Oxford University said in a statement. “It confirms there were bacteria at this time, living without oxygen.”

In a field where announced findings are routinely debunked, origins-of-life researchers praised the extensive testing performed by Brasier and his colleagues.

“This is as a good as a shot as any I’ve seen for evidence of microfossils,” said Marilyn Fogel, a biogeochemist at the Carnegie Institution of Washington who is conducting her own search for ancient life.

“The degree of preservation of these microfossils is pretty spectacular for rocks of this age,” said Mike Tice, a geobiologist at Texas A&M University who was not involved in the new research. “They’re certainly the best-documented, most convincing examples we have” of microfossils from the early, oxygen-starved era known as the Archean.

Earth formed about 4.5 billion years ago, but no one knows when life arose. Any evidence of that event — or events — was likely erased by the heavy meteorite bombardment and geologic upheaval that wracked the young Earth.

About 3.4 billion years ago, though, colonies of microbes apparently thrived on one of Earth’s first beaches, Brasier and colleagues report in the journal Nature Geoscience. Some of these microbes became embedded in the sand that later hardened into an arid region of western Australia known as the Strelley Pool Formation. The extreme age of the rocks here — and their good preservation— has attracted origins-of-life researchers for decades.

But it was Brasier and David Wacey of Western Australia University who hit the jackpot. After collecting hundreds of candidate rocks and lugging them back to their lab, they sliced one into micron-thin sections, aimed a microscope at it, and said, “A-ha!”

“It was pretty incredible,” Wacey said of the moment he first saw the circles and tubes. “Martin [Brasier] had seen huge amounts of material and he was tearing his hair out. He thought he would never find anything.”

The team saw microfossils in rocks collected at two locations. The sandstone that produced the specimens is bracketed by layers of volcanic rock that have been dated to about 3.4 billion years ago.

Because natural geologic processes can form the circular shapes the team saw, they subjected the microfossils to a battery of tests that sniffed out the chemical signatures of life.

Most convincing: Tiny inclusions of the iron-sulfur compound pyrite, known as fool’s gold. The team found pyrite near — or in some cases, on top of — the microfossils. “Some of the microfossils appear they were living on pyrite,” Wacey said.

“The sulfur in the grains of pyrite they saw contain the signatures of metabolism,” said Tice, meaning the microbes were processing sulfur and excreting it.

Even today, hardy microbes near deep-sea vents and in other oxygen-starved environments thrive by consuming sulfur compounds. With no oxygen to breathe, their ancient relatives apparently did the same and probably belched hydrogen sulfide — the rotten-smelling gas popular with high school chemistry teachers.

The finding should boost the search for life on Mars, said David Des Marais, an astrobiologist at the NASA Ames Research Center: “I mean, wow, we now know that sulfur-based metabolism happened very early on Earth. And early Mars had water and sulfur. It shared in many ways the environment of the early Earth. This gives us confidence that looking for these types of organisms on Mars is a good strategy.”

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