The oldest bug?

POSTED 30 SEP 2004


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Old = gold?
A new study of a big hunk of gold-free South African rock has revealed a different type of treasure: evidence of perhaps the earliest known life on the planet, found in samples from the ancient Buck Reef Chert in South Africa.

The rock body was called "buck" by old miners, a slur for formations that carried no gold. Although the rock was not created by coral, it is called a "reef" -- a big, continuous piece of chert. "Chert," BTW, is a fine-grained, sedimentary rock made of quartz.

the Buck Reef Chert (the gray ridge)
The Buck Reef Chert (the gray ridge) houses some of the oldest traces of life, dating to 3.4 billion years in the past.

Photos courtesy Michael Tice, Stanford University.

See the Buck Reef Chert in all it's 1.7MB glory. Link will open in new window.

What the old miners missed: The black layers in the reef contain carbon that was probably deposited by living creatures.

Small, single-celled creatures, but life nonetheless. (Multi-cellular life only appeared at the beginning of the Ediacaran Period, about 636- to 599-million years ago.)

Old = contentious?
The field of ancient life is difficult and seems to attract people who love to argue. It helps that rocks so elderly are rare, and that interpretations can go awry because many of the rocks, especially the really ancient ones in Greenland, have been intensely heated and massively deformed. In 1987, J. William Schopf of the University of California, Los Angeles, pronounced a biological origin for microscopic traces in 3.4-billion-year-old Australian rocks. He based his claim on a visual resemblance to bacteria, and on ratios of different carbon isotopes.

Through normal metabolism, living organisms can separate isotopes -- atoms with different numbers of neutrons -- creating a signature that remains even after the microbial remains become rock. In the study of early life, which hybridizes the techniques of chemistry, geology and biology, isotopes are a big deal.

Schopf's claim was hotly disputed. In 2002, for example, a British scientist argued that the traces were formed by hydrothermal fluids - chemicals dissolved in hot subsurface water.

Ecology = destiny?
Fast-forward to 2004. In the new study, Michael Tice, a graduate student at Stanford University, and his professor, Donald Lowe, concluded that Buck Reef Chert did not contain hydrothermal traces. There were no tubes carrying hydrothermal fluids, and the reef was too large and too similar to normal oceanic environments. "There's no hydrothermal system today that could produce carbon of that isotopic composition consistently," Tice says. "All of these things together told us that the carbon ... was formed by living things, not hydrothermal vents."

Instead of the remains of hot fluids, the researchers think they detected the remains of microbial mats - masses of microbes that eventually hardened into rock.

Black Reef Chert
The black gob at lower left is an old grain of organic carbon in the Buck Reef Chert, which became a framework for the newer mat (dark lines in lighter areas). Had the rock formed by deposition of fine grains, the draping and voids would be absent. Bar = 1 millimeter.

Photos courtesy Michael Tice, Stanford University.

Tice and Lowe, who has long studied ancient life, distinguished three separate environment in the reef. One area, where ripples in the rock recorded the motion of waves, resembled the lagoons, or evaporation ponds, seen around today's beaches. A second area, with deformations typical of gentler wave action, probably formed under a shallow sea. In a third area, presumably under deeper, quieter water, the rock layers were flat and regular.

Tellingly, the microbial mats appeared only in the shallows, where sunlight would have reached the bottom.

Isotopes = truth?
From the ratio of the carbon-12 to carbon-13 isotopes in Buck Reef, Tice and Lowe concluded that the black layer was deposited by an organism that used the Calvin cycle. (It was news to us too, but the Calvin cycle is one chemical pathway that microbes can use to reduce -- separate -- carbon from carbon dioxide.)

The Calvin cycle is uncommon among modern life. It is used by chemoautotrophic bacteria, which live in mine tailings and near deep-sea vents at the ocean floor. Chemoautotrophs are one of the rare forms of life does not get energy, directly or indirectly, from sunlight.

Chemoautotrophs, which have the clunkiest moniker in bacteridom, need oxygen. But was oxygen present in the ancient ocean water? Apparently not. Tice and Lowe found iron carbonate, but not iron oxide, in the chert. Today, iron in the ocean combines with oxygen -- it rusts -- to form iron oxide. So the presence of iron without iron oxide means the ancient ocean had no free oxygen. Since the chemoautotrophs required oxygen, you can rule them out, Tice says.

Let's let Tice add up the evidence: "That leaves us with something that uses the Calvin cycle, but does not need oxygen, and that lived only in shallow water. That tells us they were photosynthesizers, but were not producing oxygen." (Remember, if they had released oxygen, the rock would contain rust.)

Case = closed?
Today, such photosynthetic, "non-oxygenic" bacteria exist as a purple layer on salty beach lagoons. You might think the case is closed, that life was definitely present 3.4 billion years ago. But that would reveal you as a newcomer to the ancient-life discussion. Given the scientific and popular emotions surrounding ancient life, we don't expect the story to end here. "People care about old stuff," says Tice. "it's like the debates about the dinosaurs.... People have visions of what was going on, and they get attached to them because they care about them."

-- David Tenenbaum


Bibliography
Photosynthetic Microbial Mats in the 3,146-Myr-Old Ocean, Michael Tice and Donald Lowe, Nature, 30 Sept. 2004, pp. 549-52. See Early Options in Photosynthesis, Nicholas Beukes, p. 522-3.

Life in extreme environments.

South Africa info.

 

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