by Lucas Joel Thursday, February 16, 2017
Life emerged on land about 300 million years earlier than previously thought, according to a new study in Geology by scientists who discovered minerals in 3.22-billion-year-old rocks that they suggest could only have formed with the help of biological processes.
Researchers led by sedimentologist Sami Nabhan, of Friedrich Schiller University Jena in Germany, studied rocks from the Barberton greenstone belt (BGB) in northeastern South Africa, which contains some of Earth’s first terrestrial sedimentary rocks. It’s “one of two places in the world where you can see rocks of this age in such good preservation,” Nabhan says, the other being in Pilbara in Australia.
Nabhan and his colleagues were looking for signs of life in the ancient rocks, which is not easy; in general, the rock and fossil records are sparser and more degraded the further back in time one goes because weathering and tectonics have had more time to destroy and deform the rocks. But the BGB rocks are “beautifully preserved,” so much so that they look like they could be 3 billion years younger than they are, Nabhan says.
In the sedimentary structures preserved in the rock, “you see cross-bedding, you see ripples, you see desiccation cracks, you see these channels filled with conglomerates,” Nabhan says. All of these features point toward an ancient braided river setting where the team found ancient paleosols, or fossilized soils, that they could date.
The paleosols contain the mineral pyrite, which is made of iron and sulfur. Microbes incorporate sulfur into their biomass, Nabhan explains, and they can also use sulfur compounds like sulfate to gain energy. When they do this, they selectively use sulfate containing lightweight isotopes of sulfur, he says, which leads to sulfur isotope fractionation in minerals that precipitate in the same environment. The pyrite from the BGB paleosols is enriched in “very light sulfur isotopes,” Nabhan says, suggesting life played a role its formation.
Life is not absolutely required for the kind of isotopically light pyrite grains found in the paleosols to form, says Paul Mason, a geochemist at the University of Utrecht who was not involved in the new study; similar fractionation can occur abiotically at temperatures above about 150 degrees Celsius. But, “if the temperature of the paleosol never exceeded 150 degrees or so, then microbial pathways are the only pathways to induce [sulfur] isotope fractionations.” Another abiotic process that could be responsible is transportation and introduction of the light isotopes by hydrothermal fluids, Mason says. But this is unlikely, Nabhan says, because the paleosols bearing isotopically light pyrite are traceable over a wide area, whereas hydrothermal fluid transport probably would occur through localized networks of cracks or veins in the rock.
Nabhan says he thinks his team has found the oldest-known evidence for life on land. But, he says, it’s possible “that within the next two or three years … other scientists will publish something that pushes it back a few more million, or tens of millions of years.”
“It’s quite a logical idea that we find life on land at that time,” Mason says. “It’s a natural consequence of microbial evolution … I don’t think in itself it’s an especially surprising result, but it’s exciting to extend the boundaries” of when we know life first appeared on land.
The team has “done a lot of really good geological and sedimentological background work,” Mason says, which is essential when studying Archean-aged rocks in which direct evidence for life is often nonexistent. “Every study is an important piece of the jigsaw puzzle to build up a picture of how the early biosphere was developing.”
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