By Sid Perkins
The first life on land may not have dramatically stepped forth from the ocean to boldly claim the soil. Maybe, instead, it was left high and dry when its lake evaporated.
Regardless of how life became terrestrial, new data may push back scientists’ estimate of when that event occurred. An international team led by scientists at Pennsylvania State University in University Park says that unusually carbon-rich rocks found in eastern South Africa provide evidence of life on land around 2.6 billion years ago.
Life on Earth dates back as far as 3.85 billion years ago (see “An early cosmic wallop for life on earth?”, in this week’s issue: An early cosmic wallop for life on Earth?). However, indisputably land-dwelling organisms had been previously dated to only 1.2 billion years ago, the researchers say.
The evidence for the possibly reluctant landlubbers showed up in sequential layers of carbon-rich clays within a 17-meter-thick band of rock, says Hiroshi Ohmoto, a geochemist at Penn State and a coauthor of the report in the Nov. 30 Nature.
The upper 7 m of the band of fossil soil, or paleosol, contains more than 0.1 percent carbon, which is an unusually high concentration for Precambrian paleosol, Ohmoto says.
Because the carbon wasn’t concentrated along cracks or fractures, it’s unlikely that it came from hydrocarbons that later seeped into the paleosol. Instead, Ohmoto notes, almost all the carbon was trapped within clay-rich layers generally stretching parallel to the ancient surface of the soil.
He also contends that it’s unlikely that the carbon leached from nearby igneous rocks as the soil formed because the layers also contain significant quantities of hydrogen, phosphorus, and nitrogen. This combination of elements suggests their biological origin.
Furthermore, the ratio of carbon-13 to carbon-12 isotopes in the top 5 m of the paleosol fall within a narrow range that matches the ratio of carbon isotopes deposited by cyanobacteria in freshwater lakes. Evidence for similar organisms has been found recently in marine shales about 2.7 billion years old, he notes.
Several factors provide clues to the environment where the ancient terrestrial soil formed. The researchers found an unusually high concentration of calcium in the paleosol.
The thin layers of carbon-rich clay measured at least 30 micrometers thick and sometimes exceeded 1 millimeter. Each clay layer was typically topped by a sprinkling of quartz grains whose size, shape, and layering indicate they were deposited by the wind.
Ohmoto and his colleagues assert that the paleosol formed in a lowland basin that collected water during the rainy season. Rain and local groundwater dissolved silicates and calcium-bearing minerals from higher elevations and carried them down to the basin.
There, they seeped into the soil and crystallized when the water evaporated.
Mats of microbes—which were likely up to 1 cm thick when alive—grew on the surface of the clay-rich soil and then were buried by windblown material during the ensuing dry season, Ohmoto says. This process forms calcium-rich soils today, he notes.
“Any evidence of life on land before plants is precious information,” says David J. Des Marais, an astrobiologist at NASA Ames Research Center at Moffett Field, Calif. “This [finding] speaks to a broader ecological distribution and diversity of organisms than previously recognized.”
Because these organisms grew on the soil’s surface, Ohmoto says, they would have been exposed to ultraviolet (UV) radiation from the sun. The new research implies that Earth had an ozone layer that offered some protection from UV light about 2.6 billion years ago, he adds.
Des Marais disagrees, saying that no ozone layer was required. For example, modern microbes similar to those that Ohmoto suspects formed this paleosol shed cells that form a UV-absorbing layer, he says. “There are a lot of ways for organisms to avoid ultraviolet light,” Des Marais notes.
