Washington | Soils on Earth hosted life as early as 3 billion years ago, say scientists who examined microfossils in ancient landscapes of Australia.
The microscopic fossils probably represent whole organisms, researchers said. The 3,000 million-year-old rocks in Australia have long been thought to be of marine origin.
However, a closer look at the dusty salt minerals of the rocks suggests they had to have experienced evaporation on land, according to Gregory Retallack, from the University of Oregon in the US.
Other mineral and chemical tracers found in the rocks also required weathering in soils of the distant geological past, he said.
“Life was not only present but thriving in soils of the early Earth about two thirds of the way back to its formation from the solar nebula,” Retallack said.
The formation of the solar system – and Earth’s origin – occurred some 4.6 billion years ago, which means the organisms would have been present about 3 billion years ago.
The study outlines a microbiome of at least five different kinds of microfossils recognised from their size, shape and isotopic compositions.
The largest and most distinctive microfossils are spindle-shaped hollow structures of mold-like actinobacteria, still a mainly terrestrial group of decomposers that are responsible for the characteristic earthy smell of garden soil.
Other sphere-shaped fossils are similar to purple sulphur bacteria, which photosynthesise organic compounds in the absence of oxygen while leaving abundant sulfate minerals in the soil.
“With cell densities of over 1,000 per square millimetre and a diversity of producers and consumers, these microfossils represent a functioning terrestrial ecosystem, not just a few stray cells,” said Retallack.
“They are evidence that life in soils was critical to the cycles of carbon, phosphorus, sulphur and nitrogen very early in the history of the planet,” he said.
The new discoveries by the researchers are potentially controversial because many scientists have long pointed to stromatolites, a life form that emerged 3.7 billion years ago, and other marine life as evidence of life that evolved in the sea and found their way into intertidal rock formations.
The ancient soils with sulphate salts and microfossils come from the Pilbara region of Western Australia.
They are superficially similar to those found recently by the Mars rover Curiosity.
“They may be useful as guides for the discovery of life on other planets,” he said.