Newly discovered ancient fossils from South Africa provide strong evidence that life existed on Earth 2.2 billion years ago - four times earlier than previously believed.
Conventional scientific wisdom has it that plants and other creatures have only lived on land for about 500 million years, and that landscapes of the early Earth were as barren as Mars.
Now, a new study, led by geologist Gregory J Retallack of the University of Oregon, has presented evidence for life on land that is four times as old.
The evidence involves fossils the size of match heads and connected into bunches by threads in the surface of an ancient soil from South Africa. They have been named Diskagma buttonii, meaning "disc-shaped fragments of Andy Button," but it is unsure what the fossils were, the authors said.
"They certainly were not plants or animals, but something rather more simple," said Retallack, professor of geological sciences and co-director of paleontological collections at the UO's Museum of Natural and Cultural History.
The fossils, he added, most resemble modern soil organisms called Geosiphon, a fungus with a central cavity filled with symbiotic cyanobacteria.
"There is independent evidence for cyanobacteria, but not fungi, of the same geological age, and these new fossils set a new and earlier benchmark for the greening of the land," he said.
"This gains added significance because fossil soils hosting the fossils have long been taken as evidence for a marked rise in the amount of oxygen in the atmosphere at about 2.4 billion to 2.2 billion years ago, widely called the Great Oxidation Event," he added.
Demonstrating that Diskagma are fossils, Retallack said, was a technical triumph because they were too big to be completely seen in a standard microscopic slide and within rock that was too dark to see through in slabs.
The samples were imaged using powerful X-rays of a cyclotron, a particle accelerator, at the Lawrence Berkeley National Laboratory in California.
The images enabled a three-dimensional restoration of the fossils' form: odd little hollow urn-shaped structures with a terminal cup and basal attachment tube.
In their conclusion, the researchers noted that their newly named fossil Diskagma is comparable in morphology and size to Thucomyces lichenoides, a fossil dating to 2.8 billion years ago and also found in South Africa, but its composition, including interior structure and trace elements, is significantly different.
The new fossil, the authors concluded, is a promising candidate for the oldest known eukaryote - an organism with cells that contain complex structures, including a nucleus, within membranes.