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2.2 Billion Year Old Fossils Show Signs of Early Terrestrial Life

Jul 23, 2013 02:23 PM EDT

A new study led by University of Oregon geologist Gregory Retallack presents evidence that challenges the conventional scientific wisdom that plants and other creatures have only lived on land for 500 million years.

In a paper published in journal Precambrian Research, Retallack and his colleagues describe 2.2 billion year old fossils about the size of match stick heads connected in bunches by threads. Retallack unearthed the fossils in ancient soil from South Africa. The find opens new doors of inquiry into ancient life on Earth.

Evidence for life on land dating back 2.2 billion years is more than four times as old as as prior fossilized evidence of land life, and nearly half as old as Earth itself.

While it is unclear exactly what they are, the ancient fossils resemble a modern soil organism called Geosiphon, a fungus with a central cavity filled with symbiotic cyanobacteria. Only about about 0.3 - 1.8 mm long, the tiny fossils represent a new benchmark for the age of the earliest land-dwelling fossils known.

They have been named Diskagma buttonii, meaning "disc-shaped fragments of Andy Button," Retallack said. Using X-ray imaging to document and verify that the Diskagma buttonii were indeed fossils, the researchers then were able to describe the fossils as strange, little "hollow urn-shaped structures with a terminal cup and basal attachment tube."

"At last we have an idea of what life on land looked like in the Precambrian," Retallack said. "Perhaps with this search image in mind, we can find more and different kinds of fossils in ancient soils."

While they know what it looked like, the researchers are still trying to determine what the ancient fossils were.

"They certainly were not plants or animals, but something rather more simple," Retallack said.

"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," Retallack said. "This gain 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."

At the time of Diskagma buttonii, the Earth's air rose about 5 percent oxygen, which is low compared to the 21 percent oxygen content in Earth's air today, but prior to the Great Oxidation Event, there were was hardly any oxygen in the air.

Diskagma buttonii fossils, the researchers report, hold similarities to three living organisms: Leocarpus fragilis as found in Oregon's Three Sisters Wilderness; the lichen Cladonia ecmocyna gathered near Fishtrap Lake in Montana; and the fungus Geosiphon pyriformis from near Darmstadt, Germany.

The fossils are also similar in morphology and size to the 2.8 billion year old Thucomyces lichenoides fossil, also found in South Africa. But the chemical composition of the two fossils are significantly different.

Based on what they know so far, the researchers concluded the fossils are "are a promising candidate for the oldest known eukaryote -- an organism with cells that contain complex structures, including a nucleus, within membranes."

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