Two Billion Year Old Fossils Point to Early Terrestrial Life

Jul 23, 2013 by Sci-News.com

A new study, published in the journal Precambrian Research, describes evidence that primitive forms of life existed on land 2.2 billion years ago.

Bright, irregularly shaped structures are Diskagma buttonii (Gregory J. Retallack et al)

Bright, irregularly shaped structures are Diskagma buttonii (Gregory J. Retallack et al)

New evidence involves very small fossils connected into bunches by threads in the surface of an ancient soil from Hekpoort Formation near Waterval Onder, South Africa.

“They have been named Diskagma buttonii, meaning ‘disc-shaped fragments of Andy Button,’ but it is unsure what the fossils were,” explained lead author Prof Gregory Retallack of the University of Oregon.

“They certainly were not plants or animals, but something rather more simple.”

Diskagma buttonii are very small – about 0.3 – 1.8 mm long. They most resemble modern soil organisms called Geosiphon, a fungus with a central cavity filled with symbiotic cyanobacteria.

Prof Retallack said that 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.

“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.”

By modern standards, in which Earth’s air is now 21 percent oxygen, this early rise was modest, to about 5 percent oxygen, but it represented a rise from vanishingly low oxygen levels earlier in geological time.

Prof Retallack added: demonstrating that Diskagma are fossils 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 images enabled a three-dimensional restoration of the fossils’ form: odd little hollow urn-shaped structures with a terminal cup and basal attachment tube.

Reconstruction of Diskagma buttonii (Gregory J. Retallack et al)

Reconstruction of Diskagma buttonii (Gregory J. Retallack et al)

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

“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 is a promising candidate for the oldest known eukaryote – an organism with cells that contain complex structures, including a nucleus, within membranes,” Prof Retallack and his colleagues concluded.

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Bibliographic information: Gregory J. Retallack et al. 2013. Problematic urn-shaped fossils from a Paleoproterozoic (2.2 Ga) paleosol in South Africa. Precambrian Research, vol. 235, pp. 71 – 87; doi: 10.1016/j.precamres.2013.05.015