A 3.2 billion year old fossilized ecoystem

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Rasmussen, B. 2000 Filamentous microfossils in a 3,235-million-year-old volcanogenic massive sulphide deposit. Nature 405:676-679

"Fossils" are the traces of previous life. Fossils can range from the actual remains of an organism (e.g. an amberized insect) to scant traces of unusual isotope ratios (e.g. the Greenland banded iron formations). The further back in time you wish to examine, the harder it gets to obtain and interpret these fossils. Samples are harder to get because of plate tectonics - really old rocks are actually quite rare, since most have been heavily or completely transformed by passage into the mantle. Before about the Cambrian explosion, when all or nearly all of the animal phyla appeared a short 540 Mya, only microbial fossils exist. In later formations these are common, but very old fossils are few and far between. Most of these have been found in Australia and Greenland, where the oldest untransformed deposits reside. Here we'll talk about evidence for microbial life early in Earth's history.

This paper describes fossil filamentous microbes from the geological remains of a hydrothermal vent that existed over 3.2 Bya and about 1 kilometeer below the surface of the Archaean ocean. These are not the oldest microbial fosssils known, there are a few from between 3.5 and 3.6 Bya . However, these are of more typical rods and cocci from shallow sediment deposits, and are much less visually striking and with less of their environmental context clear. There is also some reason to doubt the origin of many of these earlier fossils.

This paper contains a great deal of geological and geophysical description that is difficult for a typical biologist (or any biologist) to decipher. Suffice it to say that this site has been described in great detail by previous geologosts and there is a clear picture of the environment that the organisms that became these fossils lived in. This was a deep-sea hydrothermal vent system, much like those that exist today, with hot (ca. 300°C) mineral-laden hydrothermal fluid rising through the fractured rock to the floor of the ocean. As this water neared the top of the sediment, it was mixed with cold ocean water, and the minerals (mostly silicates and sulfides) precipitated to form the 'massive sulfide deposit'. There were probably parts of this deposit that reached the surface of the ocean floor, to emerge as chimneys and black smokers of the sort we se today. Conversely, it is also true that today, underneath any hydrothermal field there lies a massive sulfide deposit of the sort whose remains are described, in fossil form, in this paper. Of course, there were no vent worms, clams, or scaly smails as there are in modern deep-sea vents; animals of any complexity being more than 2.5 billion years in the future. This vent, like many modern vents, contains bitumen deposits, tar, and oil. Although the authors don't point this out, these are all signs of life, since they are produced from the transformation of organic remains. The fossils are found on the surface of what aappear to have been cracks, fissures, and open spaces is the volcanic rock. Probably the water flowing through these open spaces was hydrothermal fluid mixed with the surrounding ocean water at less than 100°C, providing a rich supply of geothermal energy and even organic material (oil). These vents, then and now, are prime opportunities for organisms. The hydrothermal fluid might be in chemical equilibrium deep in the crust, but when cooled or mixed with cold ocean water (at its own, very different equilibrium), it will no longer be in chemical equilibrium, and organisms can step in, facilitate favorable reactions, and capture the energy released.

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Fig 3. Photomicrographs of filaments from the Sulfur Springs VMS deposit. Scale bar, 10um. A-F, striaght, sinuous and curved morphologies, some densely intertwined. g, Filaments parallel to the concentric layering. h, Filaments oriented sub-perpendicular to banding.

The filaments are clearly the remains of microbial life. They are nearly, but not quite, uniform in diameter and length, and are oriented preferentially rather than randomly. The filaments cross crystal boundries in the rock, and, perhaps most convincingly, they are not branched. The organisms seem to have first nucleated the precipitation of silica around themselves, become 'petrified', and then later the silica was replaced with pyrite. The fossils are threads of pyrite in the original shape of the organisms. The morphology and habitat of the organisms is strikingly similar to the filamentous sulfur-oxidizing Bacteria that currently inhabit hydrothermal vents, although because there was no oxygen they would most likely have had some other metabolism.