Archaea: Fossil Record

Not so inhospitable : It used to be unthinkable that life could exist at temperatures near boiling, but some intrepid archaeans thrive under these conditions. Geysers, like the one shown above, are home to such microbes and may help us understand how life existed when the Earth was young.

The search for fossils of Archaea faces a number of problems. First of all, they're very tiny organisms and so will leave microscopic fossils. Any search for fossilized archaeal cells would require a lot of time spent with a microscope and a lot of patience. In fact, there are fossil microbes known from throughout the Precambrian, but here a second problem surfaces -- how do you distinguish fossil archaeans from fossil bacteria?

Archaea and Bacteria cells may be of similar sizes and shapes, so the shape of a microbial fossil does not usually help in determining its origin. Instead of physical features, micropaleontologists rely on chemical features. Chemical traces of ancient organisms are called molecular fossils, and include a wide variety of chemical substances. Ideally, a molecular fossil should be a chemical compound that (1) is found in just one group of organisms, (2) is not prone to chemical decay, or (3) decays into predictable and recognizable secondary chemicals.

In the case of the Archaea, there is a very good candidate to preserve as a molecular fossil from the cell membrane. Archeal membranes do not contain the same lipids (oily compounds) that other organisms do; instead, their membranes are formed from isoprene chains. Because these particular isoprene structures are unique to archaeans, and because they are not as prone to decomposition at high temperatures, they make good markers for the presence of ancient Archaea.

Molecular fossils of Archaea in the form of isoprenoid residues were first reported from the Messel oil shale of Germany (Michaelis & Albrecht, 1979). These are Miocene desposits whose geologic history is well known. Material from the shale was dissolved and analyzed using a combination of chromatography and mass spectrometry. These processes work by separating compounds by weight and other properties, and produce a "chemical fingerprint". The fingerprint of the Messel shale included isoprene compounds identical to those found in some archaeans. Based on the geologic history of the Messel area, thermophiles and halophiles are not likely to have ever lived there, so the most likely culprits to have left these chemical fingerprints behind are archaeal methanogens (methane-producers).

Since their discovery in the Messel shales, isoprene compounds indicative of ancient Archaea have been found in numerous other localities (Hahn & Haug, 1986), including Mesozoic, Paleozoic, and Precambrian sediments. Their chemical traces have even been found in sediments from the Isua district of west Greenland, the oldest known sediments on Earth at about 3.8 billion years old. This means that the Archaea (and life in general) appeared on Earth within one billion years of the planet's formation, and at a time when conditions were still quite inhospitable for life as we usually think of it.

The atmosphere of the young Earth was rich in ammonia and methane, and was probably very hot. Such conditions, while toxic to plants and animals, can be quite cozy for archaeans. Rather than being oddball organisms evolved to survive in unusual conditions, the Archaea may represent remnants of once-thriving communities that dominated the world when it was young.

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