New research questions evidence for earliest life on earth

May 23, 2002

WASHINGTON - New geological and geochemical data call into question recent claims for fossil life on Earth greater than 3.8 billion years ago, say researchers from The George Washington University and the Swedish Museum of Natural History in the May 24 issue of the journal Science. Such claims have been based on interpreting the sensitive biochemical behaviour of carbon, the principal element of life, and its relationship with the rocks in which the carbon is found.

The rocks examined are located on Akilia, a remote island off the coast of southwest Greenland, about 18 miles (30 kilometers) south of Nuuk, the capital city of Greenland. Earlier reports claimed that a peculiar green-and-white layered rock found there, formed as a Banded Iron Formation (BIF), a type of sediment commonly preserved in very ancient ocean basins. Tiny particles of simple organisms, such as bacteria, living in the ocean were thought to become trapped in the sediments at the time of formation. With time, heat, and pressure, the organisms were converted into graphite, a type of carbon, thus erasing any "shapes" that could be recognized as fossils. While fossil shapes cannot be seen, the process of metabolism in living organisms separates different weights of carbon, leaving what has been thought to be a unique chemical "fingerprint" of past life.

However, the article by Christopher Fedo, from The George Washington University's Department of Earth and Environmental Sciences, and Martin Whitehouse, at the Swedish Museum of Natural History, reports that the story about the rocks is much more complicated, and the carbon isotopic basis for interpreting life is questionable. In a detailed study of the rocks on Akilia, the authors demonstrate that much of the fine layering in the banded rocks formed as a result of great stresses that lasted discontinuously for about two billion years, and not as thin layers of sediment deposited on a long vanished sea floor as had been suggested. Fedo and Whitehouse liken the process of making the fine layering akin to stretching a lump of bubble gum into a thin strand.

A chemical study of the rocks also points away from a BIF origin for the banded rock. Analyses of the green bands show that the composition is very similar to a primitive rock called komatiite, a relative of basalt, which formed as molten rock solidified. Such rocks form at temperatures far too high for living organisms, hence there is little chance that the preserved graphite represents past life. The white layers are thought to be veins of quartz that were subsequently introduced into the green bands.

Recent studies of hydrothermal vents on mid-ocean ridges have also shown that basalt-like rocks can interact with water to form carbon compounds by non-biological processes, leaving an isotopic signature similar to that of metabolic function. While it is possible life existed on Earth when the rocks on Akilia formed, direct evidence for life older than approximately 3.8 billion years ago is still lacking.

No one knows when life on Earth originated. The earliest history of the Earth is one dominated by many violent asteroid impacts, some of which would have been capable of entirely boiling off all water on the planet, including the oceans, and sterilizing the planet of all life.

Such a scenario suggests that life may have originated and been exterminated a number of times before the last major impact, which perhaps occurred around 3.8 billion years ago, near the time when the rocks on Akilia solidified.

From a first impression of their great age, these rocks then represent an ideal target for studying the remains of the earliest life on Earth.

However, the study by Fedo and Whitehouse, along with a recent study about the claimed earliest body fossils, indicates that discovering a preserved relic of the earliest life on Earth remains elusive, and that great caution is warranted in working with samples of such antiquity.

The difficulty of identifying life the earliest geologic record also has profound implications concerning our ability to recognize life in any Martian rock samples.
This research has been supported by grants from the National Geographic Society, The George Washington University, and the Swedish Research Council.

George Washington University

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