The rocks found on the surface of the Earth did not form all at the same time. Geologists schematize this different order of formation using a column, of which the lower layers represent older rocks and top layers represent younger rocks. Archean rocks are the ones at the very bottom of this geologic column, and are therefore interpreted as the oldest rocks. They are often heavily deformed and altered and, because they do not contain fossils visible with the naked eye, have generally been considered as not preserving a record of life. In recent decades, however, this notion has changed and numerous publications have presented evidence implying the existence of microorganisms at the time of formation of Archean rocks.
The big question: life-related or not?
Scientists trying to demonstrate that some Archean rocks contain material or structures of biological origin face a significant challenge. The criteria that can be used to support a biological origin of a microscopic feature of a rock, such as shape or chemical composition, are not so unequivocal because there could be non-biological processes that can produce similar structures. As a consequence, claims for life-related material preserved in Archean rocks are considered robust only when multiple lines of evidence are presented. What are, then, the main clues used to point to a record of past life in the Archean rocks?
Evidence based on chemical composition
One of the elements that make up living organisms is carbon. In nature, there are different variants of carbon atoms, some lighter and some heavier. While performing their life-functions, microorganisms preferentially incorporate lighter rather than heavier carbon atoms. If some biological material derived from these microorganisms is preserved in a rock, it will have a composition richer in lighter carbon atoms in comparison with material of non-biological origin. This enrichment of lighter carbon has indeed been observed in numerous samples from Archean rocks. However, some studies have shown that the enrichment could also be the result of non-biological processes . Furthermore, some rock samples can be contaminated by more recent microorganisms, with the effect of altering the original ratio of light and heavy carbon in the sample .
Another type of evidence used to show that Archean rocks preserve a record of life are chemical compounds interpreted as the remnants of the membrane of cells of microorganisms. In some cases, successive studies indicated these compounds probably represented contamination by more recent organisms , but in others they appear to be genuinely coeval with the formation of the rocks containing them [e.g., 4].
Evidence based on physical structures
Microbes in shallow water or in moist environments can cover surfaces in sheet-like aggregations called microbial mats, often characterized by a slimy consistency. The stickiness of these mats can trap or make loose sediment more cohesive, generating specific macro-structures known as microbially induced sedimentary structures (MISS) . For example, microbial mats may cause sand to break in chips instead of washing away in individual grains. Mats can also cover and consolidate delicate ripples on sand produced by waves and currents, making their preservation more likely. MISS have been identified in Archean rocks, but the exact interpretation of their origin is difficult because they could also have formed by purely chemical or physical processes.
Another interesting structure, which has been related to biological activity, consists of microscopic borings of tubular shape found in some Archean rocks . These tubules seem analogous to modern ones produced by microbes on the oceanic floor. However, it has been shown that comparable structures can also be formed through inorganic processes.
Perhaps the most well-known Archean microfossils are carbon-rich filaments found in northwestern Australia, originally interpreted as fossilized bacteria . Their biological affinity, however, has recently been strongly questioned .
At times, it can be difficult to distinguish between real microfossils and non-biological material that looks like fossils. Contamination by younger microorganisms can also be a problem . Nevertheless, there are numerous published examples of structures currently interpreted as real microfossils from Archean rocks .
Stromatolites are structures in size ranging from a few centimeters to meters, with very thin internal lamination and forming, in modern settings, through the growth and biological activity of microbial mats. Stromatolites are locally abundant in Archean rocks, but undisputable fossilized microbes preserved in Archean stromatolites have not been found. Some authors have questioned that Archean stromatolites should be considered of biological origin, suggesting that they could have been formed by inorganic precipitation of minerals . Other authors still favor a biological origin for Archean stromatolites .
Implications for origins models
For evolutionary scenarios, the fact that evidence for life found in the lowermost rocks belongs to the simplest biological forms (unicellular microbial organisms) seems to fit well the expectations of the evolutionary narrative. On the other hand, finding evidence of life in the earliest rocks also creates a problem, because it shortens the time available for the supposed emergence of life from inorganic matter. In the words of S.J. Gould , “the notion that life has been found in the oldest rocks that could contain evidence of it, forces us, I think, to abandon the view of life’s slow, steady, and improbable development.”
The significance of Archean fossils for creationist models depends on the view on the origin of Archean rocks themselves. Most of those who think that planet Earth existed in an uninhabited state for some time before creation week would probably place the formation of Archean rocks in this period. For this reason, however, they would most likely evaluate with skepticism any purported evidence for life in Archean rocks. Doing otherwise would require admitting that bacteria existed on Earth before creation week, with a related discussion on how the Biblical concept of death and its connection to sin applies to modern biological categories [on this general subject, see 15]. Those who believe Archean rocks formed during or after creation week would most likely be open to the recognition of fossils in those rocks.
The quest for evidence of life in Archean rocks is a good example of the difficulties inherent to the practice of historical sciences, where data are often incomplete and altered and interpretations are necessarily based on what we observe in the present. They also illustrate how data are more intensely scrutinized when they do not seem to fit with pre-existing expectations (whether the idea that life needed a long time to arise on Earth or that Archean rocks formed in the time before creation week). In the study of evidence for life in Archean rocks, this skeptical approach has been shared by secular and creationist scientists. Both groups, for example, highlight the risk of external contamination of Archean samples by more recent microbial material [e.g., 16].
For those who value Scripture and science as sources of knowledge, the experience of uncertainty and the stance of caution could have the positive effect of avoiding dogmatism in trying to relate specific categories of current descriptions of nature (such as “Archean” or “bacteria”) to the Biblical text.
Geoscience Research Institute
November 1, 2013
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