Size Limits of Very Small Microorganisms

Panel 3

Can we understand the processes of fossilization and non-biological processes sufficiently well to differentiate fossils from artifacts in an extraterrestrial rock sample?

DISCUSSION

Summarized by Andrew Knoll, Panel Moderator


Recognition of a Biological Signature in Rocks

Dr. Knoll opened the session by summarizing the challenges of recognizing biological signatures in extremely old rock samples from Earth or Mars. Everyday experience suggests that the gulf between biology and the physical world is conspicuous. This impression arises, however, because the biology most familiar to us is principally that of organisms found on distal branches of the tree of life. The difficulty in distinguishing between biogenic and abiogenic features lies at the other end of the tree. Life arose as a self-perpetuating product of physical processes, and it is likely that the characteristics of Earth's earliest organisms—their size, shape, molecular composition, and catalytic properties—bore a close resemblance to the products of physical processes that gave rise to biology. For this reason, detecting the remnants of early life in terrestrial rocks is difficult. In martian or other extraterrestrial samples, it is doubly challenging. Given the evidence in hand from ALH84001 and the prospect of analyzing intelligently chosen samples from Mars within a decade, how do we fashion ground rules for recognizing the unambiguous signal of past (or present) biology?

Organisms have structure, they have a chemical composition, and they affect their environment; thus, paleontological evidence of ancient life can be morphological, geochemical, or sedimentological. Experience with terrestrial rocks makes it clear that features found in ancient samples can be accepted as biological only if they satisfy two criteria. They must be compatible with pattern generation by known biological processes. And they must be incompatible with formation by physical processes. This is straightforward in principle, but it requires that we understand the limits of pattern formation by biological and physical processes.

Dr. Knoll illustrated the challenge for this panel by drawing a Venn diagram in which biological and abiological patterns were depicted as distinct but overlapping fields. As yet, our understanding of the limits on both biological and abiological pattern generation is incomplete, making it difficult to understand the dimensions of the "gray zone" of overlap. Recognition of biological pattern in extraterrestrial samples will require the identification of structures or molecules that reside in the biological field, but not in the zone of overlap. On the other hand, there is no assurance that terrestrial life exhausts the possibilities of biological pattern generation; therefore, knowing the limits of pattern formation by physical processes may provide the best yardstick for evaluating martian or other extraterrestrial samples. Dr. Benner suggested an alternative depiction of the Venn diagram in which the biological field is completely encompassed within the physical field—the point being that biological processes are a subset of a larger and more inclusive set of physical processes. As a depiction of process, this view is unimpeachable; nevertheless, the patterns generated by biological processes include structures and molecules not known to form under strictly physical conditions. Bones, radiolaria, and red algal thalli are examples of biologically diagnostic morphologies; cholesterol is a biologically diagnostic molecule.


Lessons from Earth

Panel 3 members agreed that our collective experience with Earth's geological record provides an important guide to fossil recognition and interpretation on Mars. Dr. Farmer demonstrated that processes of mineral precipitation can preserve biologically interpretable microfossils and sedimentary fabrics. This increasing knowledge of fossilization processes not only sheds light on the postmortem information loss that attends fossilization, but also focuses attention on martian environments most likely to preserve a biological record. Preservation of terrestrial remains is selective, with some organisms—and some parts of organisms—more likely to escape decay than others. During fossilization, cells can also shrivel or collapse, resulting in fossils that are much smaller than the organisms from which they are derived.

Dr. Schopf summarized experience in interpreting Earth's early fossil record, stressing the early phase of discovery, when reports of objects that proved to be abiological outnumbered those of genuine fossils. Not everything that is small and round is biological, and the rigorous criteria for biogenicity developed over the past three decades by paleontologists can be useful in the evaluation of extraterrestrial microstructures. Dr. Schopf emphasized the need to conduct interdisciplinary studies of petrology, micropaleontology, isotopic geochemistry, and molecular organic geochemistry, developing multiple lines of evidence for interpreting potentially biological patterns.

Although most of Panel 3's discussion focused on micron-scale structures, the distinctive macroscopic structures known as stromatolites were also considered. Stromatolites are laminated structures found in chemical sedimentary rocks, especially but not exclusively carbonate rocks. These structures, which can be flat-laminated, domal, columnar, or conical, are commonly interpreted as the products of sediment trapping, binding, and/or precipitation by microbial communities; however, it is apparent that comparable structures can be generated without the need for microbial templates. Indeed, such structures actually occur in the early geological record. This being the case, images of laminated precipitates that may be transmitted by a Mars rover cannot be construed as unambiguous evidence for biological activity (Farmer, Knoll). Micro- and mesoscale fabric studies on returned samples will be necessary to confirm or reject hypotheses of biological origin. Dr. Farmer suggested that specific microscopic textures may provide the biological fingerprint needed to be confident of stromatolite biogenicity in the terrestrial or martian rock record.

Dr. Bradley focused attention on abiologial pattern formation, suggesting that non-biological processes may be sufficient to explain a number of micro- and nanoscale features sometimes interpreted as biological, including those reported from Mars meteorite ALH84001. Dr. McKay vigorously disputed some of these conclusions but agreed that much clearer criteria for biological pattern formation in extraterrestrial samples are needed.


Summary and Consensus

General consensus was reached on the following points:

Terrestrial rocks contain an observable and interpretable record of biological evolution, but as we recede further back into time, that record becomes attenuated and difficult to interpret in detail. Martian samples may actually be better preserved than terrestrial sediments of comparable age, but lack both modern martian organisms for comparison and a more or less continuous fossil record that connects the present with early planetary history.

A better understanding of biological signatures in sedimentary rocks is needed, and it is needed before intelligently collected martian samples are returned to Earth. These signatures certainly include fossil morphologies, but they must also include biomarker molecules, isotopic fractionation, and biological mineralization and trace element concentrations. In all cases, improved understanding of biological pattern formation must proceed in tandem with better knowledge of the generative capacity of physical processes.

There is both a need and an opportunity to more effectively integrate laboratory and field observations of fossilization processes with investigations of Earth's early sedimentary record. Multidisciplinary investigations are required in exopaleontological research, and there is a need for new technologies that will enhance our ability to obtain chemical information from individual microstructures.





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