structural information, requirements for sample preparation, and degree of reliability in distinguishing biogenic materials from those produced by nonbiological processes.

METHODS FOR DETECTING FOSSILS

Evidence of extinct life can be sought in detail at various levels ranging from macroscopic stromatolitic structures to microfossils to the intramolecular distribution of carbon isotopes in organic compounds. Depending on their depositional environment, fossilization mechanism, and diagenetic history, both macro- and microscale biogenic structures, including biofilms, can be preserved with varying amounts of their original organic contents. In all cases, especially in the absence of organic matter, the field context of the sample site and the texture and fabric of the structures are critical in determining biogenicity. Since the latter also undergo degradation over time, laboratory and field studies of both fossilization and subsequent diagenesis processes are needed to determine the time scales over which biogenic signatures are lost or preserved under various environmental conditions (see the paper by Cady in Session 4).

The paper by D. McKay, also in Session 4, describes a variety of electron beam techniques. Scanning and transmission electron microscopy combined with optical microscopy provide powerful tools for characterization of putative fossil structures, often in three dimensions, with respect to their location within the mineral matrix, morphology, texture, and size. The energy-dispersive x-ray analyzer (EDXA) and electron energy-loss spectrometer (EELS) attachments and electron microprobes (EMs) provide essential chemical and mineralogical measurements that may support a biogenic origin. Similarly, ion-beam techniques such as time-of-flight–secondary ion mass spectrometry (TOF-SIMS) afford high-spatial-resolution imaging of organic matter and even stable isotopic measurements of specific structures or locations within structures.

No one of these techniques, however, can unambiguously address the question of biogenicity. In light of the continuing controversy surrounding martian meteorite ALH84001, it remains unclear whether the use of an array of these methods can provide unequivocal proof of biological structures (see the paper by Kirschvink in Session 3). The absence of organic remains within the structures makes the problem even more difficult. Highly relevant in this context are studies aimed at determining what biological morphologies, fabrics, and features cannot be produced by inorganic processes.

MOLECULAR AND ISOTOPIC METHODS FOR DETECTING EXTINCT LIFE

Developments in the chemistry of natural products and organic geochemistry over the past decades have yielded molecular structural, isotopic, and stereochemical attributes that are common features in compounds of biological origin. These properties have been instrumental in establishing the antiquity of life on Earth and in tracking the early evolution of biological innovations in the geological record. The use of these traits for discerning extraterrestrial life hinges on how common they are to all biochemistries, earthly and alien. Studies of the organic chemistry of meteorites and laboratory simulations of planetary chemistry provide criteria for characterizing products of nonbiological processes. In the latter case, the value of abiogenic criteria depends on their absence in all biochemistries. Application of both sets of criteria to the organic matter in extraterrestrial samples holds the promise of distinguishing biological from nonbiological materials, as Becker's paper argues (see Session 4).

A lesson of evolution on Earth is that a small number of universal enzymatic processes govern biosynthesis at the cellular level in all life. To fulfill requirements for structure and function, these processes impose distinctive patterns of restricted variation in molecular structure on the building blocks of membranes, proteins, and nucleic acids, the major components of living systems. Moldowan describes these patterns in his paper in Session 4. For membrane lipids, enzymatic pathways preferentially synthesize a small number of specific carbon chain isomers over a broad distribution of chain lengths. Repeating isopentenyl or acetyl subunits are a structural motif, chirality occurs at quaternary (or tertiary-substituted) carbon sites, and—where branched isomers occur—the branching exhibits positional preference. In peptides, only 20 amino acids among the multitude of possible structures are commonly employed and because of their biosynthetic pathway, all are levorotatory α-amino acids with an α-hydrogen. Enantiopurity, characteristic of virtually all quaternary carbon centers in biology, represents a



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