and subsequently have been transported to Mars by impacts (or vice versa). A converse difficulty, of course, is that extraterrestrial life could be so different from life on Earth that modern methods would fail to detect it.

EARLY STAGES IN THE EVOLUTION OF LIFE

The papers by both Benner and Deamer (see Session 1) underscore the need to better understand the origin of life and the early evolution of biomolecules in order to prepare for the detection of extraterrestrial life. Earth life today is the product of 4 billion years of evolution. An essential property of cellular life is that it uses linear polymers such as nucleic acids and proteins for, respectively, information storage or transfer and catalytic or structural functions. Little is known about the origin of life or the early stages of evolution that resulted in genetic complexity. One hypothesis is that prior to the appearance of life based on nucleic acids and proteins as the fundamental polymers, a simpler form of life may have consisted of a single fundamental biopolymer resembling RNA. This polymer would have the dual functions of catalytic activity and information storage. Whether singleor dual-polymer life would be more common beyond Earth is an open issue (see the paper by Benner).

The early stages of the origin of life presumably included the self-assembly of organic compounds into more complex structures, perhaps encapsulated molecular systems capable of catalyzed polymer synthesis. In the laboratory, lipids and other compounds can assemble into membrane-bound vesicles that are able to encapsulate proteins and nucleic acids. These systems are in a sense models of primitive or “proto-” cells, but at present they lack the capability to host metabolism. As Deamer argues in his paper, such systems incorporate many of the processes defining life and are worthy of continued study to determine just how closely model systems could be made to simulate living cells.

GENERAL CONSIDERATIONS REGARDING THE DETECTION OF EXTRATERRESTRIAL LIFE

Detection of extant carbon-based life can be attempted at the level of simple organic molecules or at the level of more complex macromolecular biopolymers. Highly sensitive methods for the detection of simple biochemical compounds produced by metabolic processes are nonspecific and hence require few assumptions about the nature of the fundamental biopolymers of life. However, as Pace notes, the interpretation of the detection of simple organic compounds is ambiguous, since carbonaceous meteorites contain amino acids and other compounds that might mistakenly be considered indicative of life. Further testing of such molecules to look for properties such as chirality could help resolve the ambiguity, at least for extant life. Signs of extinct life, which degrade with time (e.g., racemization of the chiral amino acids in the case of the above example), present their own difficulties, which are discussed further in Chapter 3. In his paper, Benner sketches a case study of just such a problem —namely, trying to distinguish organics associated with hypothetical martian life against a background of abiotic organics delivered by meteorites.

Given that life on Earth has at its core polymers that replicate and provide structure and function, a more specific approach to the detection of life is to look for linear ionic polymers. Both Pace and Benner argue that such polymers would be a nearly unambiguous signature of extant life, since there are no known examples of the abiotic production of linear ionic polymers with the complexity of DNA or RNA and proteins. The problem is that techniques that aim to amplify small amounts of genetic material require some a priori knowledge of the nucleic acid sequences. Thus, as Pace cautions in his paper (see Session 1), molecular probes based on terrestrial gene sequences may not detect extraterrestrial life unless it is very closely related to life on Earth. Molecular probes such as the polymerase chain reaction do, however, provide exquisitely sensitive tests for the presence of terrestrial organisms and hence are useful in testing the level of sterilization of spacecraft prior to launch.

An alternative is to try to detect single biopolymers. In his paper, Benner sketches an approach based on the property that such polymers should have regularly spaced positive or negative charges. Single macromolecules, such as nucleic acids and proteins, can be detected by various so-called nanotechnologies under development. One of these—nanopore detection—is highlighted by Deamer and described in detail in the paper by Meller and Branton (see Session 3). Because such technologies are in their infancy, their utility in the search for extraterrestrial



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