Detection of Life
In 1976, NASA's two Viking landers made the first attempts to search for life on the martian surface (see Session 1 paper by Soffen). Since then, we have seen the development of new spectroscopy and microscopy techniques for detecting extremely low levels of organic compounds and determining the isotopic signature and chirality of organic molecules. Most of these technologies are not ready for miniaturization and spacecraft delivery but could be applied to samples returned to Earth. Other new technologies including nanotechnology and microsensors are still in the development stage. One of the most dramatic changes since Viking has been the growth in understanding of the nature of life and the concomitant power of analytical tools in the biological sciences. New molecular techniques have helped to identify a huge diversity of new microorganisms not previously detected by culture methods, and to give new insights into the evolutionary history of microorganisms and their importance in the evolution of eukaryotes and biocomplexity. It is clear that a better understanding of the origin of life and the evolution of nucleic acids and proteins will greatly aid in developing strategies and methods to detect biosignatures on other solar system bodies.
WHAT IS LIFE?
The detection of extraterrestrial life first requires an answer to the question, What is life? We make the assumption that if life exists on other planets or moons, it will be carbon based and dependent on liquid water. It will also be self-replicating and capable of evolving (see Session 1 paper by Pace). Carbon is the best element for creating macromolecules; it can form chemical bonds with many other atoms to produce biochemical complexity. This complexity consists of thousands of catalytic and structural proteins and nucleic acids, the informational macromolecules involved in protein synthesis. All life on Earth evolved from a single type of cell, referred to as the last common ancestor, and thus shares the same genetic code and central biochemistry. Consequently, all terrestrial life can be compared via phylogenetic trees based on small-subunit ribosomal RNA sequences. These trees also indicate the importance of lateral gene transfer or intermixing of genomes as mechanisms for creating evolutionary diversity. Organisms that do not fit into the tree of life as currently understood (e.g., being sufficiently different so as to constitute a fourth domain of the phylogenetic tree) almost certainly would be extraterrestrial. However, this does not necessarily imply a separate origin: Such life might have a common origin with Earth life
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
life can be gauged only after extensive development. However, nanotechnologies constitute a very active area of research, and more than one novel approach to single molecule detection is under development.1
SEARCHING FOR EVIDENCE OF LIFE BEYOND THE SOLAR SYSTEM
While most of the efforts to search for extraterrestrial life are currently focused on either sample return or in situ experiments on Mars, Europa, and Titan (see Chapter 2), there is growing interest in the possibility of detecting habitable planets around nearby stars. One of the strategies for detecting habitable planets is to obtain low- to moderate-resolution thermal-infrared spectra of their atmospheres (see the Session 1 paper by Kasting).
A spectroscopic examination of Earth's atmosphere would reveal the presence of CO2, H2O, and O3 (ozone), the last as a proxy for the spectroscopically inactive O2. The analyses would also show that atmospheric O2 is orders of magnitude out of thermodynamic equilibrium with reduced gases such as CH4 and N2O. Extreme disequilibrium in these gases may signify the presence of living organisms and therefore could be useful for detecting life on extrasolar planets.
The presence of ozone in the atmosphere of an extrasolar planet is a particularly interesting bioindicator since it could signify the presence of O2 at concentrations indicative of photosynthesis. However, it is conceivable that there are planets inhabited only by anaerobic microorganisms or that O2 produced by photosynthesis is titrated in situ by reduced metals such as iron. The geologic record tells us that Earth's atmosphere had a very low O2 content during the first 2 billion years. Thus, for almost half of its history, Earth would have appeared lifeless by the “ozone criterion,” even though it supported life. Furthermore, a planet in the process of rapidly losing its water by atmospheric escape, as Venus might have early in its history, would show a strong signature of ozone even though life might not be present.
Methane could also be a bioindicator, particularly if found in high concentrations. Anaerobic microorganisms produce most of the methane on Earth. These methanogens are strict anaerobes and are believed to be evolutionarily ancient. There are caveats for using methane as a bioindicator. It is produced by abiotic sources as well, and very little is known about the compositions and chemistry of early Earth-type anoxic atmospheres.
Another technique that could be used to detect evidence of extraterrestrial life outside the solar system is radio astronomy. With this method, many organic molecules have already been detected and their abundances determined in interstellar and circumstellar gas. The bases of identification are the unique rotational spectra of these chemical compounds. With the continuing improvement in detector sensitivity, more complicated species, including isotopic and isomeric variances, may be detected in the interstellar medium that could be bioindicators.
Deeper understanding of the evolution of the planets in our own solar system, particularly Mars and Venus, will provide some ground truth on the possible evolutionary paths that planets may take away from habitability, and the consequent spectroscopic signatures.
1. H. Craighead, “Separation and Analysis of DNA in Nanofluidic Systems,” AAAS Annual Meeting, San Francisco, 2001.