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s The Evolution of Cellular and Mullicellular Life INTRODUCTION Our perspective on biological evolution is that it is a cosmic phenome- non, born of galactic and solar-system processes and influencing the further development of planetary surfaces where it occurs. Although traditional studies of terrestrial evolution have considered biology to be a system apart from and, in many respects, independent of the physical Earth on which it resides, increasing evidence compels us to reject this view in favor of a concept of life as intimately linked with the crust, sediments, oceans, and atmosphere, through an interacting series of biogeochemical cycles. In- deed, life is an outgrowth of solar-system and planetary evolution. The characteristic feature of the evolutionary process is its dependence on context. The unfolding of terrestrial life must be understood as contin- gent on the particular course of this planet's development. Both the origins of life on Earth and its subsequent evolution have been influenced strongly by events in the evolution of the physical Earth and by extraterrestrial phenomena (such as halide bombardment) that have impinged the Earth throughout its history. To understand the evolution of terrestrial life, a much more integrated understanding of Earth's biological and physical his- tory must be developed. Such an understanding is requisite to determining the extent to which the course of biological evolution on Earth can be regarded as a general feature of life and, thereby, likely to be representative of life throughout the universe. The earlier phases of evolution are most likely to share common charac- teristics on different planets. As biological systems on Earth became more complex, they came to have a greater influence on their own evolutionary development and that of the planetary surface as well. The major determi- 91
92 THE SEARCH FOR LIFE'S ORIGINS nants of early evolutionary change on Earth, however, were changes in the physical environment. Hence, although the course of evolution of more complex organisms elsewhere in the universe is difficult to predict using Earth as a model, the major transition from physical to biological evolution of organic matter, similar to that thought to have occurred on Earth, is expected to characterize all planets whose early physical evolution is com- parable to that of our own. In recognition of the need for an integrated approach to evolutionary problems, another report of this committee (SSB, 1981) recommended the following: "It is essential that data obtained using molecular methods with live organisms be evaluated in the context of the sedimentary rock record." Since the preparation of that report, methodological progress has been so substantial that it has become possible, indeed necessary, to expand and refine the recommendations of that report. Integration of the Earth's biological and physical history now seems to be attainable: molecular approaches permit the inference of evolutionary relationships for all extant life. This advance is complemented by advances in electron microscopy, making it possible to define ultrastructural pheno- types and trace their development in microorganisms. Renewed exploration of diversity in prokaryotic metabolism, spurred by the recognition of the archaebacteria as a distinct prokaryotic kingdom, has demonstrated the ex- istence of evolutionarily important bacterial metabolisms that were unknown a decade ago. Knowledge of the early Earth has expanded in concert with this biological progress and, for the first time, evidence has accumulated that forces us to consider the role of extraterrestrial factors in determining patterns of terrestrial evolution. With this in mind, the committee has articulated four goals for future research on the evolution of cellular and multicellular life. These four goals, which are components of a larger primary goal of understanding the interrelationships between physical and biological evolution on planetary surfaces, seem appropriate for NASA sponsorship and coordination. These goals and objectives have not been prioritized because all are necessary for the integrated understanding to which we aspire. Balance, rather than pri- oritization, is the key to a successful research program in cellular and mul- ticellular evolution. In delineating the specific objectives of these goals and the recommended research, it is important to note the critical role of NASA in coordinating and catalyzing the interdisciplinary study that will be necessary. No other agency is capable of providing the conceptual or intellectual umbrella for the evolutionary research advocated in this chapter. The goals defining a strategy for research on cellular and multicellular life, together with their component objectives, are described below.
THE EVOLUTION OF CELLULAR AND MULTICELLULAR LIFE 93 GOAL 1: To develop a universal understanding of the temporal se- quence and evolutionary relationships of life on Earth. Recent molecular data support the view that all extant (living) organisms on Earth descend from a common ancestor. At this time, there are three principal lines of evolutionary descent from this common ancestor, namely, the three major phylogenetic groups: the archaebacteria, the eubacteria, and the eukaryotes. Evolutionary relationships within and among these groups can be examined by a variety of biological, paleobiological, and geological means. Traditionally, evolutionary relatedness has been assessed by com- parison of phenotypic characters. This system of inquiry has worked rea- sonably well for plants and animals, but it has been of only limited value in defining relationships among fungal, protistan, and prokaryotic microorgan- isms because of their simple morphology (phenotype) and lack of fossil preservation. The committee suggests that a different combination of in- sights (items 1 to 3 below) will provide a markedly improved understanding of archaebacterial, eubacterial, and eukaryotic evolution: 1. Molecular phylogeny: All living organisms contain an extensive record of their own phylogenetic history. Nucleic acid sequencing technol- ogy now provides ready access to much of this biologically incorporated history. From sequence comparisons (for homologous functions), quantita- tive evolutionary relationships can be inferred, and these serve as concep- tual frameworks within which to relate phenotypes and their temporal evo- lution, as well as the ecological and geological conditions surrounding the evolutionary process. 2. The characterization of phenotypes: The characterization of pheno- types provides an independent set of biological data that can indicate path- ways and possible causes of morphological and/or biochemical evolution. Among prokaryotes (archaebacteria and eubacteria), many of the most im- portant characters currently used in phenotypic characterization are meta- bolic, whereas in eukaryotic protists they are predominantly ultrastructural. In plants, animals, and fungi, these are largely anatomical and morphologi- cal. Consideration of phenotypes and ecology in the context of molecularly derived phylogenetic relationships permits the generation of hypotheses concerning the conditions under which evolutionary innovations arose. 3. Paleontological and geological record: The paleontological and geological record provides a testing ground for these hypotheses and can further illuminate the causes of environmental changes or occurrences asso- ciated with significant evolutionary events. Paleontology traces the actual course of terrestrial evolution, indicating the sequence of appearances and disappearances of preservable phenotypes and placing constraints on the timing of evolutionary origins. Equally important, the analysis of sedimen-
94 THE SEARCH FOR LIFE'S ORIGINS TABLE 5.1 Information Content of a Precambrian Fossiliferous Rock 1. Physical environment of deposition Bedding and sedimentary structures 2. Chemical environments of deposition and diagenesis Petrology and geochemistry 3. Biota living in, or transported to, the site of deposition (incomplete sampling) Micropaleontology Chemical indicators of metabolism (e.g., carbon isotopes of carbonate and organic matter; sulfur isotopes of pyrite and sulfate; molecular fossils) Traces of microbial activity (stromatolites, microphytolites, oncolites) 4. Chemical indices of global tectonic and biogeochemical systems Strontium isotopic ratios Carbon isotopic ratios Sulfur isotopic ratios Rare earth element abundances Oxidation state of weathered surfaces tary rock sequences in which fossils are found provides important clues to the environmental evolution of the Earth's surface. A given sedimentary sample contains information on the sedimentary, tectonic, and geochemical environments in which organisms lived and/or were buried. It also records many features of its subsequent diagenetic history and may contain isotopic indices to the pulse of biochemical cycles and tectonic activity (see Table 5.11. From the examination of samples along environmental gradients in a single time plane come paleoenvironmental and paleogeographic reconstruc- tions, as well as the determination of paleoecological distributions of coex- isting organisms. Analyses of time sequences of samples (normalized for environment) provide the geological evidence for both biological and physi- cal evolution and the means of relating the two. OBJECTIVE: To study a wide variety of organisms by using the tech- niques of molecular phylogeny and biochemical and morphological charac- terization. A decade ago biologists regarded our understanding of eukaryotic phylo- geny as fairly complete, whereas evolutionary relationships among bacteria were considered unknown quantities. Today bacterial relationships at higher taxonomic levels are regarded as well known, whereas increasing data have exposed our ignorance about eukaryotic phylogeny. The committee be- lieves that the time is ripe for concerted effort on fundamental questions of eukaryotic cell evolution. Speculation has long focused on certain bacterial characters of the major
THE EVOLUTION OF CELLULAR AND MULTICELLULAR LIFE 95 organelles the mitochondria and chloroplasts. It is now abundantly clear, from molecular phylogenetic comparisons particularly of rRNAs that mitochondria and chloroplasts are derived from specific eubacterial groups. However, there are many morphologically and biochemically distinct ver- sions of these organelles, only a few of which have been inspected in terms of molecular phylogeny. Questions therefore arise as to whether mitochon- dria and chloroplasts each arose only once or many times. Moreover, it is now clear that substantial genetic exchange has occurred between mito- chondria and chloroplasts and their host eukaryote nuclei. The mechanisms and rationale for the genome mixing are not understood, but the occurrence has important implications for the evolution of eukaryotic cells. The con- tinuing molecular investigations of eukaryote and prokaryote diversity should include analysis of the organelles as well, so that their origins and coevolu- tion with host cells can be evaluated. Nucleated organisms have been thought to be descendants of forms re- lated to present-day prokaryotes and, hence, viewed as more recently evolved taxa. However, the emerging phylogenetic framework inferred from com- parisons of small-subunit (~16S) RNAs shows that the sequence diversity of eukaryotic RNAs eclipses that of archaebacteria or eubacteria. Such a finding is consistent with the hypothesis that the eukaryotic nuclear line of descent represents an extremely old superkingdom, derived directly from the "progenote" rather than from prokaryotes belonging to the other two superkingdoms. The eubacterial line of descent, however, contributed the two major organelles, the mitochondria and the chloroplasts. The breadth of genotypic and biochemical diversity of the eukaryotes observed thus far is represented by members of the Protista; the microsporidians, euglenoids, and trypanosomatids have been identified as particularly early branches on the eukaryotic tree. Other eukaryotic divisions especially plants, animals, and fungi appear to have arisen nearly simultaneously during a relatively recent radiation. Despite these revelations, the understanding of eukaryotic evolution is limited by the small number of taxa examined to date and by the inability to bring genotypic phylogenies into juxtaposition with relationships inferred from comparisons of phenotypes and the fossil record. Several other protis- tan groups, including the dinoflagellates and oxymonads, may represent additional early branches in the eukaryotic line of descent, but comparative morphology cannot be solely relied upon for inferring branching order. The uncorroborated assignment of primitive status to particular characters is frequently difficult to determine. Recent progress in ultrastructural research, coupled with molecular phylogeny, promises to clarify these issues. No- table successes include the new phylogeny of green algae, which differs appreciably from traditional phylogenies, and which greatly clarifies the evolutionary history of this group and its descendants, the land plants.
96 TlIE SEARCH FOR LIFE'S ORIGINS Metabolic Diversity Molecular data show that the main phylogenetic diversity of life on Earth is in the microbial world, both prokaryote and eukaryote. Traditional micro- biological investigations have relied on laboratory cultivation for the char- acterization of organisms. Yet, it is known that many, perhaps most, organ- isms in natural microbial populations have not been, and perhaps cannot be, maintained as pure laboratory cultures. This is attested to by the continued discovery of novel organisms, the abundant occurrence of symbiotic asso- ciations, and the recognition of large, so far uncultivated populations of organisms. An outstanding example of the latter is the marine planktonic microbiota, which was long considered sparse on the basis of laboratory cultivation. Over the past decade, however, direct microscopic investiga- tions have revealed an abundance of marine "picoplankton" (organisms less than 2 sum in diameter), including eukaryotes, prokaryotes, phototrophs, and heterotrophs. Cultivation attempts have repeatedly failed to retrieve repre- sentative forms; thus, a potentially major influence on the biosphere re- mains uncharacterized. Methods for analyzing phylogenetic and quantita- tive aspects of natural microbial populations, without laboratory cultivation, are now available. These methods use RNA gene cloning and sequencing approaches. The expansion of such approaches to many populations and environments should be encouraged so that a full representation of phylo- genetic diversity may be achieved. It is of critical importance to the understanding of eukaryotic evolution that the sequence sampling be integrated with expanded studies of the ul- trastructure, ultrastructural development, and biochemistry of sequenced taxa. A fuller understanding of eukaryotic evolution is necessary if data from this superkingdom are to be pooled with those from eubacteria and archaebacteria to make informed inferences about the nature of the "progenote." Just as electron microscopic studies have revealed a hitherto unappreci- ated diversity of eukaryotic phenotypes in terms of ultrastructure, so too have recent biochemical studies of prokaryotes revealed a metabolic diversity significantly broader than previously thought. The isolation and characterization of microorganisms having previously unknown metabolic capabilities (especially among the archaebacteria, cyanobacteria, and sulfur- reducing prokaryotes) have markedly revised some of the traditional "meta- bolic dogma." The discovery of these metabolic potentials has allowed the expansion of our understanding of modern ecosystems, both aerobic and anaerobic, and thus has enlarged the cast of characters available for modeling ancient ecosystems. The phylogenetic characterization of these new organ- isms will provide further insights into the evolution of metabolism and may, in conjunction with geological data, allow scientists to constrain the time of origin of certain metabolic phenotypes. Through such coupled phenotypic, molecular, and geological studies it
THE EVOLUTION OF CELLULAR AND MULTICELLULAR LIFE 97 may be possible to resolve more satisfactorily the succession of ecosystems that have characterized our planet through history. Limiting steps in such ecosystem reconstruction are the isolation and characterization of organ- isms whose unique metabolic capabilities allow them to persist in extreme environments. For example, although seemingly unusual in the context of today's envi- ronments, recently discovered thermophilic, anaerobic, sulfur-dependent archaebacteria may provide important clues to the nature of the early bio- sphere. The expanding repertoire of prokaryotic metabolisms can be used to guide further research. For example, among presently known archaebacteria, manybut not allof the complementary metabolisms needed to complete biogeochemical cycles are known to exist. The question of whether archaebacteria could maintain element cycles in the absence of eubacteria is especially important for an understanding of biological diversification on the early Earth. GOAL 2: To determine the properties of the universal ancestor of extant organisms. Understanding the universal ancestor of life on Earth is critical to under- standing evolution on this planet. Because of the general evolutionary principle of descent with modification, the nature of the earliest biological entities constrained subsequent evolution and, in the attempt to link biologi- cal to prebiological chemical evolution, also constrains our views of how life arose. The earliest life forms on Earth must have been far simpler than any organism now alive. Although these primordial organisms are extinct, clues to their genetic organization, metabolic capabilities, and other phenotypic characteristics are retained in the biological traits of unicellular organisms that represent early branchings in each of the primary lines of descent. By comparing features common to these evolutionary lineages, it may be pos- sible to infer the phenotypes of the earliest organisms; it is probable that features common to early branching groups will reflect features possessed by their common ancestors. As mentioned above, studies of molecular phylogeny have brought the universal phylogenetic "tree" within reach. In addition to providing the starting point for inquiries into the course of biological evolution within the three primary kingdoms, this set of phylo- genetic relationships provides a framework for asking about life's earliest history before segregation of the three extant lineages. OBJECTIVE: To root the universal phylogenetic tree. Our understanding of the ancestor of all extant life rests heavily upon knowing the root of the universal "tree" of phylogenetic relationships. One needs to know, for example, whether the archaebacteria and eubacteria are
98 THE SEARCH FOR LIFE'S ORIGINS specifically related to one another to the exclusion of the eukaryotes. One also needs to know whether the archaebacteria are more primitive in pheno- type than the other two major types. Although the position of the root of the universal tree must be known to answer both questions, a partial answer to the second can be given in the absence of this knowledge. Although phylogenetic relationships are customarily rooted by invoking known outgroup species, that is not an option for the tree that encompasses all life. However, this does not mean that the root of the universal tree is unknowable. Another way to determine the rooting is through comparisons among sequences of genes that have doubled and functionally separated while still in the universal ancestry state, provided that their sequences have retained a sufficient degree of similarity. Thus, the search for gene families overlaps the problem of rooting the universal tree. The universal ancestor, the so-called progenote, is considered to have been a distinct entity ancestral to, but qualitatively different from and more rudimentary than, any of its daughter lineages. It was sufficiently primitive that its capacity to transmit information from genotype to phenotype would have been more limited than that of its descendants; that is, translation at this stage was presumably more inaccurate (prone to errors) than in later organisms (see also Chapter 4~. Four salient characteristics follow from such translational inaccuracies: (1) proteins at this time would not have been typical of proteins seen in cells today (e.g., they were probably smaller than modern proteins if they were accurate translations of genetic messages); (2) the genes the organism carried would have been fewer in number than prokaryotes now carry; (3) genes at this stage were perhaps not arranged in large linear arrays (genomes); rather, they may have been physically separate entities, probably composed of RNA, not DNA; and (4) in all respects the level of biological specificity for the progenote was presumably lower than now exists. As living systems evolved from this state to those represented by the three major kingdoms, they would have evolved a more varied set of genes coding for proteins of ever-increasing variety and specificity. Traces of this evolution are preserved in gene families of living organisms. For this reason, therefore, the committee recommends as a principal objective of research the recognition and evolutionary evaluation of gene families. GOAL 3: To understand what factors drive the biosphere. As emphasized in an earlier report of this committee (SSB, 1981), bio- logical evolution has not proceeded independently of planetary evolution, nor has it been immune to influences from Earth's cosmic environment. The fossil record documents numerous episodes of evolutionary radiation; mo- lecular phylogeny also suggests that evolution occurred in bursts. Why does evolutionary history appear to have this pattern? Traditional explana-
THE EVOLUTION OF CELLULAR AND MULTICELLULAR LIFE 99 lions have stressed the evolution of new phenotypic characters (innova- tions) that confer the ability to utilize an underexploited resource or to compete for ecologic success, and some radiations may indeed represent such phenomena. However, many of life's major radiations appear to be related to environmental changes that presented new opportunities or re- moved long-standing constraints. Mammals, for example, radiated early in the Tertiary period not so much because of any innovation on their part as because the previous ecological dominants in many terrestrial niches, the dinosaurs, became extinct at the close of the Cretaceous period. Among eubacteria, evidence begins to suggest that oxygen utilization arose at about the same time in different lineages, probably during the late Archean to Early Proterozoic, which geochemical markers indicate was a time of rising atmospheric oxygen tensions. An interesting example highlights these close interrelationships among biological, tectonic, and environmental changes. Approximately 600 mil- lion years ago, after more than 3 billion years of microbial evolution, macro- scopic animals radiated over the face of the Earth. Why did this evolution- ary burst occur 600 million years ago rather than 1 billion years ago or some other time? Biologists have long stressed that animal life requires certain minimum oxygen tensions to support exercise metabolism, to ensure the diffusion of oxygen to internal cells in organisms having multiple cell layers, and to complete certain biochemical reactions. Recent geochemical evidence suggests that just prior to the observed radiation of the Ediacaran fauna, significant amounts of oxygen may have accumulated in the atmo- sphere a consequence of abnormally high rates of organic carbon burial. The anomalous carbon burial, in turn, correlates well with sedimentary and geochemical evidence for continental breakup and the opening of Late Pre- cambrian ocean basins and suggests a relationship similar to that docu- mented for the Permian through Early Cretaceous periods, when the breakup of Pangaea promoted the accumulation of economically important concen- trations of organic matter. The model that is emerging involves a tectonic event, its biogeochemical consequences, and attendant changes in the com- position of the atmosphere that remove an environmental constraint to the evolution of tissue-grade multicellularity. This model and others like it must be tested and refined in light of new geological and geochemical data, but they underscore the point that biological evolution cannot be understood outside of the context of physical Earth history, and vice versa. OBJECTIVE 1: To integrate the biological accounting of the Earth's historical development with that obtained from studies of the geological record. Substantial progress in unraveling the environmental evolution of our planet will require detailed sedimentological, stratigraphic, paleontological,
100 THE SEARCH FOR LIFE'S ORIGINS and geochemical analyses of well-preserved sedimentary basins, particu- larly those of Archean and Proterozoic age. Global syntheses are only as useful as the data that go into them, and at present the base of carefully collected and analyzed data is insufficient to tie the geological record to the biological record in the way the committee believes is possible. OBJECTIVE 2: To determine the influence of Earth's cosmic environ- ment on evolution. In 1985, NASA published an important workshop report: The Evolution of Complex and Higher Organisms (ECHO). The object of the ECHO report was to explore the last 600 million years of biological evolution on Earth in the context of the Earth's cosmic environment. The report con- cluded with the recommendation that NASA initiate a new study program designed to link existing programs in planetary biology and thereby to in- clude in NASA's overall research effort the important evolutionary events that took place in the interval between the appearance of multicellular life and the evolution of man. During the last 600 million years of Earth history (the Phanerozoic eon), animals, plants, and fungi have diversified on this planet, initially in the oceans and then on land. Complex social behavior has arisen in several phyla and technology in one. In short, the modern biota has taken shape during the last 15 percent of the Earth's development. This evolution was once seen as an orderly progression from simple to complex, with more complex organisms being selected in favor of their more primitive ances- tors. The seeming inevitability of this progression was used as a model for the evolution of life in any planetary system. The evolution of life on Earth was also seen as operating in a closed system not significantly influenced by events and processes in space. The general view of the evolution of advanced life now appears to be grossly oversimplified, to the point of being essentially wrong. The fossils preserved in the Phanerozoic record show evolutionary change (sometimes gradual, sometimes spasmodic), but they do not conform to predictions of linear models of progressive evolution. There is no evidence, for example, that the extinct trilobites of the Paleozoic era were simpler or less special- ized than their modern counterparts. Tropical reefs have been in existence and have flourished throughout most of the Phanerozoic, yet the framework builders of these reefs have varied markedly through time; the coral reef of modern seas is just the current version of a recurrent ecosystem. There is no reason, from first principles, to argue that mammals should have ap- peared when and how they did. Humanoid intelligence evolved only once, but there is no reason it could not have evolved several times in separate lineages or not at all. It is becoming increasingly clear that the Earth's cosmic environment has
THE EVOLUTION OF CELLULAR AND MULTICELLULAR LIFE 101 important influences on biological evolution. The history of life on Earth can no longer be seen as operating in a closed system. During the Earth's history, the Sun's luminosity has increased about 30 percent, day length has increased, ocean tides have decreased, the planet has been bombarded by comets and asteroids, and the solar system has undoubtedly been influenced by the gravitational effects of randomly passing field stars and by occa- sional supernovae. Phanerozoic time has included three galactic years, and the Earth has passed through the plane of the galaxy perhaps 20 times. The ECHO report considered these and other effects while focusing on two questions. (1) Do events and processes in our cosmic environment leave recognizable signatures in rock or fossil records? (2) How have these events and processes influenced the evolution of advanced life? On the one hand, innovative geochemical analyses will be necessary to constrain better the history of atmospheric oxygen or ocean chemistry. On the other hand, better constraints on temperature history, solar radiation, and the like may require information from improved models of solar-system evolution based on comparative planetology and observations of the Sun. There have recently been a number of striking research successes in relating past global biology to solar-system~or galactic influences. Work with the marine micropaleontology of the past 700,000 years has shown rather decisively that cycles of climatic change, including the several pulses of continental glaciation, can be tied directly to Milankovich cycles of or- bital change in the Earth-Moon-Sun system. This work is being extended to recognition of Milankovich cycles deeper in the geologic past. It also shows promise of having important implications for problems of global climate and predictions of future climatic change. Another success is the discovery of geochemical and geophysical evi- dence for a major comet or asteroid impact 65 million years ago, at the time of the terminal Cretaceous mass extinction. Although there is still consid- erable controversy over the role of this impact in the mass extinction of dinosaurs and other organisms, the work has dramatically increased the attention being paid to large-body impacts as influences on past life. At the very least, research on the possible effects of large-body impacts has sensitized the scientific community to think more in terms of cosmic influences on Earth systems. Current estimates of comet and asteroid im- pact rates call for about 12 impacts of objects 10 km or larger, and up to 3600 impacts of objects 1 km or larger, during the Phanerozoic. Although the environmental consequences of these impacts are still poorly known, there is an intriguing possibility that the large number of smaller impacts has been responsible for the lesser regional extinctions that punctuate the history of life. It seems likely that research to date has barely scratched the surface of a new and exciting field of science. Regular and irregular events in space
102 THE SEARCH FOR LIFE'S ORIGINS may be crucial elements in our evolutionary system. In fact, arguments can be made that in the absence of this sort of physical disturbance, biological evolution would have reached steady state hundreds of millions of years ago, thereby preventing the evolution of advanced life as we know it. The most likely scenario is one in which externally induced environmental shocks eliminated dominant organisms at certain times, thus accommodating the innovations so important to long-term evolution. Research on the evolution of advanced life may also have direct benefits in other aspects of space science. For example, if solar-system and galactic history can be documented from the geologic and fossil records, astronomy will have, for the first time, a means of empirical verification of time- dependent processes that can otherwise be treated only theoretically. The foregoing discussion must remain open ended because of the fledg- ling nature of the field. Emphasis is given to Milankovich cycles and large- body impacts because these are areas in which there has been some prelimi- nary success; however, totally different aspects of the cosmic and planetary environment may prove to be important to the global biology of the past, present, and future. To improve techniques for the quantitative understand- ing of environmental conditions on the early Earth, NASA should continue to take the intellectual lead in fostering interdisciplinary research and com- munication among scientists having disparate specialties. It is particularly important that NASA encourage improved communication among molecu- lar or evolutionary biologists, paleontologists, Precambrian geologists, and planetary modelers by sponsoring workshops, symposia, and innovative inter- disciplinary research projects. GOAL 4: To generalize our understanding of environmental and early cellular evolution on Earth by comparative studies of Mars. Because no other planet in the solar system appears to harbor living sys- tems, most scientists have assumed that any comparative study of biologi- cally active planets will necessarily involve other solar systems at great distance from the Earth. This may be true if only present planetary surfaces are considered, but if we look at the geological records of ancient planetary conditions, this assumption may prove to be wrong. The case for exobiol- ogical input into Mars sample return missions has been made by the SSB Task Groups on Planetary and Lunar Exploration (SSB, 1988b, pp. 99-106) and Life Sciences (SSB, 1988a, pp. 47-51~. This committee simply under- scores the importance of exobiological research in any and all future Mars . . missions. OBJECTIVE: To investigate the sedimentary record of Mars which, because of similarities to Earth in its early stages of planetary develop- ment, offers a unique opportunity to expand and generalize our understand- ing of environmental and early cellular evolution.
THE EVOLUTION OF CELLULAR AND MULTICELLULAR LIFE 103 According to the logic developed at the beginning of this chapter, if biological processes arise from physical ones under a given set of physical conditions, and if early stages of evolution on different planets appear, in principle, to be more likely comparable than later stages, then there are compelling intellectual reasons to conduct detailed investigations of Mars for possible evidence of prebiotic and early biological evolution. For these purposes, studies on Mars should concentrate on early supracrustal succes- sions and include well-designed site and sample selection strategies that maximize the potential for evaluating the environmental and possible early cellular evidence of evolution on Mars. PRIORITY CONSIDERATIONS This chapter has stressed the goal of developing an integrated under- standing of the evolution of life on this planet, as well as the exciting prospect of testing the universality of early events in this history through the nonchemical and naleontolo~icn1 examination of ~nri~.nt ~~,nr~r~r'~t~1 rocks from Mars. The research advocated here is highly interdisciplinary, with the consequence that program balance must take precedence over strict prioritization. Nevertheless, the committee can highlight three principal components of any successful research program aimed at understanding the course of evolution on our planet: 1. development of robust phylogenies relating living microorganisms, through the comparison of sequences in informational macromolecules, especially small subunit ribosomal RNAs; 2. elucidation of the biochemical and ultrastructural characters of micro- organisms in order to relate patterns of phenotypic diversity to phylogeny; and 3. development of improved data on the biological and physical devel- opment of the Earth through careful sedimentological, geochemical, and paleontological analysis of ancient sedimentary basins. Geological research should be aimed not only at the elucidation of environmental evolution, but also at understanding the cosmic influences on terrestrial environments and evolution. As discussed in the final chapter of this report, much of the planning for Mars sample return missions will be spearheaded by groups outside of the exobiology research community; however, the committee views the partici- pation of exobiologists in mission planning and execution as essential. It is difficult to imagine more exciting and fundamental questions that can be addressed by such a mission than those concerning the early surficial envi- ronment and the possibility of chemical or even biological evolution on the early surface of our neighboring planet.
104 THE SEARCH FOR LIFE'S ORIGINS Some areas of exobiological research are supported by other agencies in addition to NASA, especially NSF. NASA's continuing support is critical, however, because only it can provide the programmatic integration that promotes the necessary cross-fertilization of the various disciplines relevant to exobiology. Given the structure of NSF, the search for interstellar mole- cules, Archean geochemistry, and microbial metabolism are necessarily viewed as unrelated topics. Only under NASA's aegis are they integrated as components of a single research effort. This fact cannot be overempha- sized.