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1 Overview In the 1970s, the major focus of the Committee on Planetary Biology and Chemical Evolution of the Space Studies Board (SSB) centered on the planet Mars. Initially, attention was directed at assessing the prospects for finding extant organisms on that planet and developing guidelines for the biological containment both of terrestrial materials that might be transported to Mars and of Martian material that might be returned to Earth. Later, as data from the Viking spacecraft and from ground-based simulations of the Viking biological experiments were made available, the committee became involved in the analysis and interpretation of the Viking Mars biological data. In the past decade, the scientific thrust of the committee shifted substan- tially from solar-system exploration to studies of the Earth with the publica- tion of two documents. The first of these, Origin and Evolution of Life Implications for the Planets: A Scientific Strategy for the 1980s (SSB, 1981), while discussing the state of knowledge in the field of planetary biology and chemical evolution, proposed as its main recommendation the develop- ment of an integrated new program to study the global interactions between terrestrial organisms and this planet. More recently, this theme was further elaborated with the publication of Remote Sensing of the Biosphere (SSB, 1986a). Other recent reports of the Space Studies Board that relate to this field are A Strategy for Earth Science from Space in the 1980s, Part I (SSB, 1982) and A Strategy for Earth Science from Space in the 1980s and 1990s, Part II (SSB, 1985~. Superficially, it might appear that the search for life on Mars and studies of the Earth's biosphere are remotely related areas of scientific inquiry. In fact, both avenues of study have, as their basis, a desire to gain an under- standing of the processes involved in the development and maintenance of a biota on a planet, which requires insight into the interactions between an 16
OVERVIEW I 7 evolving biological system and its planetary environment. Thus the overall theme of studies on chemical evolution and planetary biology is one of evolution. A considerable amount of new information and new methodology has accrued since publication of these earlier reports. The purpose of the pres- ent report is to review the current state of this field, which has expanded into several new areas of inquiry. This report indicates objectives of future research in the various aspects of planetary biology and chemical evolution and makes detailed and comprehensive recommendations for accomplishing them. Issues that were inadequately discussed in earlier reports are empha- sized here, and issues that have been treated in detail before are omitted. For example, this report does not consider current interactions between the Earth and its biota, because detailed attention was given to various aspects of this issue in the 1986 SSB study. As can be seen in the following chapters, the subject matter of this field which is generally referred to as "exobiology" involves a very wide spectrum of disciplines from astronomy to paleobiology. Some of the re- search described requires direct analysis of solar-system objects through the use of spacecraft; other areas utilize both ground-based and space-borne observational techniques to probe the solar system and beyond; and still other lines of investigation are to be carried out in the field and in terrestrial laboratories. Despite their seemingly disparate natures, the different ele- ments of planetary biology and chemical evolution constitute an integrated whole. They can be viewed as a continuum in which the areas of study are bound together by one underlying goal: to understand the evolution of liv- ing systems. In addition, what makes this field the domain of the space sciences is its explicit reliance on spacecraft and space technology for im- plementation of this major goal. This report, then, assesses the status of our knowledge concerning the history of living things from prebiological epochs to the present. As such, the field extends backward in time to when the elements necessary for life (the so-called biogenic elements) were incorporated into the organic com- ponents of the gases and grains of the interstellar medium from which our solar nebula was formed, and it attempts to trace the history of these materi- als. Chapter 2 discusses the area of chemical evolution, whose goal is to understand the nature of the chemical and physical transformations of the biogenic elements from their nucleosynthesis to their ultimate incorpora- tion into planetary bodies. To achieve this understanding requires informa- tion from diverse sources, involving not only ground-based observations and laboratory studies but also space-borne observations and analyses of objects such as comets, asteroids, other solar-system bodies, and different stellar and protostellar systems. In addition, investigators studying the ori- gins of the molecules that ultimately become the precursors for biology
18 THE SEARCH FOR LIFE'S ORIGINS must interact with the astronomical, astrophysical, and planetary science communities and participate with them in the planning and execution of relevant programs. Chapter 3 discusses a cardinal problem in the history of biological evolu- tion, namely, elucidating the processes by which materials came together on the primordial Earth to form the first living organisms. Were the biogenic elements and organic molecules on the newly accreted Earth already present and adequate for the subsequent emergence of life? What, if any, were the contributions of comets, meteorites, and cosmic dust? What was the envi- ronmental "envelope" on this planet that permitted, first, the emergence of life and, then, its subsequent maintenance? For answers to these questions, the geological record on Earth for this period of its history is essentially unavailable, owing, in part, to weathering and to tectonic and volcanic activity. Indeed, the cumulative effects of billions of years of metabolic activity on the part of the Earth's biota itself have also contributed to the erasure of earlier records of the Earth's history. However, records do go back to this epoch elsewhere in the solar system. Portions of the surfaces of the Moon and Mars apparently have survived since the formation of the solar system and are, in principle, available for detailed analyses. The outer planets and their satellites, as well as comets and asteroids, hold additional clues on the nature of the primordial solar nebula from which the Earth was formed. It should be apparent, therefore, that one very important source of data with which to model the environment of the Earth during the period when life was emerging is the knowledge gained from intensive exploration of the solar system. Laboratory studies aimed at finding mechanisms to explain the specific chemical processes that produced replicating molecules on Earth constitute an integral and essential part of the research activity involved in decipher- ing the course of chemical evolution. The current status of this area of research and recommended future studies are detailed in Chapter 4. Since the pioneering experiments of S. Miller and H. Urey in the early l950s, great strides have been made in elucidating plausible reactions by which most of the biologically important monomers could have been produced on the prebiotic Earth. Many of these processes were discussed in the 1981 SSB document. However, new insights have been obtained in more recent years in a number of areas relevant to studies on the origin of life. These include the demonstration of routes by which polymerization of nucleic acid monomers may have occurred on the primitive Earth, new ideas about the possible role of RNA as a self-replicating molecule, and new theories about the possible role of clays. All of these require further intensive investigation and indicate that fresh approaches to research on the origin of life are in order. When the first replicating system appeared on this planet is unknown at
OVERVIEW 19 present, although it probably occurred early in our planet's history. Sedi- mentary rocks deposited 3.5 billion years ago contain paleontological and geochemical evidence that microbial communities were already well estab- lished at that time. Could the origin of life have occurred soon after the Earth was formed, that is, during the postaccretionary period when large objects were impacting the Earth? What would have been the effects of these large impacting bodies on the stability of the Earth's biota? Some insight into these questions has already been obtained from studies of the impact basins on the Moon, because much of its early record still remains. However, more comprehensive studies of impact craters on the Moon, Mars, and other solar-system bodies aimed at characterizing the size and velocity of the impacting bodies are required to understand how these large projec- tiles may have influenced early life on Earth. Scientists interested in chemi- cal evolution and planetary biology clearly must interact with geologists and geophysicists in these studies, and their interests must be represented in the planning of lunar planetary missions. As discussed in Chapter 5, the nature of the first replicating system on this planet is also unknown. The fossil record, which goes back 3.5 billion years, provides only general clues concerning the properties of the organ- isms of that period. Moreover, the nature of the organisms that must have existed even earlier can be constrained in only a general way by this evi- dence. To address this problem, a powerful new approach has come into extensive use in the past decade. This involves techniques of comparative molecular biology, in which large molecules, particularly nucleic acids, from diverse organisms are sequenced in order to compare their relatedness. From such determinations, evolutionary "trees" can be deduced, and the sequence of appearance of different types of organisms can be inferred. From this line of investigation, it has been possible to trace evolutionary lineages back to very early points in the course of biological evolution, when different kinds of organisms diverged from each other. Further re- search along these lines may enable a characterization of the so-called uni- versal ancestor of all extant organisms. In this regard, any meaningful inferences about the nature of this ancestor must be made within the context of reasonable models of the terrestrial environment of that period. As a corollary, studying the phylogeny of organisms can also lead to inferences about the environment of the early Earth. Deducing the early environmental history of the Earth, although largely the province of geologists and planetary modelers is thus of great impor- tance to biologists interested in early evolution. Indeed, evolutionary biolo- gists are becoming increasingly aware that the course of biological evolu- tion, over its entire history, simply cannot be viewed as operating indepen- dently of planetary conditions. Variations in the Earth's climate due to orbital perturbations, changes in solar luminosity, and impacts of comets or
20 THE SEARCH FOR LIFE'S ORIGINS asteroids on the Earth are among the extrinsic factors recently proposed by evolutionary scientists as agents that could substantially modify the Earth's climate, with resulting changes in the direction and pace of evolution of terrestrial biota. This new view of the potential role of extraterrestrial events portends increased interdisciplinary activity on the part of the biological and space science communities. The interplay between these sciences is creating a synergism of benefit to both. For example, paleobiological data indicating periodicity in mass extinctions of terrestrial organisms have stimu- lated inquiry into possible periodic astronomical phenomena and their ef- fects on the environment. The intellectual content of chemical evolution and planetary biology cannot be complete without a consideration of whether the processes thought to have led to the origin and evolution of terrestrial life also occurred on other bodies in the universe. Recent astronomical observations suggesting the formation of planetary systems around other stars point to the very real potential for discovering extrasolar-system planets in the not-too-distant future. In an upcoming SSB study, A Strategy for the Detection and Study of Extrasolar Planetary Materials: 1990-2000 (in press), the current status of this field is reviewed and techniques to implement a search for other planetary systems are discussed. In Chapter 6 of the present report, the detection of extrasolar-system planets is also considered, but the focus is on whether other planets exist that have undergone chemical evolution to the point of producing living systems and on techniques that might be used to determine the presence of organisms on such bodies. As this report attempts to show, research in chemical evolution and plane- tary biology is addressing one of the most fundamental and historically persistent questions humans have asked: how did living things come to be on this planet? A broad outline of the processes by which primordial mat- ter, derived from ancient lineages of stars, ultimately was transformed into living organisms is reasonably well understood. However, many of the details and, more important, many of the key questions involved remain for future study and discovery. Only an interdisciplinary attack, pursued along many different lines of research, is likely to resolve the major issues. The recommendations in the ensuing chapters delineate specific research objec- tives over the broad range of disciplines that constitute this field.