In this paper, I discuss contributions that the new field of astrobiology can make to questions of life’s origins. I am an astronomer and space scientist, not a biologist or biochemist. My perspective is therefore that of an interested outsider. But as an astrobiologist, I look at the state of knowledge in the field and try to make some judgment about directions in which current research seems to be taking us.
The paper has 3 parts. First is a discussion of the nature of astrobiology, using the NASA Astrobiology Roadmap as a way of organizing the subject. Second is a review of the conditions on earth when life began. Third is a perspective on current origins research.
The nature of astrobiology
The United States National Aeronautics and Space Administration (NASA) has encouraged the new discipline of astrobiology by organizing workshops and technical meetings, establishing a NASA Astrobiology Institute, providing research funds to individual investigators, ensuring that astrobiology goals are incorporated in NASA flight missions, and initiating a program of public outreach and education. NASA’s role comes from its history of studying the origin of life and searching for evidence of life on Mars and elsewhere in our solar system. These studies have traditionally been called “exobiology”. Under the broader umbrella of astrobiology, however, research has expanded to include the search for life within other planetary systems, as well as investigation of the response of terrestrial life to global changes on the earth and to exposure to conditions in space and on other worlds. Astrobiology addresses not only our origins, but also our aspirations to become a space-faring civilization.One description of astrobiology is provided by the NASA Astrobiology Roadmap (available at http://astrobiology.arc.nasa.gov/roadmap/). This Roadmap, completed in 1999, defines the content of astrobiology as perceived by scientists at its birth. It is a starting point only, and astrobiology is maturing as new information is obtained and diverse scientists bring their own perspectives to this discipline.
Astrobiology addresses 3 basic questions, which have been asked in some form for generations.
- How does life begin and evolve? (Where did we come from?)
- Does life exists elsewhere in the universe? (Are we alone?)
- What is life’s future on earth and beyond? (Where are we going in space?)
1. Understand how life arose on the earth.
Terrestrial life is the only form of life that we know, and it appears to have arisen from a common ancestor. How and where did this remarkable event occur? We can now perform historical, observational, and experimental investigations to understand the origin of life on our planet. We should determine the source of the raw materials of life, either produced on this planet or arriving from space. We should seek to understand in what environments the components may have assembled and what forces led to the development of systems capable of deriving energy from their surroundings and manufacturing copies of themselves.2. Determine the general principles governing the organization of matter into living systems.
To understand the full potential of life in the universe, we must establish the general physical and chemical principles that lead to the emergence of systems capable of energy extraction and growth (catalysis and metabolism), generating offspring (reproduction), and changing as conditions warrant (evolution). Must all life be based on something similar to terrestrial biochemistry and molecular biology? How can laboratory experiments and computational simulations help us to understand life as a more general phenomenon?3. Explore how life evolves on the molecular, organism, and ecosystem levels.
Life is a dynamic process of changes in energy and composition that occurs at all levels of assemblage, from the molecular level to ecosystem interactions. Much of traditional research on evolution has focused on organisms and their lineages as preserved in the fossil record. However, processes such as the exchange of genetic information between organisms and changes within DNA and RNA are key drivers of evolutionary innovation. Modern genetic analysis, using novel laboratory and computational methods, allows new insights into the diversity of life and evolution at all levels.4. Determine how the terrestrial biosphere has co-evolved with the earth.
Just as life evolves in response to changing environments, changing ecosystems alter the environment of earth. Scientists can trace the co-evolution of life and the planet by integrating evidence acquired from studies of current and historical molecular biology (genomics) with studies of present and historical environments and organismal biology. We seek to understand the diversity and distribution of our ancient ancestors, to identify specific chemical interactions between the living components of the earth (its biosphere) and other planetary subsystems, and to trace the history of earth’s changing environment in response to external driving forces.5. Establish limits for life in environments that provide analogs for conditions on other worlds.
Life is found on the earth anywhere liquid water is present, including such extreme environments as the interior of nuclear reactors, ice-covered Antarctic lakes, suboceanic hydrothermal vents, and deep subsurface rocks. To understand the possible environments for life on other worlds, we must investigate the full range of habitable environments on our own planet, both today and in the past.6. Determine what makes a planet habitable and how common such worlds are in the universe.
Where should we look for extraterrestrial life? Based on our only example (life on earth), liquid water is a requirement. We must therefore determine what sort of planets are likely to have liquid water and how common they might be. Studying the processes of planet formation and surveying a representative sample of planetary systems will determine what planets are present and how they are distributed, essential knowledge for judging the frequency of habitable planets.7. Determine how to recognize the signature of life on other worlds.
We are poised on the brink of searching for life, past or present, on a variety of worlds. This search requires that we be able to recognize extraterrestrial biospheres and to detect the signatures of extraterrestrial life. We must learn to recognize structural fossils or chemical traces of extinct life that may be found in extraterrestrial rocks or other samples. And we must develop a catalog of possible signatures of life that can be identified astronomically in planets circling other stars.8. Determine whether there is (or once was) life elsewhere in our solar system, particularly on Mars and Europa.
Exciting data have presented us with the possibility that at least two other worlds in our solar system have (or have had) liquid water present: Mars and Europa. Extensive exploration of the Martian surface will be required to evaluate the total potential for life on that planet, both past and present. Furthermore, exploration of the subsurface probably offers the only credible opportunity to find extant life on either Mars or Europa.9. Determine how ecosystems respond to environmental change on time scales relevant to human life on earth.
Research at the level of the whole biosphere is needed to examine the habitability of our planet over time in the face of both natural and human-induced environmental changes. To help to ensure the continuing health of this planet and to understand the potential long-term habitability of other planets, we need predictive models of environment–ecosystem interaction.10. Understand the response of terrestrial life to conditions in space or on other planets.
What happens when terrestrial life is moved off its home planet and into space or to the moon or Mars, where the environment is very different from that of earth? Can organisms and ecosystems adapt to a completely novel environment and live successfully over multiple generations? Are alternative strategies practical, such as bioengineering organisms for specific environments? The results from attempting to answer such questions will determine whether earth’s life can expand its evolutionary trajectory beyond its place of origin.Although it is defined in terms of a research agenda, astrobiology also lends itself to education and outreach. The three theme questions strike a chord of interest among both students and the public. Courses built around these questions offer a powerful platform to discuss issues such as deep time, astronomical and biological evolution, and our place in the universe. On a slightly more sophisticated level, this multidisciplinary field illustrates different styles of approaching science such as contrasting the historical versus experimental research and exploratory versus hypothesis-driven research. A new NSF-supported upper-school curriculum, “Voyages Through Time”, provides a highly appealing introduction to evolution on multiple levels: evolution of the universe, planets, life, and intelligence. At the college level, many astronomers (in particular) have begun to offer general-education courses on “astrobiology” or “life in the universe”. Two new college-level textbooks have been published, and the popularity of such courses is rapidly growing.
The origin of life on earth: Context
The first goal of astrobiology discussed above is to understand the origin of life on earth. Such a study requires that we look at the astronomical and planetary evidence concerning the early environment of earth, as well as the likely chemical pathways that led to life. This study overlaps with the two goals that deal with the general conditions for the origin of life in the universe and with understanding the evolution of life on earth, especially in the microbial world.Let us be clear at the beginning that we do not understand the origin of life on earth in any detail. Indeed, we are not even sure that life began here. There are some arguments that Mars might have been a more suitable environment for the origin of life 4 billion years ago. Since Mars and Earth have exchanged materials throughout their history, it is possible that life has migrated from one planet to another. This modern form of panspermia has its advocates, but the simplest hypothesis is that life formed on earth. If it began on Mars instead, the processes are probably similar to those that we hypothesize for our own planet.
The solar system formed 4.5 billion years ago from a collapsing cloud of gas and dust that already contained a rich complement of organic material. Astronomical investigations of similar “molecular clouds” that exist today have revealed more than 120 molecules, including such complex substances as ethyl alcohol. The so-called biogenic elements (oxygen, carbon, nitrogen, sulfur, and phosphorus) are among the most common interstellar constituents, once we get beyond hydrogen and helium, which make up 99% of the visible universe. Given the abundance of hydrogen and oxygen, water is one of the main molecules. The simple building blocks for life were thus readily available even before the formation of the planets.
The planets themselves condensed from a disk of gas and dust spinning around the protosun. Some of the pre-existing organic chemicals probably survived this formation process, but most may have been destroyed and then reconstituted within the cooling disk, and perhaps destroyed a second time as the planets coalesced. We know from the study of the oldest meteorites that organics were abundant in the disk; the common carbonaceous meteorites are composed of a few percent carbon by weight, partly elemental and partly in the form of organic compounds. One of these, the Murchison meteorite, yielded 74 separate amino acids. Most of these included equal amounts of right- and left-handed molecules, indicating their non-biological origin. The earth and other rocky planets accreted a veneer of volatiles (including water) and organics from the rain of comets and meteorites that continued for the first half-billion years after the surface cooled. These external sources may have been a more important source of organics than Miller-Urey–type synthesis in the atmosphere and ocean, especially as the initial atmosphere of earth is now thought to have consisted largely of carbon dioxide and been neither strongly reducing nor strongly oxidizing.
What were conditions like on the early earth? Since no rocks have survived from that era, we do not know for sure, but some generalizations seem robust. Although initially hot, the surface layers cooled quickly, and oceans formed. The hot interior undoubtedly contributed to a high rate of volcanism, but surface conditions were then, as now, dominated by solar heating, not volcanism. From their study of stellar evolution, astronomers are confident that the early sun was about 35% less luminous than today (a condition called the “faint young sun paradox” by those who note the contradictory evidence for a relatively constant surface temperature over the history of the earth). Therefore either the earth had a large atmospheric greenhouse effect to maintain surface temperatures above freezing or else the primitive oceans froze. We can imagine an initial carbon dioxide greenhouse effect that partly compensated for the faintness of the sun but left frozen oceans like the Arctic Ocean today. The marine environment thus paradoxically included both a relatively cold surface and an abundance of volcanically-driven hydrothermal systems in the depths. However, there probably were not any of Darwin’s “warm little ponds” on the surface, and the surface might have been bathed in ultraviolet light, depending on the mass and composition of the early atmosphere.
Other external agents in addition to the faint sun influenced the environment of the early earth. The lunar cratering history, among several lines of evidence, shows that the rate of asteroid and comet impacts on the earth was much higher before 3.9 billion years ago. Although it is unclear whether there was a short-lived burst of impacts (a “late heavy bombardment”) or a steadily declining impact rate dating all the way back to the accretionary period, the impacts were sufficient to influence the surface environment. Then as now, the greatest effects are from the rarest, largest impacts, happening at intervals of millions of years. It is likely that the earth was struck several times with sufficient energy to boil away most or all of the oceans. Although the surface would cool and the oceans recondense within a few thousand years of such an impact, the effects on any nascent life would have been catastrophic. This bombardment by a few projectiles hundreds of kilometers in diameter has been termed the “impact frustration” of the formation of life. It suggests that life might have formed several times and then been wiped out in such a sterilizing catastrophe. It also suggests the presence of one or more thermal bottlenecks in the early evolution of life, a topic I will return to below.
The origin of life on earth: Evidence
There is very little surviving geological evidence from the first 500 million years. What we know of impact history, for example, is derived from studies of the moon, not directly of the earth itself. The earliest fossils date from sometime after the end of the heavy bombardment.Study of the early geological record of life dates back half a century, when Stanley Tyler, Elso Barghoorn, and William Schopf identified fossil microbes in the 2.1 billion–year-old Gunflint chert. By 1993, Schopf had found what appeared to be the oldest fossils in the Apex chert of Western Australia at 3.46 billion years. Schopf also suggested on the basis of morphological evidence that these fossil microbes were probably photosynthetic cyanobacteria. However, this work has recently come under attack, and at this writing the situation remains unresolved. In particular, the crucial conclusion that photosynthesis was operative on earth 3.5 billion years ago is in dispute. In any case, there seems to be no question that microbial fossils can be dated to at least 3.0 billion years. Macroscopic fossils in the form of stromatolites — layered constructs built up by generations of microbial mats — have also been found with similar ages.
A complementary approach is to look for an isotopic signature that indicates the presence of life in sufficient quantities to influence the global chemistry of the planet, even if individual fossils have not survived. Stephen Mojzsis and others argue on this basis for the presence of diverse bacteria on earth before 3.85 billion years. If these interpretations are correct, the interval between the end of the late heavy bombardment and the development of a robust global biota is remarkably short.
The major alternative way to study early life is to examine genomic evidence. Similarities and differences in DNA and RNA sequences illustrate relationships related to their lineage. In the case of the metazoans whose fossil remains dominate natural history collections, genomic analysis is a powerful supplement to more traditional studies of evolution. In the microbial world, such studies provide us with almost our only access to the lineages of life. Given that life on earth was exclusively microbial for the first 85% of its history, and that microbes still dominate in terms of biomass and range of habitats, these tools are invaluable for the astrobiologist. Much of astrobiology research is focused on the smallest but most numerous of life’s creatures.
Carl Woese pioneered the comparison of 16s mitochondrial RNA, a highly conserved sequence that can be found in almost every living thing. By the late 1980s, he had established the division of life into 3 domains, Bacteria, Archaea, and Eukarya. The molecular phylogenetic “tree of life” based on mitochondrial RNA provides us with an entirely new way to look at the diversity of earth’s biota. This diversity, and by implication its evolutionary history, is dominated by microbes within all three domains; the metazoans that have evolved since the Cambrian explosion are banished to a few outlying twigs. Although we do not know the rate of change for mitochondrial RNA in any absolute sense, the conclusion is clear that natural selection has been at work throughout the development and diversification of the microbial world. Today’s microbes should not be called primitive; they are in fact highly versatile creatures that occupy a much greater range of ecological niches than do the more familiar Cambrian metazoans.
Molecular phylogeny is based on the relationships among extant biota. It cannot be used to analyze the mineralized fossils that make up most of the historical record of life on earth — we cannot, for example, use gene mapping to compare an Eohippus with a modern horse, as we can a human and a chimpanzee. Still less are we able to determine the genomic content of ancient microbes, which must have been quite different from anything that survives today. But it is possible to determine which extant microbes are probably similar to the inferred precursors of modern life. This is sometimes ambiguous, especially when we consider that there has been a history of gene transfers among different lineages that can shuffle the deck in ways that make reconstruction nearly impossible. With these caveats, however, a number of suggestions have been made that the most “primitive” organisms today are anaerobic thermophiles — that is, microbes that are happy in oxygen-free environments at high temperatures. Many are also methanogens, microbes that generate methane. These studies suggest, even if they do not prove, that our earliest common ancestors had similar properties.
Even if the common ancestor or ancestors of today’s life were high-temperature, methane-producing microbes, this does not mean that these are representative of the first life. Almost certainly there were many precursors that existed and evolved before the invention of DNA. In addition, however, the last common ancestor is likely to have been the survivor of a “bottleneck” resulting from a catastrophe that wiped out its predecessors. One such possibility is the largest impacts of the heavy bombardment. If the surface and upper layers of ocean were sterilized by an impact, the most likely survivors would be thermophiles from the ocean depths, and it is these survivors who could have repopulated the planet.
The origin of life on earth: Theory
Putting the pieces together to form the first life is a daunting problem. Many scientists who look at the great progress that has been made in understanding the chemical steps along the road toward life are justifiably pleased and optimistic. Others look at the huge gaps that remain and are more cautious. The following is the briefest overview of many complex issues. In preparing this summary, I have been influenced by Belgian Nobel laureate Christian de Duve (Vital Dust: Life as a Cosmic Imperative [New York: Basic Books, 1995]; Life Evolving: Molecules, Mind and Meaning [New York: Oxford University Press, 2002]) and by Australian physicist Paul Davies (The Fifth Miracle: The Search for the Origin and Meaning of Life [New York: Simon & Schuster, 1999]).The earliest life needed to acquire several basic capabilities. These include assembly of the necessary raw materials within a structure, metabolism (extracting useable chemical energy from the environment), and reproduction, which ultimately involved information-storing molecules such as RNA and DNA that were themselves capable of replication. Each of these is a challenge, and they can hardly have appeared simultaneously.
The first step was surely the chemical factory that extracts energy and uses it to assemble complex molecules. Many such chemical reactions were possible, especially in a rich organic “soup” of amino acids and other organic chemicals. The key was to be able to select and control the rate of these reactions using the biological catalysts called enzymes. The energy sources could have included the conversion of sugars to alcohol or lactic acid by fermentation, or the formation of methane from carbon dioxide and hydrogen by oxidation, depending on available raw materials.
As chemical synthesis became more important, it was necessary to segregate different materials physically. Such segregation can be accomplished by membranes composed in part of lipids, which react with water to form nearly impenetrable barriers. A number of recent experiments and computer simulations have studied simple membranes and the ways they can incorporate proteins to permit partial permeability. A successful cell (or protocell) must eventually develop the ability to admit food and expel waste. Such simple membranes can readily form closed quasi-spherical chambers. In one interesting experiment, organic materials extracted from a meteorite spontaneously formed such closed systems when exposed to water.
Today, DNA is life’s primary information storage and retrieval system, but we also use a simpler system based on RNA, which has the property of participating in protein synthesis as well as storing information. Most workers now think that an RNA world must have preceded the development of DNA. Gerald Joyce of the University of California, San Diego, among others, has carried out extensive experimental studies of the RNA world, demonstrating the ability of RNA to evolve in the test tube. DNA could later have been developed as a more stable information storage system by something akin to the reverse transcription process that can still be observed today.
What processes brought these components together? De Duve makes the case that it cannot have been chance — the probabilities are far too small for self-assemblage of even the simplest such systems in the lifetime of the universe. To paraphrase de Duve, the process must have involved many chemical steps that had a high probability of taking place under prevailing conditions. This progression must have led from prebiotic organic chemistry to biochemistry, and selection effects must have been important in favoring certain chemical pathways. If this is correct, the process could have been rapid, and life should have been able to start in the few million years of stable conditions that separated major impact catastrophes.
The product of this sequence of events — the first protocells — may have been quite different from life that survives on earth today. Even the oldest common ancestor of today’s life probably represents a much more sophisticated system than the first recognizable life. But once natural selection came into play, the means existed for life to evolve. The key challenge, it seems to me, is to understand the selection processes that acted before the formation of the first protocell. It is these processes that must have guided the complex sequence of chemical changes that gave birth to life on the early earth, and, if life exists there, in the rest of the universe, too.