Prospects for Life on Other Planets

This book describes a hypothetical process in which populations of protocells can spontaneously assemble and begin to grow and proliferate by energy- dependent polymerization. This might seem to be just an academic question pursued by a few dozen researchers as a matter of curiosity, but in the past three decades advances in engineering have reached a point where both NASA and the European Space Agency (ESA) routinely send spacecraft to other planetary objects in our solar system. A major question being pursued is whether life has emerged elsewhere than on Earth. The limited funds available to support such missions require decisions to be made about target priorities that are guided by judgment calls. These in turn depend on plausible scenarios related to the origin of life on habitable planetary surfaces. We know that other planetary bodies in our solar system have had or do have conditions that would permit microbial life to exist and perhaps even to begin. By a remarkable coincidence, the two most promising objects for extraterrestrial life happen to represent the two alternative scenarios described in this book: An origin of life in conditions of hydrothermal vents or an origin in hydrothermal fields. This final chapter will explore how these alternative views can guide our judgment about where to send future space missions designed as life-detection missions. Questions to be addressed: What is meant by habitability? Which planetary bodies are plausible sites for the origin of life? How do the hypotheses described in this book relate to those sites? There is healthy public interest in how life begins and whether it exists elsewhere in our solar system or on the myriad exoplanets now known to orbit other stars. This has fueled a series of films, television programs, and science fiction novels. Most of these feature extrapolations to intelligent life but a few, such as The Andromeda Strain, explore what might happen if a pathogenic organism from space began to spread to the human population. There is a serious and sustained scientific effort—SETI, or Search for Extraterrestrial Intelligence—devoted to finding an answer to this question.

Cycles, Compartments, and Polymerization

The two quotes in the epigraph, in juxtaposition, always make me smile, and I tried to keep them in mind while writing this chapter. The first eight chapters of this book have the effect of eliminating the impossible by investigating the facts to which Twain is referring. Perhaps he would consider them trifling, but I doubt that Twain ever performed an experiment to test an idea. Every working scientist knows that science is not just a set of facts but is also a set of questions. The best way to begin answering a question is to pose a hypothesis and that hypothesis begins as a conjecture. Only when we have a hypothesis, can we design experiments to test it, and if we are lucky, the results of those experiments lead us a little closer to the truth. This chapter summarizes facts that lead to an alternative scenario for life’s origin in freshwater hydrothermal conditions rather than a marine origin in saltwater hydrothermal vents. As stated in the introduction to this book, when assumptions are part of the story they will be made explicit so that the logic that arises from them will be clear. What follows in this overview is a list of ten prerequisites we assume are necessary for cellular life to begin, followed by eight assumptions underlying the scenario to be presented here. Prerequisite conditions for life to begin: Dilute solutions of potential reactants are available, together with a process by which they can be sufficiently concentrated to react. Energy sources available in the environment can drive reactions such as carbon fixation, primitive metabolism, and polymerization. Products of reactions accumulate within the site rather than dispersing into the bulk phase environment. Amphiphiles assemble into membranous compartments over the range of temperatures, salt concentrations, and pH values related to each site. Biologically relevant polymers are synthesized with chain lengths sufficient to act as catalysts or incorporate genetic information. A plausible physical mechanism can produce encapsulated polymers as protocells then subject them to combinatorial selection. Organic solutes in aqueous solutions become biochemical solutes within protocells and then substrates supporting a primitive metabolism.

Condensation Reactions Synthesize Random Polymers

Over the past half century of serious research on the origin of life, several schools of thought have emerged that focus on “worlds” and what came first in the pathway to the origin of life. One example is the RNA World, a term coined by Walter Gilbert after the discovery of ribozymes. Other examples include the Iron-Sulfur World of Günther Wächtershäuser and the Lipid World proposed by Doron Lancet and coworkers. Then we have a competition between “metabolism first” and “replication first” schools. The worlds and schools have the positive effect of sharpening arguments and forcing us to think carefully, but they also can lock researchers into defending their individual approaches rather than looking for patterns in a larger perspective. One of the main themes of this book is the notion that the first living cells were systems of functional polymers working together within membranous compartments. Therefore, it is best not to think of “worlds” and “firsts” as fundamentals but instead as components evolving together toward the assembly of an encapsulated system of functional polymers. At first the polymers will be composed of random sequences of their monomers, and the compartments will contain random assortments of polymers. Here, we refer to these structures as protocells which are being produced in vast numbers as they form and decompose in continuous cycles driven by a variety of impinging, free-energy sources. This chapter describes how thermodynamic principles can be used to test the feasibility of a proposed mechanism by which random polymers can be synthesized. There is a current consensus that early life may have passed through a phase in which RNA served as a ribozyme catalyst, as a replicating system, and as a means for storing and expressing genetic information. For this reason, we will use RNA as a model polymer, but condensation reactions also produce peptide bonds and oligopeptides. At some point in the evolutionary steps leading to life, peptides and RNA formed complexes with novel functional properties beyond those of the individual molecular species.

The Early Earth: An Ocean with Volcanoes

Malcolm Walter was talking about the Pilbara region of Western Australia where some of the oldest known biosignatures of ancient life, in the form of extensive fossilized stromatolites, are preserved. The first potential stromatolite was discovered by graduate student John Dunlop, who was studying barite deposits at the North Pole Dome. Roger Buick went on to investigate the biogenicity of the stromatolites for his PhD (Buick, 1985) and Dunlop, Buick, and Walter published their results (Walter et al., 1980). In a prescient paper, Walter and Des Marais (1993) proposed that the ancient stromatolite fossils could guide the search for life on Mars. I have walked with Malcolm Walter through the Dresser formation where the fossils were found. It is humbling to realize that if time passed at a thousand years per second, it would take 41 days to go back in time to the first signs of life on our planet. In any description of events that occurred some 4 billion years ago, certain assumptions must be made. I will try to make assumptions explicit throughout this book, beginning here with the geochemical and geophysical conditions prevailing on the early Earth and Mars. I am including Mars not as an afterthought but because both planets had liquid water 4 billion years ago. Most of our understanding of planetary evolution comes from observations of our own planet, but it is now clear that the Earth and Mars were undergoing similar geophysical processes during the first billion years of the solar system’s existence, with an equal probability that life could begin on either planet. In a sense, the surface of Mars is a geological fossil that has preserved evidence of what was happening there at the same time that life began on the Earth. For instance, Martian volcanoes offer direct, observable evidence that volcanism was occurring nearly 4 billion years ago; making it plausible that similar volcanism was common on Earth even though the evidence has been completely erased by geological and tectonic processes.

Geochemical and Geophysical Constraints on Life’s Origin

Bernal's quote is a bit wordy, but he was basically saying that life can be understood as a continuous chemical reaction, and I agree. Throughout this book I will be describing ideas about how life can begin on habitable planets, which are defined as planets with orbits not too close and not too far from a star so that the temperature permits liquid water to exist. The conditions in which life can begin must have sufficient complexity to permit primitive life to assemble from organic chemicals dispersed in a sterile environment which then begin to react and evolve into more complex structures. This chapter will describe the main parameters of geochemical and geophysical complexity, and then consider them in terms of scales from the nanoscopic to the macroscopic. Questions to be addressed: What scales must be considered to understand how life can begin? What are the properties of the scales? How do the scales relate to the origin of life? The physical dimensions related to the origin of life can be described in terms of four scales—global, local, microscopic, and nanoscopic— and these dimensions must be related to the chemical and physical properties of each scale. The global scale is easiest to understand because the parameters are averages of very broad variables. For instance, we can state that the global temperature today is 15° C and even follow changes in the temperature to accuracies of a tenth of a degree on a year to year basis. However, within the global scale are extreme variations between winter temperatures of - 60° C at the poles and summer temperatures of 50° C in Death Valley, California. Of course, even higher temperatures are associated with hydrothermal fields, up to boiling at 100° C, but sometimes nearer to 90° C because the fields are usually at higher elevations associated with volcanoes. Table 2.1 summarizes the main parameters of the global scale on Earth and Mars today and compares their values with those near the time that life began on the Earth 4 billion years ago.

Testing Alternative Hypotheses

Narratives related to the origin of life all incorporate the assumption that the first living microorganisms emerged in a sterile planetary surface after the ocean condensed over 4 billion years ago. This means that even though global conditions can be deduced from our growing understanding of planetary science and the early solar system, laboratory simulations of localized prebiotic sites remain the only way to guide virtually all of the attempts to reproduce the chemical and physical processes by which life could emerge. Because simulations are used to test the hypotheses described in this book, it seems essential to review representative examples and consider their strengths and limitations. Laboratory simulations of the prebiotic environment incorporate specific sets of factors chosen to reflect what we can deduce about the atmosphere, lithosphere, and hydrosphere of the early Earth and Mars. This chapter attempts to sort out the main factors that are incorporated into simulations. These can be conveniently divided into the drivers—physical and chemical processes—and the emergent complexity that is a product of the drivers. In order to describe a given system, it is helpful to first provide abbreviations for the component factors that define the degree of complexity, then compare that complexity to the complexity that seems to be required for life to begin, as is done in the next section. Questions to be addressed: What factors and assumptions provide a foundation for a given simulation? How do the factors interact to produce increasing complexity? How can the factors guide the design of experimental approaches? What combination of factors would be an adequate simulation of the prebiotic condition? Laboratory investigations related to origin of life research often begin with mixtures of simple organic molecules assumed to be available on the prebiotic Earth. These are then allowed to undergo reactions driven by a source of energy, either impinging on the system such as ultraviolet light or electrical discharge or contained within chemically activated compounds that can undergo spontaneous reactions. The mixture typically becomes more complex in some interesting way, such as the synthesis of polymers described in earlier chapters.

Bioenergetics and Primitive Metabolic Pathways

It seems inescapable that at some point primitive cells incorporated chemical reactions related to what we now call metabolism. In all life today, metabolic reactions are driven by sources of chemical or photochemical energy, and each step is catalyzed by enzymes and regulated by feedback systems. There have been multiple proposals for the kinds of reactions that could have been incorporated into early life, but so far there is little consensus about a plausible way for metabolism to begin. This chapter will briefly review the main ideas that are familiar to chemists as solution chemistry but then ask a new question from the epigraph: how can reactions in bulk aqueous solutions be captured in membranous compartments? This question is still virtually unexplored, but I can offer some ideas in the hope of guiding potentially fruitful approaches. Because metabolism is such a complex process, it is helpful to use bullet points to help clarify the discussion. The first is a list of questions that guide the discussion, the second is list of facts to keep in mind, and the third is a list of assumptions that introduce the argument. Questions to be addressed: What are the primary metabolic reactions used by life today? What reactions can occur in prebiotic conditions that are related to metabolism? How can potential nutrient solutes cross membranes in order to support metabolism? How could metabolic systems become incorporated into primitive cellular life? Metabolism can be defined as the activity of catalyzed networks of intracellular chemical reactions that alter nutrient compounds available in the environment into a variety of compounds that are used by living systems. Most of the reactions are energetically downhill, so there is an intimate association between the energy sources available to life and the kinds of reactions that can occur. Here is a summary of energy sources used by life today: Light is by far the most abundant energy source, totaling 1360 watts per square meter as infrared and visible wavelengths. Chemical energy in the form of reduced carbon compounds is made available by photosynthesis.

Self-Assembly Processes Were Essential for Life’s Origin

In the absence of self-assembly processes, life as we know it would be impossible. This chapter begins by introducing self-assembly then focuses on the primary functions of membranes in living cells, most of which depend on highly evolved proteins embedded in lipid bilayers. These serve to capture light energy in photosynthesis and produce ion concentration gradients from which osmotic energy can be transduced into chemical energy. Although lipid bilayer membranes provide a permeability barrier, they cannot be absolutely impermeable because intracellular metabolic functions depend on external sources of nutrients. Therefore, another set of embedded proteins evolved to form transmembrane channels that allow selective permeation of certain solutes. The earliest life did not have proteins available, so in their absence what was the primary function of membranous compartments in prebiotic conditions? There are three possibilities. First, the compartments would allow encapsulated polymers to remain together as random mixtures called protocells. Second, populations of protocells that vary in composition would be subject to selective processes and the first steps of evolution. Even though any given protocell would be only transiently stable, certain mixtures of polymers would tend to stabilize the surrounding membrane. Such an encapsulated mixture would persist longer than the majority that would be dispersed and recycled, and these more robust protocells would tend to emerge as a kind of species. Last and perhaps most important, there had to be a point in early evolution at which light energy began to be captured by membranous structures, just as it is today. Bilayer membranes are not necessarily composed solely of amphiphilic molecules. They can also contain other nonpolar compounds that happen to be pigments capable of capturing light energy. This possibility is almost entirely unexplored, but the experiments are obvious and would be a fruitful focus for future research. Questions to be addressed: What is meant by self-assembly? Why is self-assembly important for the origin of life? What compounds can undergo self-assembly processes? How can mixtures of monomers and lipids assemble into protocells? We tend to think of living cells in terms of directed assembly.

Integrating Chemistry, Geology, and Life’s Origin Coauthored with Bruce Damer

Chapter 8 recalled John Platt’s recommendation that testing alternative hypotheses is a preferred way to perform research rather than focusing on a single hypothesis. Karl Popper proposed an additional way to evaluate research approaches, which is that a strong hypothesis is one that can be falsified by one or more crucial experiments. This chapter proposes that life can begin with chance ensembles of encapsulated polymers, some of which happen to store genetic information in the linear sequences of their monomers while others catalyze polymerization reactions. These interact in cycles in which genetic polymers guide the synthesis of catalytic polymers, which in turn catalyze the synthesis of the genetic polymers. At first, the cycle occurs in the absence of metabolism, driven solely by the existing chemical energy available in the environment. At a later stage, other polymers incorporated in the encapsulated systems begin to function as catalysts of primitive metabolic reactions described in Chapter 7. The emergence of protocells with metabolic processes that support polymerization of self-reproducing systems of interacting catalytic and genetic polymers marks the final step in the origin of life. The above scenario can be turned into a hypothesis if it can be experimentally tested— or falsified, as described in the epigraph. The goal of falsification tends to be uncomfortable for active researchers. It’s a very human tendency to be delighted with a creative new idea and want to prove it correct. This can be such a strong emotion that some fall in love with their idea and actually hesitate to test it. They begin to dislike colleagues who are critical and skeptical. However, my experience after 50 years of active research is that we need to think of our ideas as mental maps and expect that most of them will not match the real world very well. And so, I say to my students, “When you have a new idea it’s OK to enjoy it and share it with others, but then you must come up with an experiment that lets you discard it.

Hydrothermal Conditions are Conducive for the Origin of Life

Alexander Ivanovich Oparin was first to consider the origin of life in strictly scientific terms. Oparin published The Origin of Life in 1924, in his native Russian language, and was active in the field for the next 50 years. During my initial field work in the volcanic regions of Kamchatka, organized with Vladimir Kompanichenko, we visited the Institute of Volcanology and Seismology in Petropavlovsk, and I happened to see the above quote painted on a wall near the entrance. Oparin’s proposal about how life can begin was intuitive because he had no experimental evidence as a foundation, but as our party rode in helicopters up and down the peninsula from one volcanic site to the next, I began to share his intuition. The focus of this chapter concerns the properties of water in contact with mineral surfaces heated by volcanism, inspired by what we saw in Kamchatka. Four billion years ago, as the global temperature decreased following the condensation of the ocean, there came a point at which the components required for the origin of life could assemble into systems of encapsulated polymers. Two alternative hydrothermal conditions have been proposed as sites where this could have occurred: salty seawater at submarine hydrothermal vents and freshwater circulating in hydrothermal fields associated with volcanic land masses. To weigh the alternatives, this chapter considers the chemical and physical properties of hydrothermal vents and hydrothermal fields and how each could contribute to the origin of cellular life. Questions to be addressed: What are the chemical and physical properties of hydrothermal vents? How do the properties of hydrothermal fields differ from those of vents? How are these properties related to the way that organic solutes can undergo physical and chemical interactions related to the origin of life? Suppose that an organic chemist decides to synthesize a new compound that involves making an ester bond. The chemist is provided with a solution of the two reactants such as acetic acid and ethanol, and then is given a choice: should the reaction be run in an ice bath or instead heated to boiling and refluxed?