Why is this question important?

In observing our vast Universe thus far, we have encountered life only on or near the surface of our home planet. Yet life in its properties and behavior is so different from the barren realms that we have surveyed elsewhere, that we cannot help but wonder how it first took root here, and whether things that we would consider alive exist elsewhere. The fossil record on Earth appears to extend to 3.5 Gyr (Schopf et al., 2002) and isotopic evidence suggests the presence of life several hundred million years earlier than that. Recently, however, this evidence has come into question (Brasier et al., 2002), so caution should be used in relying on these conclusions (Section 12.2.1). No hard evidence exists at all, however, concerning the mechanism by which life first began here.

Every human culture has felt the need to address this question, considering its importance in defining our place in the cosmos. In the absence of firm evidence, the door has been left open to a variety of answers from science, mythology, and religion, each defining our place in the Universe in different ways. I will follow a scheme put forward by the scientist and philosopher Paul Davies (1995: 21) and separate the competing points of view into three groups, called Biblical–Creationist, Improbable Event, and Cosmic Evolution.

“Astrobiology” was originally defined as “the consideration of life in the Universe elsewhere than on Earth” (Lafleur, 1941). But as the field has advanced, we have learned to place no artificial barrier between the study of life on Earth and life that may exist elsewhere in the Cosmos. Astrobiology today is “the study of the living Universe” (NAI, 2004), be it here or elsewhere. It would be foolish to narrow the definition, for the approaches we take in searching for extraterrestrial life are strongly informed by our understanding of life on Earth, and our understanding of the origin and evolution of terrestrial life is informed both by the study of other planetary environments and by Earth's environment within the Solar System and Galaxy. As Carl Sagan (1974) remarked decades ago, we are able for the first time in human history to assess life on Earth “in a cosmic context.” The assessment is still nascent and inchoate, but as the chapters in this book illustrate, the floodgates have opened and our knowledge is expanding quickly now. We will soon know much more.

Besides “astrobiology,” the study of life in the Universe has also been called “cosmobiology” (Bernal, 1952), “exobiology” (Lederberg, 1960), and “bioastronomy” (IAU, 2004) (see Sections 2.3.1 and 2.4 for discussion). Under its exobiological label, the entire field was famously criticized by the biologist George Gaylord Simpson (1964), “in view of the fact that this ‘science’ has yet to demonstrate that its subject matter exists!” If astrobiology meant only the study of extraterrestrial life itself, Simpson's criticism would still have weight, four decades later.

Introduction

The process of metabolism, in which cells carry out biochemical reactions, is a hallmark of all living organisms. Catabolic reactions generate energy for the organism while anabolic reactions are used for the synthesis of cell material. Metabolic pathways in today's living organisms have been evolving for more than 3.5 Gyr. In fact, since metabolism would have been necessary even for the earliest organisms, its evolution cannot be separated from the origin of life. Contemporary metabolic pathways are presumed to be much more elaborate and sophisticated than those that first evolved. Indeed, metabolism today is extraordinarily rich and diverse, ranging from the use of various inorganic chemicals such as hydrogen or sulfur for nutrients and energy, to several forms of photosynthesis, to the metabolism of hundreds of organic compounds. It is impossible for us, at least at this time, to know which pathways originated first and how they evolved. Nonetheless, because metabolism is essential to life, understanding how metabolism evolved is of considerable importance. Furthermore, we have good grounds to speculate on which of life's diverse metabolisms evolved earliest and which could only have come later. Microorganisms, most likely resembling present day Archaea and Bacteria, were the first organisms, so it is their metabolism that is of relevance. Indeed, all basic metabolic pathways on Earth today can be traced to microorganisms.

The goal of this chapter is to describe, insofar as possible, the evolution of metabolism. Although there are several principles that guide our considerations, two are predominant.

Problems with the record

Astrobiology has only a single successful experiment in planetary life available to investigate: that on the Earth. Hence, the history of terrestrial life must act as the archetype, albeit an ever more contingent and unique one, for astrobiological models of the appearance and radiation of life anywhere in the Universe. Indeed, it could be argued that all habitable planets would have had similar environmental constraints and pathways of physical and chemical development, so the process of biological initiation elsewhere should be broadly reminiscent of Earth's experience of the phenomenon. If so, astrobiology is saved from the challenges of studying things far away, but is instead faced with the difficulties of examining events here long ago.

Unfortunately, and perhaps surprisingly, the origin and early evolutionary history of terrestrial life is poorly known, as is the corresponding record of environmental conditions on the early Earth. There are many reasons for this. Firstly, like all old things, ancient rocks are rare (Fig. 12.1). Almost all potential information about the first half of Earth's history is contained in geological materials. But rocks of such great antiquity have mostly been hidden or destroyed by geological processes like burial, erosion, or subduction back into the mantle via plate-tectonic recycling of crust along ocean trenches. Even ejection into space by catastrophic meteorite impacts, of which there were plenty during the heavy bombardment that occurred over the first billion years of Earth history (Chapter 3), is a viable mechanism for destruction of the earliest crust.

Introduction

The appeal of astrobiology lies in two primary features, the first being the important questions astrobiologists ask: “Are we alone? How did life begin? How will it all end?” The second attractor is the opportunity to work across many disciplines. Astrobiology challenges us to draw from many intellectual resources in the attempt to answer these questions – biology, chemistry, physics, astronomy, geology, and engineering are all required. Life detection, in particular, requires a strong interdisciplinary approach. In this chapter we focus on life detection within the context of Solar System exploration; techniques for detecting planets and possible associated life beyond the Solar System are discussed in Chapters 21 and 26. Within our Solar System life detection efforts have been and still are primarily focused on Mars (Chapter 18), and so we will use Mars exploration as a model for discussion, though our approach is applicable to any potential habitat for life.

The success of a life detection mission to another planet should not be focused solely upon whether or not live organisms are detected, but rather it must be able to correctly classify observations as evidence of (a) life, (b) non-life, (c) once-alive-but-now-dead, or (d) made-by-life (biogenic). It is essential that the scientific community agree on which measurements should be made, and then how to interpret those measurements.

Communication across the many disciplines involved in astrobiology is fraught with difficulty on many levels, including even the seemingly simple matter of units and usage of terms and abbreviations. When first introduced in any chapter, unusual units often not known to those outside the field are defined. In this appendix we give conventions and conversions for various units used throughout the book.

Astrobiologists hope to understand the origin, history, extent, and future of life in the Universe. This is a huge task, considering that two of the terms in this mission statement are difficult to define. The definition of “life” is itself worth a chapter in this book (Chapter 5). Here in this chapter, we must not only concern ourselves with a definition of life, but also with the concept of “understanding.” What does it mean to say that we “understand life”?

Any attempt to understand life soon engages organic chemistry. Biology today is increasingly focused on the molecular scale. Indeed, it is difficult to find a biologist today who is not attempting to put a molecular structure on the phenomenon that they are studying, so much so that biology can be (provocatively!) viewed as the subfield of chemistry dealing with chemical systems capable of Darwinian evolution.

Some illustrations make this point. The human genome is nothing more (and nothing less) than a collection of chemical structures, recording how carbon, oxygen, nitrogen, hydrogen, and phosphorus atoms are bonded in the natural products directly responsible for heritance. Molecular evolution uses organic chemistry to describe the Darwinian evolution of species, the process that drives biology. Neurobiologists are attempting to describe the inventory of molecules, including messenger RNA, that allow neurons to learn and remember.

Nowhere is this more evident than in the segment of astrobiology that investigates the origin of life.

Earth is not the only body in the Solar System that is habitable. Life as we know it requires liquid water and free energy gradients, both of which probably also exist on Mars and Europa, although liquid water on those bodies is restricted to the subsurface. Earth is, however, the only planet in the Solar System that has liquid water at its surface. Similar planets may exist around other stars (Chapter 21) and would be of profound interest for two reasons. First, biology on such planets might resemble life on Earth. Second, the biosphere on such planets would interact with the planet's atmosphere and could modify it in a way that may be detectable remotely. Today, life may be thriving on Mars or Europa but its discovery will require subsurface exploration. In contrast, we might be able to tell whether a distant Earth-like planet is inhabited by measuring the spectrum of its atmosphere.

Thus, from an astrobiological standpoint, one of the most fundamental characteristics of a planet is its surface temperature Ts. If Ts is not within the range in which liquid water can exist, remotely detectable life will probably not exist there. Consequently, the first part of this chapter is concerned with planetary surface temperatures. The constraint on temperature is not as obvious as 0 < Ts < 100 °C.

Introduction

The search for extraterrestrial life is intimately linked with our understanding of the distributions, activities, and physiologies of Earth-life. This is not to say that only Earth-life could exist on other planets and moons but it is important to know the extent of environmental conditions that can support terrestrial organisms as a first-order set of criteria for the identification of potential extraterrestrial habitats. Even though the life forms may have different biochemistries and in fact may have had different origins, the limits of life on Earth may help define the potential for habitability elsewhere. It is also likely that many of the limits of Earth-life could extend out of the bounds of extreme conditions found on modern-day Earth. This is the case for the bacterium Deinococcus radiodurans that can tolerate levels of radiation beyond those found naturally on Earth, and also for the apparent tolerance by Escherichia coli to hydrostatic pressures that exceed by more than ten times the pressures in the deepest ocean trenches (Cox and Battista, 2005; Sharma et al., 2002).

Since Earth is the only planet that unequivocally supports modern, living ecosystems, it is logical to first look for life elsewhere that resembles Earth-life. Earth-life requires either light or a chemical energy source, and other nutrients including nitrogen, phosphorus, sulfur, iron, and a large number of elements in trace concentration; 70 elements in all are either required or are targets of interaction by various species of Earth-life (Wackett et al., 2004).

Introduction

The question “What is life?” is foundational to biology and especially important to astrobiologists who may one day encounter utterly alien life. But how should one approach this question? One widely adopted strategy among scientists is to try to define ‘life.’ This chapter critically evaluates this strategy. Drawing from insights gained by philosophical investigations into the nature of logic and language, we argue that it is unlikely to succeed. We propose a different strategy, which may prove more fruitful in searches for extraterrestrial life.

We begin in Section 5.2 by reviewing the history of attempts to define ‘life,’ and their utility in searches for extraterrestrial life. As will become apparent, these definitions typically face serious counterexamples, and may generate as many problems as they solve.

To explain why attempts to define ‘life’ are fraught with so many difficulties, we must first develop the necessary philosophical background. Therefore, in Sections 5.3 and 5.4 we discuss the general nature of definition and of so-called theoretical identity statements. Section 5.5 then applies the material developed in these sections to the project of defining ‘life.’ We argue that the idea that one can answer the question “What is life?” by defining ‘life’ is mistaken, resting upon confusions about the nature of definition and its capacity to answer fundamental questions about natural categories (Cleland and Chyba, 2002).

To answer the question “What is life?” we require not a definition but a general theory of the nature of living systems.

Introduction

Ten times farther from the Sun than the Earth is, shrouded in an orange haze, preserved at temperatures near 100 K, Saturn's moon Titan seems an unlikely astrobiological target. In fact, its extremes suggest images of death rather than life.

Yet this planet-sized moon possesses a dense atmosphere of nitrogen and methane, which, over time and with the action of ultraviolet radiation, may have generated 1016 tons of hydrocarbons and nitriles – the constituents of life – that were then deposited on its surface in liquid and solid form. Within these vast organic deposits, at those times when water ice might liquefy because of volcanism or of impact heating, some of the organic chemical steps leading toward the origin of life might be replicated and then preserved for study on the surface of Titan.

Titan's dense atmosphere has easily won it the high status we reserve for terrestrial planets with atmospheres (Venus, Earth, and Mars) although it is consigned to the outer Solar System. It possesses methane-driven meteorology, and its large size for an ice-rock world likely means a wealth of tectonic activity in its interior. A variety of observed surface landforms produced, apparently caused by erosional processes driven by winds and surface liquid hydrocarbons. In short, other than the likely absence of extant life because of the extreme cold, Titan exhibits a broad range of atmospheric and geologic processes that rival in complexity those of Mars and perhaps of Earth.

Exploring knowledge space for microbes

Astrobiology and biology more generally are integrating new visions of biodiversity with evolutionary and ecological processes. The body of knowledge about hundreds of thousands of microbial species is huge, and involves data on ontogenetic transitions and intraspecific variation; encompasses scales of biology ranging from molecular variation in a particular kind of cell to the role of individuals in complex ecosystems; and accommodates the biology of individuals whose identity and role change as a function of time and place, as well as in response to biotic and abiotic interactions. The knowledge may be either digital or in traditional media, such as often found in libraries, museums, and herbaria. Researchers need solutions that lead to a comprehensive and evolving “knowledge space” (which includes information and its interpretation). The solutions will include tools to empower experts to transfer knowledge from traditional to contemporary media, as well as allow them to integrate old with new. An approach that is universal, inclusive, scalable, and flexible can evolve into a comprehensive Encyclopedia of Life (Wilson, 2003).

The absence of websites offering comprehensive treatments of microbial diversity is a serious impediment for students and investigators who are in need of morphological, physiological, and lifestyle information about microbes. In fact, this is a problem not only for microbial life but for all life. There are no robust standards for indexing phenotypic and biodiversity data, it is difficult to parse and recompile existing relevant data resources.

Introduction

Astrobiology is a discipline that is best enjoyed in the field. What follows is a series of short descriptions by University of Washington students and faculty of selected astrobiological destinations that our planet offers. We cannot hope to provide a comprehensive list – with more space and time we might have included the Burgess shale of Canada; the Atacama desert of Chile; the Cretacious/Tertiary boundary at Gubbio, Italy; Louis Pasteur's home and lab in Paris; Witwatersrand mine in South Africa; the channelled scabland of eastern Washington state, to name but a few. Nevertheless, the ten selected locales have played primary roles in determining how we have come to view the phenomenon of life, and how we have placed constraints on its potential occurrence both on our own planet and elsewhere.

From boiling microbial ponds in Yellowstone to frozen wastes of Greenland harboring Earth's oldest sedimentary rocks, a lifetime of exploration awaits you.

Technology, not intelligence

SETI (Search for Extra Terrestrial Intelligence) can be defined as the branch of astrobiology looking for inhabited worlds by taking advantage of the deliberate technological actions of extraterrestrial organisms. This definition usually draws a chuckle during public lectures, but it underscores why this chapter is somewhat different from the preceding ones. As in other parts of astrobiology, one must consider the diversity of physical environments in the cosmos, and the limitations imposed by them. But with SETI one must also consider modifications to the environment that are not just the byproduct of life, but the result of deliberate actions by intelligent organisms intended to achieve some result.

For millennia people have speculated about the existence of other habitable worlds, and their inhabitants (Chapter 1), but the rules of the game underwent a profound change in the second half of the twentieth century. The publication of the initial scientific paper on SETI (Cocconi and Morrison, 1959) and Drake's (1961) first radio search (Project Ozma, described in Section 1.9) turned speculation into an observational science. No longer were priests and philosophers the sole respondents to the “Are we alone?” question; scientists and engineers could work on finding an answer empirically. Following the first flurry of observing programs in the US and the Soviet Union (Chapter 2), the acronym SETI became the accepted name for this new exploratory activity. But, in fact, SETI is a misnomer because there is no known way to detect intelligence directly across interstellar distances.

People have wondered for centuries whether we are alone or share this Universe with extraterrestrial beings. Questions about the origin of life and the possible existence of extraterrestrial life have deep roots in the history of both science and culture (Chapters 1 and 2; Dick 1997, 1998). In ancient times, the debate centered mainly on our uniqueness versus the plurality of worlds. Following the advances of the Copernican era, scientists gradually accepted notions about the plurality of solar systems and recognized the large-scale nature of the Universe. Today, cosmic evolution, from the Big Bang to the evolution of intelligence, has become a working hypothesis for astrobiology, one that combines characteristics of both biological and physical universes into a “biophysical cosmology.” This new world view recognizes the enormity of the Universe and hypothesizes that life is one of its basic and essential properties, a cosmic imperative rather than an accidental or incidental property found only on Earth. The current key questions in astrobiology are whether biological laws reign throughout the Universe, whether Darwinian natural selection is a universal phenomenon rather than simply a terrestrial one, and consequently whether there may be other biologies, histories, cultures, religions, and philosophies beyond Earth. In short, is the ultimate outcome of cosmic evolution merely planets, stars, and galaxies – or life, mind, and intelligence? The answers to these questions raise a multitude of issues in the realms of both science and society.