Introduction

While the notion of worlds beyond our Earth is ancient, the specific idea of planets orbiting distant stars is relatively new. Over two millennia ago Epicurus stated “there are infinite worlds both like and unlike this world of ours,” but he was not speaking of Earth-like planets orbiting Sun-like stars. Indeed, planets orbiting stars would have been a meaningless issue to the Greeks, as the Sun was not recognized as a star, nor the Earth as a planet (Chapter 1).

One of the earliest and most eloquent spokespersons for what is now called astrobiology, and among the first to grasp the implications of the Sun being a star and the Earth a planet, was the mystical Roman Catholic monk Giordano Bruno. In On the Infinite Universe and Worlds (1584) he wrote:

Bruno then concludes with the revolutionary slogan:

Bruno's reward for this prescience and for other heresies was condemnation by the Church, followed by immolation in a public square in Rome in 1600.

Introduction

Prokaryotic microorganisms were the only form of life for at least 80 percent of our evolutionary history (Schopf and Packer, 1987). Multicellular organisms including plants, animals and fungi evolved a mere 0.5–1.0 Ga from single-celled eukaryotic ancestors. Geologists and paleontologists debate the age of life on this planet and when the major microbial lineages first diverged (see Chapter 12 for details). Cyanobacterium-like fossils suggest that life emerged at least 3.45 Ga (Schopf et al., 2002; Schopf and Packer, 1987), but the biogenic origins of these structures are contested (Brasier et al., 2002; Section 12.2.1). The chemical record documents prokaryotic metabolisms that may have existed 3.47–3.85 Ga (Mojzsis et al., 1996) and eukaryotic biosignatures that may be as old as 2.7 Gyr (Brocks et al., 1999). Yet, these are still imprecise interpretations (some might be more recent microbial contamination) and do not set absolute limits on the possible origins of life on Earth. Early periods of heavy bombardment between 4.1 and 3.8 Ga might constrain when life first appeared on Earth, although microorganisms living off chemical energy at kilometer depths could have survived even the largest impact events.

By the standards of multicellular plants and animals, single-cell organisms look relatively simple (Patterson and Sogin, 1993), yet they transformed the atmosphere, the waters, the surface, and the subsurface of the Earth.

Introduction

Picture a future triumph in robotic space exploration: in a complex mission to Mars, a sample has been collected from the martian subsurface near a newly discovered hydrothermally active site at 30°N latitude. Ten years in the detailed planning and execution, the mission's Earth return capsule with its sample canister has landed in the Utah desert. The sample is now under extensive analysis and testing in an ultra-clean containment facility – and initial observations have shown positive indications that it contains life. Only after later testing is completed, checked, rechecked, and repeated is it shown unequivocally that the lifeform contained within the sample is a soil bacterium common to the dirt of an old Soviet launch facility in Baikonur, Kazakhstan, and which has apparently been alive on Mars since a spacecraft crash-landed there in 1972 …

Or picture, as did novelist Michael Crichton (1969) in the very year of the first lunar sample-return mission (Apollo 11), a spacecraft returning to Earth containing a dangerous extraterrestrial organism – The Andromeda Strain – not related in any way to Earth-life and operating by rules scarcely understood even after hundreds of humans have met their grisly demise …

Once you have those events in mind, you are developing a feel for what planetary protection might be, and what it is meant to prevent.

Introduction

How does life begin? Can life arise elsewhere than the Earth? These questions are among the most fundamental and challenging in all of biology. Charles Darwin once wrote to a friend, “It is mere rubbish, thinking at present of the origin of life; one might as well think of the origin of matter.” (Letter to J. D. Hooker, March 29, 1863.) Darwin made this comment when the knowledge required to think about the origin of life and matter simply did not exist. Now, 150 years later, we understand much more. We know that new elements are constantly being synthesized by nuclear fusion of hydrogen and helium in the interiors of stars, then expelled into interstellar space when stars reach the ends of their lives. This matter is the source of new stars and planetary systems, and it is literally true that planets like the Earth and the biogenic elements that give rise to life are composed of “star dust” (Chapter 3). We also know that liquid water once existed on Mars, and perhaps still does beneath the martian surface, suggesting that microbial life may exist elsewhere than on the Earth. Probably most important is that we understand living cells in unprecedented detail, even to the point of knowing the entire sequence of three billion nucleotide bases in the human genome, and we have begun to manipulate the genetic blueprint of life.

Introduction

Habitable planets are those bodies that provide environments, materials and processes that are advantageous for the formation and long-term evolution of life. Understanding the processes that lead to the formation of such planets is a central issue in astrobiology. We are of course handicapped in this quest since Earth is the only example of a planet with proven habitability – the only one known to have provided thermal, chemical, and other physical conditions that allowed life to form and survive for ~3.5 Gyr.

This chapter emphasizes the formation of Earth-like planets, those with environments capable of supporting complex life comparable to Earth's plants and animals. The focus on life comparable to Earth's multicellular organisms is partly due to the practical consideration that we better understand the environmental constraints of such life. Despite this restricted focus, note that most astrobiologists consider that the dominant form of life in the Universe, as it has been on Earth over most of its history, is probably far simpler and more rugged, analogous to Earth's bacteria and archaea (Section 3.2).

This interpretation of habitability is highly Earth-centric and assumes that life elsewhere is similar to terrestrial life and requires environments similar to those of terrestrial organisms. The actual cosmic limits of life are of course unknown, but the Earth-centric view is a reasonable, albeit conservative, place to start. Until there are detailed data on other inhabited planets, discussions of extraterrestrial life would be prudently biased by what is known from our Earth experience.

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.