Introduction

It is easy to envision our earliest ancestors, hands under chins, staring out into starlit space, wondering about their existence and how they came to be. With infinite possibilities it is no wonder that a myriad of gods could show their face in the galaxies and set the stage for the birth of religions, their theology, a moral code of ethics, a special and exclusive position for humans, and eventually a method of inquiry that has led to modern science. Furthermore, the early history of thought about life elsewhere in the universe was linked to more of a theological perception of man's relationship to gods, and Earth as the exclusive domain of life – exemplified in the thirteenth-century Aristotelian–Thomistic synthesis of science and religion.

It is particularly interesting and germane to astrobiology today, that it was astronomers, and particularly those in the fifteenth to seventeenth centuries, that were the first to make quantitative observations that questioned the orthodox view of man's place in the universe. One looked to the heavens for answers to the most profound philosophical questions and in doing so helped establish the scientific method as another method of inquiry. Perhaps a lesson from this early philosophical-based history is that the drive to grow knowledge is a fundamental and perhaps evolutionary characteristic of humans. The implication is that our survival is likely to be dependent on our exploring all possibilities that help us to understand the how, why, and uniqueness of our existence. Perhaps, no topic inspires this need to explore more than the search for life elsewhere with its implications for the origin of life and the possibility of life forms unrelated to Earth life.

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.

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).

The emerging field of astrobiology encompasses a daunting variety of specialties, from astronomy to microbiology, from biochemistry to geology, from planetary sciences to phylogenetics. This is both exciting and frustrating – exciting because the potential astrobiologist is continually exposed to entirely new ways to look at the world, and frustrating because it is difficult to understand new results when venturing outside the confines of one's own specialty. There are now many excellent popular books on astrobiology, but a scientist wants more details and more sophistication than these afford. Where can an astronomer without any formal biology since high school learn the basics of cellular metabolism? Or the principles of evolution? Or notions about alternative forms of life? And where can a microbiologist with little physics and no astronomy learn the basics of how a planetary atmosphere works? Or how the Earth formed? Or how planets are detected around other stars? This book is designed to fill these needs.

We have endeavored to cover all the important aspects of astrobiology at an advanced level, yet such that most of the contents in every chapter should be understandable to anyone versed in any relevant science discipline. We envision our youngest readers to be science majors near the end of undergraduate study or the beginning of graduate study. And at the other extreme, we aim to serve scientists who haven't taken an academic course for forty years, but are intrigued by the nascent field of astrobiology.

From Lamarck to Darwin to the central dogma

The basic notion of evolution is that inherited changes in populations of organisms result in expressed differences over time – these differences are at the gene level (the genotype) and/or expression of the gene into an identifiable characteristic (the phenotype). The important underlying fact of evolution is that all organisms share a common inheritance, or, put more dramatically, all extant organisms on Earth evolved from a common ancestor. We see this in the universal nature of the genetic code and in the unity of biochemistry: (a) all organisms share the same biochemical tools to translate the universal information code from genes to proteins, (b) all proteins are composed of the same twenty essential amino acids, and (c) all organisms derive energy for metabolic, catalytic, and biosynthetic processes from the same high-energy organic compounds such as adenosine triphosphate (ATP).

In On the Origin of Species Charles Darwin (1859) (Fig. 10.1) built his theory of evolution using evidence that included an ancient Earth thought at the time by many geologists to have an age in millions of years. He also took the extinction of species to be a real phenomenon since fossils existed that were without living representatives.

A new synthesis: the Biological Universe

A remarkable shift in our scientific world picture is taking place, potentially as fundamental in its consequences as the new views put forth by Copernicus in the sixteenth century, or by Darwin in the nineteenth. Although astronomers have long been involved with the prospects for extraterrestrial life, their fundamental task since Newton has been to apply physics to a lifeless Universe. On the other hand, biologists have pursued their studies for centuries in cosmic isolation, meaning that biology considered life on Earth, with no attention paid to its cosmic context. Today, however, both camps are recognizing fruitful and exciting avenues of research created by a new synthesis. Biology is vastly enriched when attention is paid to a broader context for life as we know it, as well as the possibilities for other origins of life. And astronomy is coming to realize that the themes of cosmic, galactic, stellar, and planetary evolution, which have become central over the past century, must now also incorporate biological origin(s) and evolution(s). Historian of science Steven Dick (1996) has hailed this new synthesis as the Biological Universe. Although astronomy and biology are its two primary poles, many other disciplines are also vital components, in particular Earth and planetary sciences.