Abstract
Life in the Universe: Expectations and Constraints, by Dirk Schulze-Makuch and Louis N. Irwin, 3rd ed., Springer, Cham, Switzerland, 2018, 343 pp, ISBN: 978-3-319-97657-0 (Hbk.).
Four books related to the origin of life were published within a few months of one another in 2019, including Beyond Physics by Stuart Kauffman, Symphony in C by Bob Hazen, my own book called Assembling Life, and Life in the Universe by Dirk Schulze-Makuch and Louis N. Irwin. The authors know each other personally and by reputation but did not realize that a couple of years ago all had decided it was time to write a book. Carl Jung referred to this kind of coincidence as the “collective unconscious,” and maybe he was onto something.
I was pleased to be invited to review Life in the Universe. When a review copy arrived, it turned out to be a third edition, which is pretty rare for a book with such an exotic title. The book had its first edition published in 2004 and a second edition 10 years ago, so why was it time for a third edition? The authors answer that enormous progress has been made during the past decade. This is reflected in new sections and chapters describing exoplanets, synthetic life, the search for extraterrestrial life, and a tour of the Solar System for signs of conditions conducive to life.
Then I turned to the dedication. Usually a book is dedicated to an individual human being, but the dedication instead is to all the humans populating Earth, giving a clue to the authors' perspective: “To a species of life on a water-rich planet in an otherwise unremarkable solar system at an outlying part of the galaxy in an unexceptional part of the Universe, just beginning to explore the world beyond the home on which it began to evolve about 4 billion years ago.”
I began to look through the table of contents and was in for another surprise. The titles of chapters actually seemed to live up to the ambitious promise of the book's title. Somehow, the authors managed to write a scientifically sound book about something that doesn't exist. Of course, life might exist in the Universe, but there is no evidence that it does. The authors confess this in the first sentence of their preface: “astrobiology remains the most speculative of all the sciences because it assumes a reality that has not been demonstrated conclusively: the presence of life on other planetary bodies.”
Even if life in the Universe is an act of imagination, life on Earth is a fact, so what we learn about our planet can be a guide as we look for life elsewhere. This is central to NASA's missions to Mars, Titan, and icy moons, and represents a cohesive thread that runs through the book. Although we necessarily use life on Earth as a foundation for exploring possible life elsewhere, the authors make every effort to avoid being what they refer to as Earth centric. As you read through the chapters, time and again you will see the authors illustrate a firm understanding of life on Earth but then venture beyond that core knowledge into other possible ways for life to exist. This is perhaps why the book has gone on to a third edition, because readers appreciate speculations about other forms of life that are still grounded in the laws of chemistry and physics.
The book is composed of 16 chapters with a few illustrations per chapter, half in color. The first eight chapters present biological foundation knowledge from the authors' perspective. For instance, Chapter 2 offers a definition of life, not in the dictionary sense but instead describing properties of life in such a way that anything that is not alive is excluded. I agree with this approach. There have been many attempts to put forth dictionary-style definitions of life, but they all fall short in one way or another because life's complexity cannot be confined in a sentence or two. Table 2.1 summarizes a full-page list of the properties that define life.
Chapter 3 is a knowledgeable discussion of the origin of life, which is a sine qua non for the main theme of the book, life in the Universe. If life began on Earth, similar processes presumably would have given rise to life on other habitable planets. The chapter compares various environments and proposes that three components were essential for life to begin: membranous semipermeable boundaries of cellular compartments, primitive metabolism to capture energy, and a genetic code. I would add several more, such as a mechanism for polymer synthesis, capture of light energy by a pigment system, ion gradients as an energy source, and self-replicating polymers.
Chapter 4 presents a history of life on Earth and has a useful list of how life can adapt to extreme conditions. Chapter 5 discusses energy sources for life and characteristically doesn't just review light and chemical energy; it goes on to “life as we don't know it.” This is an exploration of alternative sources ranging from thermal energy to radioactivity. I don't know of a similar discussion in any other source.
Chapter 6 covers carbon chemistry of life but also includes an extensive discussion of alternatives to carbon, such as silicon. I often get questions about silicon-based life following a talk, and now I will be able to give a much better answer because I learned that silicon in the form of silicate is in fact widely incorporated into some forms of life today. I also learned about the chemistry of silanes and silicones and why these polymers could not serve as a chemical basis for life.
Chapter 7 introduces water as a universal solvent but again expands the discussion to alternative solvents such as ammonia, hydrocyanic acid, hydrofluoric acid, hydrogen sulfide, sulfur dioxide, and hydrogen peroxide, even liquid methane. Who would have guessed that a fungus called Fusarium alkanophilum can thrive in hydrocarbon solvents, making its own water by metabolism?
Chapters 8 and 9 are about habitats and biosignatures and represent a transition to the second half of the book in which we leave Earth and venture into space. Chapter 10 discusses life detection, using the Viking mission and the martian meteorite ALH-84001 as examples. Chapter 11 explores the Solar System from the terrestrial planets to the astonishing moons of Saturn and Jupiter, and Chapter 12 describes exoplanets.
If I were a science fiction writer, I would carefully read Chapter 13, with the title “Ideas of Exotic Forms of Life.” This is by far the most imaginary chapter and includes such far-ranging concepts as life based on spin configurations. The authors recall Fred Hoyle here, the cosmologist who invented the term Big Bang to describe George Gamow's idea that there was a beginning to our universe. Hoyle's major contribution to science was to show how the source of carbon is stellar nucleosynthesis, but he was also a science fiction author who wrote The Black Cloud, about a vast molecular cloud that had achieved intelligence. Hoyle's idea is a perfect addition to Chapter 13. The last three chapters consider the future and fate of life, the search for extraterrestrial intelligence, and finally a practical discussion of how to go about exploring space.
The bottom line of a review can be stated as a question: did the reviewer learn anything from reading the book? For me, the answer is yes. Time and again I was impressed and surprised by the breadth of information presented. The breadth extends to the references, an astonishing 45 pages of some 1200 citations. For researchers working in astrobiology and origins of life, this is a valuable resource. I can recommend the book to anyone who wants to expand their understanding of how life could exist elsewhere in the Universe.
Associate Editor: Russell Shapiro