From very tentative beginnings, astrobiology is experiencing the intellectual version of a Cambrian explosion, as interest in the field attracts talented young scientists and research centers sprout up in universities around the world. One such researcher is Lewis Dartnell, who holds a faculty position in Experimental Biology at the University College London. While some of the more speculative outputs of the field make for great popular infotainment, explaining the fundamental principles of astrobiology to public audiences, as with most of the natural sciences, presents challenges of its own. While popular television science shows prefer to focus on the outlandish shapes that life on alien worlds might take, much of what astrobiologists do is concerned with the complex metabolic processes and biochemistry at the heart of cells. With his debut book, Life in the Universe, Lewis Dartnell has done a creditable job covering the important basics of astrobiology, even though he has chosen the difficult path of focusing on the hard science of astrobiology rather than engaging in whimsical speculation.
Life in the Universe is a difficult book to review, because it is immediately clear that it is not intended for all audiences. With a heavy focus on microbial biology, the target audience of Dartnell’s book is probably limited to those who have a solid understanding of chemistry and familiarity with the natural sciences. When a winner of the Carl Sagan Medal calls your book “comprehensive and authoritative”, as David Morrison does on the dust jacket of my copy, you know that the contents are scientifically rigorous. Dartnell’s book would serve as an excellent introductory textbook for an undergraduate course in astrobiology, but it will challenge casual readers. However, those who really want to delve into the questions at the heart of this exciting new field will find lots to digest in this volume.
Some readers might be surprised that the majority of Life in the Universe is spent describing the conditions that allow microbial life to survive here on Earth. But those basics are important if we want to understand the potential limits to the evolution of life in alien environments. The book begins with a detailed explanation of the chemical processes that allow earthly cells to produce energy and store information. Dartnell explains the differences between the two main types of organisms on earth, eukaryotes and prokaryotes, and then goes on to describe the mechanisms by which these organisms gather energy and reproduce. By choosing to place the section on cellular metabolism at the beginning of the book, Dartnell may alienate some casual readers, although this section is written as clearly as possible using scientific terminology.
Later in the book, Dartnell takes us on a tour of some of the most extreme environments on earth, which might be expected to mimic the harsh environments of other planets that may fall near the extreme edge of their stars’ goldilocks zone of habitability. But even here, Dartnell explains, scientists have found abundant examples of life. From chemoautotrophs, which harvest nutrients from deep sea underwater vents, to cells which survive in near boiling conditions near acidic hot springs, life on Earth has filled just about every available biological niche, within a temperature gradient of more than 100 degrees Celsius and pressure gradients of several hundred earth atmospheres.
It is clear, when Dartnell takes the reader further out on a journey of planets and moons in our own solar system, that life would have to be very adaptable indeed to survive in any of these places. The desiccated, freezing and highly irradiated surface of Mars would seem to be the best bet, but Dartnell suggests that life is unlikely to be found near the surface, if at all. Trace amounts of methane found in the martian atmosphere may point to simple biology buried deep within the crust of Mars, kept warm by any remnant heat near the core of the planet and lubricated by liquid water. Other possible – though unlikely – candidates for life include Jupiter’s moon Europa and Saturn’s moon Titan, which are heated by gravitational interactions with their parent planets, but harbor chemistry different from that on Earth.
Finally, Dartnell explores the possibility of discovering life outside of our solar system, mainly by examining rocky planets forming within the habitable zone of observable stars. It is a testament to the speed at which this field is developing that at the time of first publication in 2007, only a few hundred extrasolar planets had been discovered. Dartnell references the Kepler mission as a possibility in the hypothetical future. Of course Kepler has since flown, and we now have a much clearer estimate on the number of planets in the Milky Way galaxy, which could number in the tens of billions.
One gets the sense from reading between the lines of Dartnell’s book that he is optimistic and excited about the possibility of discovering extraterrestrial life within our lifetimes. In a few passages he grapples with the theory that the evolution of biological complexity may be somehow hard-wired into the structure of the universe, a theme also present in Paul Davies’ recent book. Whether or not life is abundant or rare in the universe, Dartnell does an excellent job of both highlighting the miraculous combination of events which gave rise to life on earth, as well as the natural processes by which it continues. Since we know that conditions favorable to life must exist in our galaxy, by virtue of the sheer number of undiscovered planets circling their suns, the real mystery remains unanswerable to science: what spark transforms inanimate matter into thinking creatures with a will of their own? Dartnell’s book cannot answer that question, but it does contribute to our ability to wonder about the outcome.
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