Goldilocks factor makes life unique

Astrobiology is a new name given to study of the old question of whether we are alone in the universe. In their authoritative text, Peter Ward and Don Brownlee take us through all the arguments for and against the possibilities of extraterrestrial life in an immensely readable fashion.

The book's title immediately provides a clue that Ward and Brownlee might be setting off along a different track from that trodden by authors of other astrobiology texts. And in the subtitle, they make clear that they are discussing the possibilities for the evolution of extraterrestrial animal life, rather than microbial organisms. They consider the origin and evolution of life from an astrophysical and then a geological perspective: what is it about Earth that sets it apart from other planets? Each chapter takes one aspect of the Earth's evolutionary history, as we are carried from the formation of the universe in the big bang, to the explosion of life in the Cambrian period and eventually to the rise of Homo sapiens . The treatment of this enormous subject range is not simply a recitation of chronology or a planet-by-planet dissection of the solar system, but a thoughtful and thought-provoking discussion that leads, perhaps inevitably, to the conclusion that we are rare and fragile creatures, fortunate in our domicile and probably unique in our corner of the galaxy.

So what brings the authors to this conclusion? The text is prefaced by two short sections. The first lists various astrophysical locations (such as globular clusters and elliptical galaxies) in which it is unlikely that life could have arisen. The other briefly outlines the special properties of Earth that have allowed life to flourish and develop. The first chapter proper contemplates "Why life might be widespread in the universe" and considers the range of viable habitats on Earth, from the cold and dark ocean floor to hot springs and geysers. Recognition of the abundance of life in extreme terrestrial environments and the corresponding study of extremophilic micro-organisms have formed one of the most fruitful branches of astrobiology and provide a pointer towards identification of similar extraterrestrial environments.

The theme of environmental suitability is continued in the second chapter, "Habitable zones in the universe". Consideration of the Earth's unique position in the solar system (not too hot, not too cold) is extended to consideration of habitable zones around other types of stars, habitable zones in other regions of the galaxy and at other places and times in the universe.

In "Building a habitable Earth", astrophysics gives way to geology, as we race from the creation of hydrogen and helium in the big bang, to the origin of the chemical elements during stellar processing, to the accretion of Earth following collapse of an interstellar cloud, to conclude with the formation of Earth's atmosphere. Having arrived at a habitable Earth, Ward and Brownlee turn to biochemistry to look for life's first appearance on Earth, where the RNA world is introduced, then move rapidly up the tree of life for how to build animals, before returning to geology for "Snowball Earth", in which changes in the Earth's physical environment (specifically the oxygen content of the atmosphere and changes in global temperature) are discussed in terms of their effect on the evolutionary process.

All this leads up to the crux of the authors' thesis - that animal life is rare in the universe - through consideration first of "The enigma of the Cambrian explosion" and then "Mass extinctions and the rare Earth hypothesis". About 550 million years ago, a wide diversity of animals appeared in the oceans and were subsequently preserved in the fossil record. The reason for the Cambrian explosion is presumed to be a combination of several effects, both environmental and evolutionary, and might even include changes in the direction of the Earth's spin axis. If such a combination of events were necessary for animal life to arise, the rarity of higher extraterrestrial organisms should be no surprise.

Chance must also have played its part: the death of the dinosaurs and subsequent success of mammals after the impact of a bolide at the end of the Cretaceous period, some 65 million years ago, must surely count as one of humanity's most fortunate breaks. The surprising importance of plate tectonics continues the geological theme. The authors describe the carbon dioxide rock-weathering cycle, powered by tectonic movement, that controls the atmosphere's carbon dioxide content and acts as a "global thermostat", maintaining Earth's equable temperature.

The question of why Earth is the only planet in the solar system to exhibit plate tectonics is discussed in chapter ten, "The Moon, Jupiter and life on Earth", in which geology is again abandoned in favour of astronomy. The importance of the Moon and Jupiter in the development of Earth can be summed up in one word: impacts. The most widely accepted hypothesis for the formation of the Moon is that it was created during the catastrophic collision of a Mars-sized asteroid with Earth. The Moon stabilises the Earth's spin axis and rotation rate. Without the Moon, the gravitational pulls of the Sun and Jupiter would lead to changes in Earth's obliquity and uncontrolled changes in climate. The geometry of the Earth-Moon system coupled with the geography of the solar system have conspired to produce what might be a unique habitat for life.

Ward and Brownlee have taken an issue that is much in the public domain and treated it thoughtfully and thoroughly, but with a lightness of touch that draws the reader on. A list of references for each chapter allows the interested reader to follow up the general outlines conveyed in the text, and the whole is completed by a comprehensive index. Rare Earth is an excellent book for both specialists and non-specialists.

Monica M. Grady is head of petrology, Natural History Museum, London.