The pro-life movement
Chemistry & Industry, 18 September 2000
Life on other worlds
and how to find it
Ppxi+179, £16.95, ISBN 1 85233 097 X
The recent crop of books about the scientific case for the existence of extraterrestrial life could be seen as another symptom of Millennial yearnings, but it is a topic where things are happening. Not in the sense that we're any closer to actually meeting the little green men, but in that scientists from a diverse array of fields are pooling their resources and ideas.
Marine biologists are talking to astrophysicists, and geologists are talking to combinatorial chemists. From the formation of the elements in the early history of the universe, to the 'extremophiles' clustered around deep-sea vents, research in a range of fields is promising to help us answer the age-old question of whether we are alone in the universe.
Before you look for life, you obviously have to know what you're looking for. Life is like art -everyone thinks they know it when they see it, but defining it isn't always easy. A NASA workshop came up with a 'self-sustained chemical system capable of undergoing Darwinian evolution' as a working definition, which certainly beats the one about a bowl of cherries.
The big question is how likely, in a universe governed by harsh physical laws, is life? Under what conditions will a mush of cosmic material form those evolving, self-sustained chemical systems? It is a question of probabilities, but investigations are hampered by the very limited set of data. All we know for sure is that physical conditions on Earth permit the emergence of life, thanks to our distance from the sun that allows the existence of liquid water, the large moon to stir it up with tides, the bulk of Jupiter whose gravity shields us from meteorites and comets, and many, many other factors. But even given the right conditions, how likely is it that a chemical soup will assemble some of its atoms and molecules to create the basic parts of life?
Biologist AG Cairns-Smith has estimated that there are at least 140 discrete chemical steps in the creation of a useful sequence of nucleotides such as DNA. Giving each step a fairly arbitrary one-in-six chance of happening spontaneously, the chance of the random creation of DNA is 10(-109). This does not compare favourably to the estimated number of chemical reactions that have occurred in the entire history of the universe, at 10(99).
There are two conflicting conclusions to be drawn. One is that the emergence of life (assuming that any form of life would need something like DNA) is so fantastically unlikely that the Earth is its only home, and that the fact that we're here at all is just an incredible fluke. The other is that there are as yet unknown mechanisms at work that make life probable, if not inevitable. If such a seemingly-unlikely thing as life can happen once, chances are it's bursting out all over. The idea that life is such an emergent phenomenon - one that cannot be predicted from its constituent parts, but that just happens when a sufficiently complex system of disparate components begins to work together - occupies the grey area between the inanimate laws of physics and chemistry and the complex behaviour of biological systems.
Astronomer Stuart Clark is firmly, even evangelically, behind this second conclusion, and extends it to argue that every habitable planet - those with liquid water, the right chemical mix, an energy source and a stable environment - will necessarily be inhabited. But inhabited by what? There is life, and then there is life. The quest for extraterrestrial life is largely driven by the quest for extraterrestrial intelligence. The discovery of some basic form of extraterrestrial life, similar to the deep-sea extremophiles or even the controversial 'nanobacteria' seen (by some) in a Martian meteorite, will be hugely significant from the scientific point of view, but the alien culture most people want to see is the kind of culture that has languages, technology and philosophy, not the kind of culture you study in a petri dish.
Intelligence, as artificial intelligence researchers know all too well, is at least as difficult to define as life. Again, we know it when we see it but can't say exactly why, and again, there's no clear theory as to how it came about. And again, Clark claims it as an emergent phenomenon, and therefore all but inevitable.
'Life on other worlds and how to find it' presents the argument, based on scientific theories, that the universe is indeed redolent with intelligent life. The objective reader may find the case weakened by the fact that Clark so obviously sets out to prove this hypothesis. Clark assumes that the reader is also firmly 'pro-life', and the book appears to be aimed, with its 'grey' alien on the front cover, firmly at the X-Files 'I want to believe' market. More annoying is Clark's style of writing. For much of the book, he adopts a rather cloying chatty style, spotted with out-of-place attempts at jokes and discussions of the wit and wisdom of his favourite rock group (Rush), and directly addressing the reader in a manner rarely seen outside the Victorian romantic novel. It is a relief when these authorial interjections slacken off in later chapters - Clark is a lot more readable when he just gets on with the matter in hand.
But having said that, the science is clearly set out, and the book would be ideal for an intelligent teenager. Others may find more rewarding fare in Robert Shapiro's 'Planetary dreams' (John Wiley & Sons, 1999) or David Koerner and Simon LeVay's 'Here be dragons' (OUP, 2000), both of which present similar arguments in a more comprehensive and less annoying way.