The biggest problem in seeking life elsewhere is our psychological geocentricity. All the life on Earth, marvellously varied though it seems - bacteria, amoebas, mushrooms, oak trees, us - is a variation on a very tiny theme. As Francis Crick said, "DNA makes RNA makes protein": DNA is the stuff of genes, the great administrators; proteins, in the form of enzymes, are the body's operatives; and RNA acts as the executive go-between.
The system may be universal on Earth because this is the only way any mechanism as complicated as Life can possibly operate; there may be no other chemical options. More probably, however, there are many other possibilities, and the DNA-RNA-protein trinity dominates simply because it works so well and having once appeared on Earth it ousted all others by natural selection, just as cats, bears and dogs have ousted the clumsier and more stupid predators that came before them.
But if life elsewhere is not DNA-RNA-protein, how can we tell if it is there at all? What do we look for? Living things move, do they not? - but not necessarily. Besides, we should concern ourselves with life that is past, and not just with life of the present; and even the most active of creatures become irretrievably inert when once they are fossilised. Orderliness is often taken as a shibboleth of life: organisms neatly divided into cells, snails tidily twirled in geometric shells. But orderliness is deceptive. For a billion years or so before any discrete and shapely organisms evolved on Earth, "life" probably drifted on as "primeval soup" - metabolising, perhaps photosynthesing, doing many of the things that modern organisms do, but formless none the less. Indeed, as any snowflake demonstrates, non-living systems are more likely to show obvious order.
Living things are likely to be made of carbon - the principal element of DNA, RNA and proteins - so we could look for that; but then, as de Duve comments, carbon is one of the commonest elements in the Universe - more so elsewhere than on Earth. Martian life is quite likely to resemble ours for the same kind of reason that American bison resemble European bison - there could well have been some exchange between the two: two continents by landbridge, two planets by meteor. But life farther afield could be built along quite different lines. If it was, then this would be highly instructive, for we can learn much about ourselves by observing what is dissimilar - and to some extent, the more dissimilar the better. As Kipling remarked in a slightly different context, "And what should they know of England who only England know?".
But although biologists think of "life" in esoteric terms - of chemical "systems" that can replicate themselves and evolve, and process energy in particular ways - most people have more homely images in mind. What most people care about is alien life that thinks.
Alien intelligence is a few orders of magnitude less likely than mere "life" - though it probably exists somewhere, given that the Universe is so big, and offers so many opportunities. We do not know what intelligence entails but we can be sure that it requires complexity, and complex systems can evolve only from more simple ones, with many a pitfall and diversion along the way. Evolution does lead to what can properly be called "progress" (although it is very unfashionable to say so). Creatures that swim generally swim better as the generations pass; birds have become swifter and more agile; and mammals at least, lineage after lineage, have steadily become more brainy and commensurately more intelligent over the past few million years. Even so, progress does not imply inevitability.
Just because intelligence seems desirable, that does not mean it is bound to evolve. And it is not free: high intelligence requires big brains (or something equivalent) and big brains require a great deal of nourishment and maintenance. Our own brains soak up about 20 per cent of our total metabolic effort. Unless big brains really do increase an animal's ability to acquire food they will certainly not be favoured by natural selection, even if a creature has the genetic wherewithal to evolve them in the first place. Braininess, in short, is always likely to be relatively rare.
On the other hand, if other life-forms are intelligent at all, then they might be hugely brighter than we are. We are good at some things - recognising faces, for example, which computers find very hard - but dreadful at others, such as maths. There could be a race elsewhere that thinks mathematically, who would find Einstein dull and Deeper Blue trivial even if it did thump Kasparov at chess. It is hard to see how such mathematically brilliant brains could evolve but easy to envisage what they might be like. Even if such aliens were not outlandishly clever, they might still think very differently from us. This again would be instructive - just as computers are instructive, not because they are like the human brain but because they are in so many ways different. But if the aliens thought too differently, could we communicate at all? As Wittgenstein said of hypothetically talking lions, would we understand what they said? It would be fun to find out, but trans-galactic telephone conversations seem likely to be weirder than is commonly envisaged - and perhaps, as with alien life in general, we would not recognise it when we encountered it. We are much more limited than we are in a position to know, for even when we do seek to "step outside ourselves", and explore our own minds "objectively", it is still we who are doing the exploring.
But if there is life elsewhere, as there almost certainly is, how would this affect our attitude to ourselves? Just as it did in the early 16th century, presumably, when Copernicus first showed that our Earth is not the centre of the Universe. When Galileo rammed home the point a few decades later, it all seemed too much to take. Three centuries on and we know that even our Sun is a paltry star, one among thousands of millions in a galaxy that is one among thousands of millions of galaxies. Darwin in the mid-1800s knocked us further off our perch when he showed that we, like every other beast, were merely evolved, not made to Divine prescription one bright Friday morning. But we quickly get used to such shocks. Humility is not what we are good at. The proof of life elsewhere would not bother us for long.
Supposing, though, the fantasts' dreams were realised and we found wee folk on Mars, quaint and green with schnozzle noses, naive and mischievous. How would we treat them, once the surprise wore off? At what point would natural history become politics? If they were as bright as ourselves, we would soon draw up treaties and agreements, and wait to see who broke them first. If they seemed less intelligent - or simply more trusting, which is biologically feasible - then we would truly be embarrassed. If they were only as bright as, say, chimps, then doubtless we would want to experiment - perhaps test the odd vaccine. If they were more human than chimpish - like our ancient ancestors Homo erectus, say - then we would be in two minds; whether to put them in zoos or simply herd them into reserves, and allow them to sell folk art to the space tourists. And would we care for them from the outset, or wait until they were rare?
All in all, it's probably best if Pathfinder doesn't find anything at all.
Colin Tudge's latest book, `The Day Before Yesterday', is available in paperback from Pimlico.