The message from Mars

Nasa says there is extra-terrestrial life, but do we understand what life is?

Everyone - presidents, priests, scientists, shadowy denizens of Clapham's exemplary omnibuses - is rightly stirred by Nasa's suggestion of possible life on Mars; and, indeed, the putative Martian worm that has flown to us by meteor is intriguing. As usual in matters scientific, however, the excitement seems largely misguided and the responses - religious, political and scientific - have often been bizarre. A broader and longer view seems called for.

What exactly is life and how do we know it when we see it? Whether we are looking for life in the Universe at large, or asking how it began on Earth, or trying to create it from scratch (which biotechnologists will surely attempt in the next century or so), the danger is that we will miss it when it is there and see it when it is not, and look for its origins in the wrong places. Present perceptions of what life is - what Aristotle would have called the "essence" of it - are generally too narrow. The things that matter, and which we need to recognise where we see them, are not the most obvious; and the most obvious things are not necessarily crucial.

To gain insight into life's essence biologists have looked first at the life all around us - at what Star Trek's Mr Spock likes to call "life as we know it" - which indeed is the obvious place to start. What they see first of all in living things is order and pattern - floral symmetry against inchoate rock. They also see growth and reproduction where the rock stands still, and they see the living creatures absorbing energy and retaining it in chemical form, while the rock merely warms by day and cools by night. Plants illustrate the principle beautifully as they gather photons from the sunshine and use the energy of the photons to split water, stealing hydrogen to combine with carbon dioxide to make carbohydrate. They make it look simple although, thermodynamically, it is a very hard trick to pull. So the picture emerges: pattern, growth, reproduction, the concentration of energy - an eddy in the thermodynamic tide of dissipation - are the apparent shibboleths of life.

Beneath the abstractions lie chemical actualities. It has been clear for about two centuries that living things on Earth are always compounded from carbon; the stuff of soot and diamonds. To a biochemist, indeed, the much-loaded term "organic" means merely "carbon-based". More specifically, it became clear after the 1950s that all Earthly living things are variations upon the theme of DNA, RNA, and protein: DNA is the stuff of genes; protein is the stuff of which flesh is made - and also of enzymes, the catalysts that orchestrate metabolism; and RNA is the intermediary. In the words of Francis Crick: DNA makes RNA makes protein.

Such a conception of life has informed biologists in their search for life elsewhere in the Universe, and for its origins on Earth.

In particular they have looked for pattern, for signs of organic chemistry, and - at least with fingers crossed - for specific signs of DNA, RNA, and proteins. But such an Earth-bound vision is likely to be extremely deceptive.

For example, although living things on Earth seem in general to be regular in form, many boring things that by all reasonable standards are obviously dead are also intriguingly patterned, like sand dunes and crystals of common salt. The architecture of the Devil's Causeway in Ireland was created not by a diabolic intelligence but the cooling of volcanic rock. In truth, the living systems that drive the cells are not so much patterned as heterogeneous. Living systems are dialogic; many different kinds of molecules co-operate to produce a continuing interchange, and in principle this dialogue could be messy. There does not have to be a shape to it. The dialogue does not need to take place even in a recognisable organism - and indeed it is more or less certain that the systems found in present-day Earthly organisms evolved over many millions of years and to a high level of sophistication within a kind of primeval organic "soup", long before they became packaged into discrete "organisms". In short, if you see life in patterns, you are liable to be deceived; and if you look only for patterns, you are liable to miss many forms of life even when they are there.

Then again, it would be good to find the particular dialogue of DNA-RNA- protein in the dark blue yonder, but that would be a luxury indeed. Of course the trinity of DNA-RNA-protein works extremely well, and once it did evolve in the primeval Earthly soup it swamped everything alternative by natural selection, just as any advanced life-form is liable to oust its predecessors. But DNA-RNA-protein is not the only dialogue that could provide the essentials of life. Chemists might envisage many alternatives. Indeed, logic tells us that some of those alternatives did once exist on Earth for DNA, RNA and protein are all highly evolved molecules and many of those alternatives could now be operating on other planets and moons. Look for DNA, then, and you are liable to be disappointed; yet life could still be flourishing even in its absence, though of another kind.

Need life even be organic? Must it be based on carbon? If we envisage that life is bound to be complicated then the probable answer is yes. Of all the 100 or so chemical elements of which the Universe is compounded, carbon seems the most propitious. Each individual carbon atom can link up to four other atoms and so can generate a virtual infinity of intricate molecular forms. The only other element with comparable properties is silicon, but silicon has larger atoms which make it less flexible and hence less versatile: silicon is breeze-blocks compared to carbon's bricks. At the very least, carbon-based ("organic") life would be liable to oust any other forms that were importunate enough to appear. So in the search for life we should probably look for carbon. Unfortunately, carbon is one of the commonest elements in the Universe (more common in many places elsewhere than it is on Earth). It is clear, too, that a great many intricate carbon-based molecules are produced in space but there is no good reason to suppose that any of those organic molecules would ever begin the dialogues that we associate with life.

So in our search for alien life, what should we really be looking for? When you look closely, only two of the many qualities of Earthly life turn out to be essential - in the sense that we would recognise no life without them. One is replication, so the search for life is, crucially, a search for replications. On Earth, the universal replicators in all living things are the genes - DNA - but genes may be very special, and may not exist anywhere else. So we cannot look simply for DNA; we have to look instead for evidence of replication. Yet we must also recognise that the replicators need extra qualities. The ones that really deliver goods of the kind that can properly be called "life" are the ones that can expropriate energy from their surroundings and use it for their own replication, in the way that plants harvest passing photons. Thus we should be looking for replicators that do interesting chemistry. What might such a thing look like? Who knows? Has the question even been formulated?

Where does all this leave the putative Martian worm? Well, the learned journal Science has yet to publish and Nasa has yet to speak out, so perhaps we should wait and see. But a few points are clear.

First, it would be amazing if life did not exist elsewhere in the Universe - once you perceive that life is not necessarily of the Earthly kind. Neither should we be surprised if life exists on Mars even though conditions on Mars are very different from on Earth because life (defined broadly) would be able to exist in a huge range of conditions - indeed, it seems certain that life originated even on this Earth in very different conditions from those of today. It would not even be terribly surprising if Martian life was recognisably similar to Earthly life since the two planets are so close and, as the recent meteor might be demonstrating, the two could in the past have exchanged notes.

However, we would expect the resemblances between Martian life and Earthly life to be of a rudimentary kind: similarities of chemistry, conceivably including a common dedication to DNA-RNA-protein, but no more than this. There might even be some common chord between the most primitive organisms on Mars and those on Earth. But it would be very surprising indeed if Mars had ever produced anything resembling a worm for although worms might look primitive from our lofty human standpoint they are as sophisticated relative to, say, a bacterium as a jet plane is to a parachute; and the gap between the bacterium and the most primitive soups that we might legitimately consider "alive" is even greater. On the other hand, we should be suspicious of anything that merely has the form of a worm since the regularity and pattern that we associate with worminess (or snailiness or dogginess) has no ineluctable relationship with life at all.

Yet we should not easily give up. If this meteoric worm turns out to be nothing at all, there may still be life on Mars, or there may have been once; and if there is none on Mars, there is more or less bound to be some somewhere. Even if the alien life is as different from our own as it is possible to conceive then it might still have evolved to do some of the same things, since we both occupy the same Universe, and the physical conditions of the Universe constrain what can be done. For example, alien life might indeed be intelligent, even if the hardware of its intelligence is quite different from ours.

So should we drop everything and send a fleet of inquisitive rockets into space, as President Clinton seems now to be suggesting? Absolutely not. First, there is a lot of homework still to be done for it is clear that in the search for life we have not clearly defined what we should be looking for; and perhaps we will not truly get a feel for what life is, and what we should be looking for, until we set about making it in the laboratory. There is nothing like making things for finding out what really matters.

Second, and importantly, there is no urgency to find life elsewhere. If it exists at all then it has probably been there for several billion years, and it will still be there in several billion years' time. In our search for it we really do have all the time in the world. What is truly urgent is to study - and first of all to preserve - the life that we already have on Earth, much of which is liable to disappear in the next few decades, and much of which is almost certainly a lot more advanced and a lot more interesting than most of what is out in space.

This find from Nasa is exciting, despite the caveats. But the suggestion that we should launch a thousand rockets on the back of it is depressing. It shows now naive is the deployment of science. And that really matters.

The writer is a research fellow at the Centre for Philosophy at the London School of Economics.