On a fine day in late spring or early summer, preferably around sunrise or sunset, go to a patch of uncultivated or wooded land as far as possible from people and traffic. Find a comfortable place where you can remain quiet and still for half an hour. Wait, watch and listen. For a while you hear only the sounds of insects, the alarm calls of small birds and the breeze among foliage; nothing moves except leaves and clouds. But after ten or fifteen minutes there is a transformation. Birds settle and feed. Shiny beetles sidle down tree trunks and over the ground. Furry bodies dart to and fro. The world around you has come alive.

Such experiences bring us into contact with other species and seem to satisfy a deep human hunger. "Communing with Nature" is sometimes said to refresh the spirit. The sights and sounds and smells of non-human life in its natural setting arouse our curiosity. They fascinate and enchant. They are the source of much poetry, music and visual art - and of science.

Science - in this case the science of life, biology - has its roots in curiosity. What we see raises questions. These might be simple questions, such as the names of the trees and the shiny beetles and the owners of the furry bodies. Or they might be more complicated ones, such as how birds and flowers are made, how they do the things they do, why they do them; and why they exist at all. Science is a way of framing such questions and trying to answer them. It is not the only way, but it is a very informative and productive one. It works by considering things in themselves, taking no account of whether they are beautiful or ugly or good or bad. The nature and origins of science and its effects on the world are topics for a different book. For present purposes a simple definition will suffice: science is a way of satisfying our curiosity by formulating questions about what we observe and answering them dispassionately - that is, without making value judgements.

You might ask how "communing with Nature" can still enchant a person -a scientist - who devotes his or her working life to dispassionate analytical inquiry. Surely, when curiosity is satisfied, wonder is lost? In fact, for most scientists, the opposite holds. Understanding the techniques of counterpoint and sonata form can enhance our appreciation of Bach fugues and Beethoven symphonies. Analysis of literary styles can help us to relish the subtle ways in which Henry James or Charles Dickens convey character and tension and a sense of place. In much the same way, the fruits of scientific inquiry increase both our understanding of the natural world and our wonder at its workings. Framing and answering questions does not destroy our pleasure in what we see around us; quite the contrary. Knowledge (especially the acquisition of knowledge) is pleasurable in itself, and it augments other pleasures.

However, research scientists ask different kinds of questions from other people. Two individuals who witness the same burgeoning of life during a spring sunrise might experience similar feelings of wonder and excitement. But if one of them is a practising biologist and the other is not, their curiosity will take different forms. The non-specialist might ask why certain insects visit primroses but not wood anemones, or how the shiny beetle manages to feed on the unappetising trunk of the oak tree; or how swallows, swooping from the bright sunlit air into the windowless barn, adapt so quickly to the sudden darkness that they unerringly find their nestlings and never collide with beams or walls. The specialist, the scientist, might be able to answer such questions; if not, then answers will surely be found among the wealth of available wildlife documentaries, books and magazine articles. But personally, he or she will be interested in different matters: the exact mechanism, say, whereby the primrose flower synthesises its chemoattractant, and why insects of one species but not others respond to it; or precisely what place the shiny beetle has in the ecology of mixed woodland. For both individuals, the pleasure of questioning and answering enhances the immediate sensory experience. But the biologist's pleasure in knowledge is difficult to share, except with those who have the same specialist background. There is a comprehension barrier, which we need to try to cross so that scientific knowledge becomes more generally accessible.

Popular science books, television and radio documentaries, science articles in newspapers - all these have gone a long way towards overcoming this barrier. Nevertheless scientists still tend to feel, and to be, misunderstood. This is apparent in their reactions to the most general, basic-seeming questions, the sorts of questions that a child might ask. Scientists tend to consider such questions unanswerable: too vague, too resistant to accepted technical vocabulary, too remote from the rigorous demands of ongoing research; in a word, too hard. For instance, when after a quarter of an hour's stillness in a chosen rural spot you have merged into the landscape and the world has "come alive" around you, what exactly does that phrase mean? Of course it "means" that local animal life has come out of hiding and revealed itself, but what do the words "alive" and "life" really denote? What fundamental properties do the primrose, the oak tree, the beetle, the swallow and the darting weasel have in common that distinguish them from the soil and rock beneath them, the air around them, the clouds above them, or the sunlight on which they all ultimately depend?

We can broaden this question. The bodies of the oak, the swallow and every other plant and animal are swarming with microscopic inhabitants such as bacteria. So is the soil itself. There are probably more bacteria in a handful of soil than there are leaves in the entire wood. What properties do these minute living things, scraps of matter that cannot be seen without a powerful microscope, share with the primrose and the beetle and the weasel but not with anything inanimate? What is "life"? Many have asked this question. It is the main topic of this book.

Thanks to a number of excellent popular scientific publications, most people nowadays might answer "What is life?" by saying "DNA". All living things contain DNA, but no inanimate ones do. DNA is the material of the coded instructions - the genome - for making and maintaining an organism. Cracking the code, unravelling the genome sequence, helps us to understand everything there is to know about that organism. The non-living world has no genome, no coded instructions. That is the difference between the living and the inanimate. For very good reasons, this answer has become deeply entrenched in modern thought: life is DNA. The double helix has become a major cultural icon. The complete sequencing of genomes (not least the human genome) has been hailed as one of the greatest achievements of human history.

However, without belittling this achievement or doubting that DNA is indeed basic to life on Earth, we can challenge the answer. Indeed, is it an answer? DNA itself is not living. Pure DNA in a test tube does not behave like anything alive; in fact, it does not behave at all. A freshly fallen leaf contains just the same DNA as it did before it fell, but it is no longer alive. Moreover, the fallen leaf still contains the materials - the proteins and their products - that the genome instructed it to make. So non-living things such as test tubes, and once-living things such as dead leaves, can contain DNA and the substances that DNA codes for. Yet they are not alive.

There are other objections, too. For instance, there might be entities on planets far across the galaxy that we would (if we ever saw them) describe as "living" because they shared certain characteristics with terrestrial organisms. Suppose we could analyse one of these hypothetical entities, and suppose we found that it contained no DNA. Would we then declare: "Our mistake. Despite appearances, these entities aren't living after all"? Surely not. So we are back where we started. We might accept that DNA is fundamental to life on Earth (whatever we mean by "fundamental" and "life"), but neither DNA nor the materials it encodes are sufficient to define the living state. The question "What is life?" remains open.

Many biologists are impatient with the question. They point to past attempts to distinguish the living from the non-living (traditionally, organisms are said to eat, breathe, excrete, grow, move, respond to stimuli and reproduce) and tell us, quite rightly, that all such attempts have proved inadequate. The reason why they have proved inadequate is simple. "Eating" involves wildly different processes in, say, oak trees and weasels. Weasels "move" in ways that oaks do not. And so on. Any definitions of "eating" and "moving" that are broad enough to encompass such a range of meanings would be useless. They would apply to many non-living things as well as living ones; and however broad we made our definitions, there would probably still be living things to which they would not apply. The quest for a clear distinction between living and non-living has always been vain, say the sceptics, so it is a waste of time to consider the question further.

This attitude is understandable but it unsatisfactory. If biology is the study of life and we cannot define life, then we cannot define what biology is about. This elementary logic ought to make the sceptics uncomfortable. Also, if we cannot define life, what do we mean by the "origin of life"? The origin of what? Similar problems abound. One more example: an established tenet of biology is that the cell is the fundamental unit of life; in other words, every organism comprises one or more cells. (We shall start to explore what we mean by a "cell" in the next chapter.) But if we cannot define life, of what is the cell the fundamental unit?

Other biologists take a different view, less sceptical but not very helpful. The living, they say, can be distinguished from the non-living by our detailed knowledge of the workings of organisms, knowledge that we have acquired through centuries of research world-wide. In principle, this view is unexceptionable. Any definition or characterisation of the living state must be based on what we have learned through the progress of science. But the amount of published biological data is colossal. Consider cell biology alone. The workings of some types of cell, such as the intestinal bacterium Escherichia coli or a rat liver cell, are known in mind-numbing - though not yet exhaustive - detail. The existing mass of information about such cells is far too unwieldy to provide a comprehensible distinction between the living and the non-living. And what essential facts might lurk among the details we have not yet discovered? Moreover, though all living cells share many features, each type of cell is also distinctive; and the common features might not suffice to identify an object as "living". So although a general account of the living state must be firmly based on what we know about particular living cells, this approach to answering "What is life?" is impractical if we take it literally.

In this book we shall construct a provisional, somewhat abstract answer to the question "What is life?" by generalising from these masses of information. We shall express this answer in non-technical terms as far as possible. We believe that our answer is interesting enough to publish, but it is not written on tablets of stone. It will probably be challenged by other biologists; indeed, we hope it will. Science - like the communication of science - progresses by trying out ideas, finding flaws in them, and trying again. If no ideas are put forward there is nothing in which to find flaws and therefore no progress. So although we are prepared to defend our provisional answer, we want it to be a target for rational criticism. Rational criticism will lead to better answers.

The words alive and life define the main theme of this book, but we shall also look at some related issues. Some of these issues, such as the origin of life and the existence of extraterrestrial life, have received much attention from other authors. Our contribution, a small one, is to reconsider them in the light of our general "definition" (or rather characterisation) of the living state. Inevitably we shall discuss evolution - it is impossible to write a book about biology without mentioning biology's central theory - but again we shall take advantage of the excellent popular treatments of this subject that are already in print.

One question recurrently asked about extraterrestrial life is whether it might be "intelligent" in the sense that our species is intelligent. To consider this question, we shall briefly discuss the nature and evolution of the organ of human intellect, the brain. Once more we shall take advantage of popular accounts, and of the revolutionary progress made in neurobiology during the last two decades of the twentieth century; but we shall suggest a new perspective on the topic.

To summarise: we begin the book by focusing on the "fundamental unit of life", the cell, and we spend the first few chapters developing our characterisation of the living state. In chapter 11 we turn to evolution, and in the remainder of the book we consider the origin of life, the evolution of "intelligence" and the question of extraterrestrial life.

Our aim is to share ideas equally with fellow-biologists and non-specialists. We invite all our readers to challenge the central idea in this book, the fundamental difference between the living and the non-living, and to improve on it. Any reasonable attempt to answer "What is life?" will help to develop more coherent views about the origin and evolution of life on Earth, the nature and evolution of intelligence, the possibilities for extraterrestrial life, and other big topics.

It is enjoyable to debate these topics, so this seems a worthwhile aim in itself. But there is another point: to have clear ideas about such broad issues enhances the wonder and pleasure that we gain from contemplating the world around us. In consequence, our thoughts and reflections when we "commune with Nature" at sunset will continue long after the stars come out.

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