Quantum Evolution: Life in the Multiverse. Johnjoe McFaddenЧитать онлайн книгу.
each other. When he synthesized tartaric acid in the laboratory he grew crystals with approximately equal proportions of the left- and right-handed forms. However, when he extracted tartaric acid from living tissue, the crystals he grew were always left-handed. Pasteur found that the same was true for nearly all biochemicals extracted from living tissue: if the chemical came in a left- and right-handed form, then only one would be found in living tissue. Living systems were chiral. He was therefore astonished to find that growing crystals out of wine fermentations, he obtained only left-handed crystals. This convinced Pasteur that he was dealing with a biological process rather than a simple chemical reaction. He confirmed his suspicions by demonstrating that yeast was a living microbe that fed on sugar, generating both alcohol and (sour) acids as the waste products. He thereby discovered the cause of the souring in brewing, and simultaneously founded the sciences of microbiology and biochemistry.
This marriage of mechanist philosophy and reductionism led to the great triumphs of twentieth-century biology. Over many decades, the myriad of interlocking biochemical pathways forming the living cell’s metabolic skeleton were laid bare. This knowledge and capability has led to major innovations in medicine and biotechnology. It may seem churlish to question the success of the mechanist/reductionist approach. Yet, despite its undoubted success in elucidating the chemical processes that underscore life, has it really enabled us to understand life itself? It is noteworthy that several centuries since the mechanist manifestos of Descartes or La Mettrie claimed that living organisms were mere machines, we have not succeeded, despite numerous attempts, synthesizing life in the laboratory. Joyce Kilmer’s line still hold true. No one has ever made a tree, or a flower or an animal or an insect or even the lowliest bacterium.7 The sole means of making life is procreation – sowing the seeds of pre-existing life forms. Though scientists can confidently describe physical chemical reactions taking place at the centre of our sun, at the surface of black holes, or during the first millisecond of the universe’s existence they cannot achieve what the lowliest life forms on Earth manage with ease: make life.
Our failure to put the ingredients of life together and obtain anything living suggests something must be missing from our list. Perhaps we should start by examining why the mechanist/reductionist approach has failed to tackle life’s fundamental questions: there is a paradox that lies at the heart of the reductionist approach to biology. As one dissects the workings of any living creature, examining the detail of smaller and smaller components (we will be attempting to do just this in the following chapters), life itself seems to vanish before our eyes. Whilst we have no difficulty in recognizing life in a whole animal, or indeed in one of its cells; when we come to looking at the cell’s insides, the question seems to evaporate. Is a chromosome alive? What about a gene or DNA? Is a ribosome alive, or a protein or an enzyme? The question seems to lose its relevance when applied to these bits of life. The components of living cells, stripped of context, seem fundamentally no different to inanimate chemical systems. Life seems to emerge only at higher levels. It is, to use modern jargon, an ‘emergent’ phenomenon: one that cannot be entirely understood in terms of its parts. As a means to explaining life, the unrelenting reductionist approach is doomed to failure.
There are of course many phenomena, both biological and non-biological, that do not succumb to what the philosopher Daniel Dennett describes as ‘greedy reductionism’ – the mating behaviour of birds, ecology or politics – to name but a few. Each has its appropriate level of explanation and no one would attempt to analyse them at the level of fundamental particles. However, that life itself is such a phenomenon, whose appropriate level of explanation lies at cells or above, is not generally appreciated. But if we cannot hope to understand life by dissecting it, what alternative approach can we use? We should start by looking again at what we are studying. What is life? We still have not answered the question that troubled our ancestors thousands of years ago. The modern answers have suffered the fate of reductionism. Life is reduced to a collection of parts which, in isolation, have lost their essential livingness. Attempts by scientists and philosophers to identify the key properties of life read rather like a checklist: self-replication, sensitivity, evolution, heredity, metabolism, etc. etc. If you can tick more than three boxes then it’s probably alive; but in isolation none is either necessary or sufficient to define life. Consider self-replication. This is generally considered to be a key attribute of living organisms. But not all organisms that appear to be alive are capable of self-replication. No mule has ever produced offspring. Many of the hybrid varieties of garden plants are sterile. Even when we examine life at a cellular level there are many cell types (for instance, nerve cells) which are certainly alive but are unable to replicate. The same kind of arguments can be used with any of the properties said to define life. They all somehow miss the key feature.
But is it so hard to identify what is alive? When I was a child, a popular children’s game was ‘Animal, Vegetable or Mineral’.8 It is a simple guessing game in which the first player has secretly written down something – a dog, house, brick, carrot – indeed, anything at all. The object of the game is for the player’s opponents to guess what is written down. They can ask only simple questions and the reply is always yes or no – but with one exception. The first question asked is: is it animal, vegetable or mineral? In my experience of playing this game, I do not remember any player having any problem deciding whether it was animal vegetable or mineral. We would all agree that a lion was an animal, a turnip vegetable and a brick mineral. It seems that even as children, we have little difficulty recognizing the qualities that identify and characterize living things. But how?
To make the game both harder and more enlightening, we should put ourselves in the position of the alien spacecraft; imagining that the things we have to identify are completely unfamiliar. How then would we decide what is alive or dead? How would we recognize life on other planets? The Exobiology (search for alien life) Programme of the American Space Agency at NASA has the following definition: ‘Life is a self-sustained chemical system capable of undergoing Darwinian evolution.’ This strikes me as an impractical life definition, particularly for an exobiology programme. How long would any spaceprobe have to wait to detect Darwinian evolution on a planet? It is hard enough to detect Darwinian evolution on Earth. I am also sure that the NASA definition was not the one we used as children; yet we were still able to identify life. So, what did we use?
Our alien spacecraft spotted the rock pigeon’s ability to fly and this prompted the LIFE signal. Flying is a particularly impressive example of mobility, the property that Aristotle recognized over two millennia ago as the essential characteristic of life. However, as he argued, the mobility characteristic of life is far more subtle than mere movement. Sand-grains blown by the wind are not alive. Water flowing along the course of a stream is not alive, nor are the stones tumbling down the stream-bed. But a salmon leaping up a waterfall is alive, and instantly recognizable as of a quite different nature than either water or stones. It would not matter if an alien salmon were coloured green and shaped like a carrot; if it leapt upstream we would recognize it as living.
But what is it about the motion of a salmon or a bird that makes it so distinctively animate?9 It is that a fish swimming or a bird flying is initiating its own movement against the prevailing exterior forces. Water flows towards the sea under the influence of gravity. The water’s currents (mostly frictional forces) tumble a rock along a stream-bed. But the salmon’s majestic leap out of the spray of a waterfall seems to defy both gravity and current to climb upstream towards its spawning ground. This seems the crux of the matter. Inanimate objects such as water or rocks are moved by the forces surrounding them; but living organisms have a internal vitality and vigour allowing them to defy these forces of nature and perform autonomous or directed actions.
This capability to initiate actions is both more general and more fundamental to life than mere movement. Exploring this further, we return to our alien spacecraft and imagine it has landed in a forest devoid of animal life. How would it recognize immobile plants as living organisms (we will for convenience ignore the possibility of moving plants such as the Venus flytrap)? From our argument above, we would look for a plant’s ability to initiate action or movement against prevailing exterior forces. There are many ways a plant does this. The most obvious is