Buch lesen: «Quantum Evolution: Life in the Multiverse»

1 What Is Life?

Starlight glistens on a spaceship’s silvery hull as it cruises, unseen and unmanned, amongst the planets of a distant solar system. Guided by the encoded instructions of an alien civilization it glides past dark, rocky planetary outposts and bloated gas giants until it reaches its goal, and swings into the orbit of an inner planet. A probe is released. Retrorockets fire that adjust the probe’s trajectory, easing it slightly from the mother-ship’s geostationary orbit and turning its heat-resistant nose towards the ground. The grip of the planet’s gravity drags the probe inwards, through ever-decreasing orbits. Faster and faster it spins until, plunging through clouds, it finally emerges under a leaden sky. A parachute is released to halt the headlong dive, and the craft slowly descends to land on a rock-strewn landscape.

Minutes later, a metallic lid is drawn back, exposing a camera lens, and pictures are beamed back to the mother-ship. The camera pans across the rocky scene. The same rubble-strewn landscape is everywhere – rocks of all shapes and sizes lie sunken into fine grey sand. The air is still. Nothing moves. The camera scans the monotonous surface stretching in all directions towards the horizon – grey rocks, some precariously balanced atop others, others lie shattered, blasted by the forces of alien weather. The camera pans again, and then one rock, in shape and colour much like any other, spreads its wings and soars into the sky. The mother-ship sends a signal backward through the vastness of space, towards the distant home of the spaceship’s makers: LIFE!

The planet is, of course, Earth and the rock a bird, perhaps a rock pigeon, lost in barren desert. The story illustrates the wonder we should feel at the most remarkable phenomenon in the known universe – life. Our telescopes and space-probes return images of the universe’s many marvels – the twisted braids of Saturn’s rings, Neptune’s moon Mirander’s scarred and shattered surface, the birth of stars within the Crab Nebulae. Extraordinary as these are, they pale before the astonishing nature of life itself. And yet, all life forms are essentially rocks – made of the same materials, obeying the same laws, as the rocks, stone and sand that surround us. We are rocks that run and swim, climb and leap; that hear, touch and see; rocks that can look out into the vastness and grasp for an understanding of ourselves and the universe that made us.

In this book we will explore the nature of life and ask what animates living organisms. What is, in the words of Dylan Thomas, ‘The force that through the green fuse drives the flower’? To understand the nature of this force, we must explore life at its most fundamental level, examining the two key events in Earth’s history that made the act of writing these lines possible. The first took place nearly four billion years ago, when life emerged. The second took much longer. Living creatures had been swimming in Earth’s oceans for three and a half billion years before the mammals gave rise to a family of bipeds, the primates, and from their ranks emerged a thinking ape, man. Since that time, several million years ago, the mind of man has unravelled many mysteries concerning the universe’s workings. We watch the sun setting every evening and are confident of its rise the next day, because we know its rising and setting are caused by Earth spinning on its axis. We can look up into the night sky and know that each star is a sun like our own. Scientists can calculate the energy released from the fusion of hydrogen nuclei inside our sun, or use powerful telescopes to witness the birth of galaxies that existed billions of years ago. Remarkably however, the two key events that made our own existence possible – the emergence first of life and then of consciousness – still remain mysterious. Although we know now a great deal concerning both the workings of living cells and (though far less) the human brain, the spontaneous appearance of both phenomena remains a puzzle. This book’s aim is to explore this puzzle and examine the startling proposition that we already hold a missing piece of the puzzle. We will discover how, with this piece in place, enigmatic phenomena can be explained and light shed on life’s central mysteries.

To approach the answer, we must first understand the meaning of the question. What is life? What is the force that through the green fuse drives the flower? Living today inside the concrete and glass walls of urban environments, it is easy to ignore life’s astonishing nature. Our perceptions are formed within homes shared with domesticated animals and potted plants and only slightly modified during weekend excursions across forest-denuded hillsides or through fields of monoculture crops. The forces of the natural world are often perceived as problematic: mould creeping over damp patches of bathroom walls, weeds encroaching on flowerbeds or ants invading kitchens. But it is in our encounters with these weeds and vermin that we glimpse nature’s true character. The moulds, plants and insects invading our homes and gardens are heirs of the creatures that first colonized the oceans and proceeded to relentlessly invade every habitable niche on this planet. If we are to unravel life’s secrets, it is their nature we need to understand.

One starting point is to examine how our ancestors, unsullied by the preconceptions of our civilization, viewed their world. Man first walked on the planet several million years ago. For almost all subsequent history, man’s chief preoccupation was the gathering, snaring and hunting of nature’s bounty. Our ancestors’ day-to-day survival was contingent upon the ebb and flow of life through their landscape: the migration of herbivores, the ripening of fruit and the spawning of fish. To survive, man needed to exploit these resources, and he learned to lay traps to catch animals, grind tools to butcher them, fashion clothes from their skins and kindle fires to cook them. But the same reasoning that endowed Homo sapiens with his unique skills to exploit nature, condemned him to remain for ever discontented with mere exploitation. Man sought to understand his world. Our ancestors held nature’s procreative power in awe, worshipping gods and goddesses whom they represented as sexually exaggerated figures – such as often heavily pregnant ‘Mother Earth’ figures or priapic males. Life’s vitality was celebrated in the vigorous images of bison and reindeer that leap across the cave walls at Lascaux or Altamira. These two aspects of nature – its energy and its capacity to reproduce – clearly impressed our ancestors, and still remain mysteries of life today.

Much of subsequent history is a reflection of the changing pattern of man’s interaction with the rest of the natural world. After several million years as a hunter and gatherer, man turned his skills towards manipulating nature. About ten thousand years ago – apparently independently in several parts of the world – people discovered how to cultivate grain and domesticate wild animals. Man thereby freed himself from a perpetual march in search of a moving food supply, and established settlements. A surplus of plentiful crops allowed the rise of an aristocracy, who hoarded and guarded this resource. This enabled many to escape from the drudgery of tilling the land altogether. Warriors and servants could be paid from the royal coffers and thus persuaded to protect the lands of their kings, to build walls or erect palaces and temples. The level of social organization required for these tasks was previously unknown amongst the hunter-gatherer communities and a remarkable invention was devised to keep track of their transactions. Symbols and signs were scratched onto clay tablets, representing bales of wheat, jugs of beer, or heads of cattle, either paid to, or received from, the king’s subjects. From these modest beginnings, writing developed. Information and ideas encoded on baked clay tablets could be faithfully transmitted across space and through time. Fortunately, those ancient scribes turned from recording the jugs of beer paid to their workmen to more interesting information: the beliefs, hopes and dreams of their people. The stories they tell are our first detailed records of man’s thoughts concerning life.

The earliest creation myths record the belief that life represented the fundamental creative power in the universe. The universe’s origin was itself often held to be some form of birth. In the Orphic creation myth, black-winged Night laid a silver egg in the womb of darkness; Eros was hatched from the egg and set the universe in motion. Similarly, the Rig-Veda’s Hindu creation myth describes the birth of the first being from a golden egg, all other deities springing from his limbs. The authors of these myths were mostly farmers, and much mythology revolved around the seasonal cycles. They sowed their fields with seed and marvelled at its power to sprout and grow into luxuriant crops. Their myths reveal that they generally traced this power to a divine source. The ancient Sumerian sky-god Enlil is described as:

The lord (Enlil) who brings forth what is useful

The lord whose decisions are unalterable

Enlil, who brings forth the seed from the earth¹

Life is clearly considered to be apart from the rest of creation, its vitality a channelling of divine power. The cycle of growth, death and rebirth was, within agrarian societies, almost universally attributed to the death and rebirth of a fertility god or goddess. Thus, Osiris, the Egyptian god of vegetation, was said to have been slain and dismembered by his brother. His wife, Isis, gathered together his body’s scattered fragments and with magical ceremonies restored him to life. The Egyptian reapers chanted a dirge for the death of Osiris and prayed to Isis for his return. Similarly, the descent of the Babylonian goddess Ishtar into the netherworld echoed the desolation of the dry season; her subsequent rescue and emergence restored the growing season’s fertility. The cycles of human fertility were seen as under divine control. The coincidental synchronicity between the moon’s waxing and waning and the female menstrual cycle was attributed to the influence of a lunar deity, such as the Roman goddess Juno, to whom barren women would pray. The (less obvious) role of male reproductive organs in procreation was also recognized. Thus Aphrodite was born from Uranus’ testicles which had been flung into the sea by his son Cronos, who had castrated him with a saw-toothed sickle.

Each tale records a belief that life contained a divine or magical principle, absent from the inanimate world. To create life, this vital principle needed to be added, often from a living source, such as blood. Thus, in the Babylonian Poem of Creation, it is related how man was fashioned from clay mixed with the blood of a god:

‘Let him be made of clay animated by blood’.²

Although, today, it is easy to dismiss these myths, they are in reality man’s earliest attempts to find answers to the questions still plaguing us – they are the first theories of everything. Today we know where the sun goes at night and why spring follows winter. But much of our knowledge is received wisdom and this wisdom of ages was hard-won. How many of us would be able to prove that the Earth revolves around the sun, when any fool can see the sun rise in the morning, travel across the sky and descend below the horizon at night?

The dawn of the rational approach to understanding our world is usually attributed to the intellectual revolution of the sixth and fifth centuries BC which gave rise to the ancient Greek civilization. One of the earliest philosophers was Thales (born about 600BC). Although his writings have been lost, several of his sayings have survived, including, ‘the lodestone has life, or soul, as it is able to move iron’. This short phrase implies a complex set of beliefs. Firstly, that the ability to initiate movement is a key attribute of life. This is a concept we will return to as, in modern molecular interpretation, it forms a cornerstone of this book. Secondly, that this ability to make movement betrays the presence of a ‘soul’. Like the mythmakers before him, Thales considered that the phenomenon of life pointed to the presence of supernatural forces. Finally, the equation: ability to initiate movement = life = soul, has been taken to the extreme of attributing the property of life to a variety of inanimate objects, such as a magnet (lodestone). This reflects a widespread tradition of pantheism in the ancient world. As the third-century Roman chronicler, Diogenes Laertius put it, ‘the world was animate and full of divinities’.

The ancient world’s greatest biologist was undoubtedly Aristotle. Sadly, our received image of him is frozen by those chalk-white busts of venerable bearded philosophers who seem to stare into a perfect world of spheres and equilateral triangles. But Aristotle’s vision was far more earth-bound than that. Like his predecessor, Heraclitus, he believed that ‘knowledge enters through the door of the senses’, and as a young man he spent several years living on Lesbos, studying marine life. His biological writings betray the acute observation and attention to detail which is the hallmark of all great naturalists.

‘Animals also which fly and those which swim, fly by straightening and bending their wings and swim with their fins, some fish having four fins and others, mainly those which are of a more elongated form (eels for example), having two fins. The latter accomplish the rest of their movement by bending themselves in the rest of their body, as a substitute for the second pair of fins. Flatfish use their two fins and the flat part of their body, instead of the second pair.³

Instead of the venerable sage, we should imagine a younger Aristotle diving into the Aegean’s clear waters to retrieve starfish, crabs and anemones, to study their form or observe their behaviour.

‘The sea-urchin has a better defence system than any of them: he has a good thick shell all round him fortified by a palisade of spine.’⁴

Any lover of rock-pools will recognize an ally in Aristotle’s writing. But the scientist in Aristotle was not content to describe nature; he needed to explain it. Perhaps, later in the day, he would set light to driftwood to cook his catch and ponder on the ephemeral quality he roasted out of the living flesh. Like Thales, Aristotle considered that the essential quality of living creatures was that they possessed their own internal will and this allowed creatures to initiate independent movement.

‘For nature is in the same genus as potency, for it is a principle of movement – not however in something else but the thing itself.’⁵

To Aristotle, living creatures were made distinct by their ability to move themselves. His concept of movement was more subtle than simple locomotion. The shoreline of Lesbos, had taught him that clams, anemones, or indeed simple seaweed moved very little (except when pulled by the waves and the tide), but were still very much alive. To Aristotle, there were six forms of movement: generation, destruction, increase, diminution, alteration and change of place. This broader conception of movement actually reflects a more general meaning to the verb, to move, than our modern usage, one that remains apparent when we say that we found a particular piece of music to be deeply moving, or when a motion is passed by a debating society. Our modern usage is rooted in Newtonian mechanics, and a better translation of Aristotle’s concept of movement would be the term action, a word with a precise, useful meaning in modern physics, to which we will return. The essential point of Aristotle’s argument is that all living organisms possess an internal will that allows them to initiate and perform actions such as growth, regeneration, procreation and movement. Aristotle, like Thales, ascribed this internal will – the cause of independent action – to the eidos, the soul or psyche: ‘The soul creates movement’.⁶

It would be mistaken to equate Aristotle’s eidos too closely with the Christian soul. He believed all animals and plants were endowed with a ‘soul’ capable of initiating movement. To Aristotle, this soul was clearly a much more functional entity than the Christian moral guardian. However, only man possessed the highest form of soul: the source of reasoning and moral judgement.

Aristotle’s writings, lost and then found by the Arabs and passed from them to mediaeval Europe, were to form the basis of Western thinking throughout the Middle Ages. The Aristotelian concept of a soul was translated into the vitalist approach to biology. To the vitalists, life possessed a mysterious property, the élan vital, or living spirit, whose nature lay beyond the realms of science. In the words of Joyce Kilmer:

Poems are made by fools like me

But only God can make a tree.

The vitalist tradition survived until the twentieth century in many biological writings. I remember biology textbooks that described the mysterious living protoplasm inside cells with the same awe and mystery that mystics describe the aura. However, the concept has been in retreat since the dawn of the Age of Reason in the seventeenth and eighteenth centuries, and no serious scientist subscribes to it today. The opposing camp, the Mechanists, were inspired by the machines that were, by then, revolutionizing the world; and they believed that life, like machines, could be understood in terms of the laws of chemistry and physics. They rejected the vitalist argument that life required special laws beyond conventional science. René Descartes (1596–1650) was a founding figure who proposed that animals were mere automata, in principle no different from the clockwork figures which played music or danced at fairgrounds. Descartes was however unwilling to accept the full implications of mechanism and reserved man a special place amongst God’s creations. He considered man’s intellectual capabilities, his reasoning power, betrayed the presence of an immortal soul. Mechanists had to wait for another century before books such as La Mettrie’s L’Homme Machine (Man the Machine) (1748) laid bare the full force of the mechanist manifesto. La Mettrie agreed that animals were no different from machines but argued that man differed from animals only in complexity. The way was now open for science to delve into the very substance of life.

Technical advances in analytical chemistry and microscopy naturally drove the life sciences towards reductionism – the belief that complex systems can be considered as the sum of their parts. Nineteenth- and twentieth-century scientists began a reductionist dissection of the chemistry of life. In 1853, the Lille brewing industry hired Louis Pasteur to discover why their wines soured. At that time, fermentation was considered purely a chemical reaction. Brewer’s yeast was thought to be a chemical catalyst facilitating the conversion of the grape-sugars to alcohol: yeast was not recognized as a living organism (which is not so strange when you examine it in its powdery form). The brewers’ hiring of the brilliant young chemist was, thus, hardly surprising.

Pasteur had made his name demonstrating that tartaric acid crystals came in two forms, left- and right-handed that were mirror images of 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?