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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 its ability to grow. But many things grow. A mountain may grow (if you wait long enough), or a fire may grow. However, a mountain is pushed up by plate tectonics; a fire increases if the temperature of surrounding flammable material exceeds the temperature needed to ignite the material. In both these cases, the growth is in response to exterior forces. Neither possesses the ability to initiate autonomous actions. In contrast, the acorn initiates the process, culminating in the generation of a mature oak tree: it is a directed action. If we filmed the growth of an acorn and replayed the film, speeding the action so that the tree’s entire life took just a few minutes, then we would see the oak appearing to raise itself up from the forest floor – in defiance of gravity – propelling itself towards the sunlight. This ability to move against external forces is a fundamental property of life, one lost when life is lost. If we continued to run the film of our oak tree for many years, we would observe that eventually the tree would no longer sprout new growth in the spring; it would remain leafless and eventually it would lose the ability to defy gravity, falling to the forest floor.
Somehow, whilst an organism remains alive, it is able to resist external forces and perform directed actions. When a pigeon perched on a tree decides to fly, its directed action is to beat its wings, thereby creating the turbulence that lifts it up into the air. Although inanimate objects may similarly perform actions, they lack the ability to direct them. Consider a stick of dynamite. In a sense, it can perform an action by exploding and may similarly get lifted into the air. Is the dynamite any different from the bird? Yes it is. If we determined the chemical composition of the dynamite and then added the exterior forces acting upon it, we could predict the dynamite’s subsequent behaviour and the effect on its environment. We could predict when it would explode. The dynamite cannot direct its action. Its behaviour is entirely deterministic.
Determinism is one of the bedrocks of classical science. It is the principle that the future (or present) state of any system (say, the stick of dynamite) is determined solely by its past. If you know the precise configuration of any system, by adding in the laws of physics and chemistry, you can calculate its future behaviour. The principle is at the heart of Newtonian mechanics, allowing astronomers to calculate the movement of planets from their known positions and trajectories and so forecast the precise times of solar and lunar eclipses far into the future (or back into the past). It is of course entirely impractical to determine the precise positions of all particles for anything other than the simplest systems but, in principle, determinism should reign – we should be able to predict when the dynamite would explode from knowledge of existing conditions. There is nothing that the dynamite can do, no action it can take, that would affect when it is likely to explode. It does not possess the ability to direct its own actions.
However, if we similarly determined the pigeon’s precise chemical composition and added the prevailing temperature, wind conditions, etc., could we predict that it would fly up into the air? Perhaps. But then, suppose it spied a bag of seed on the ground. It would then be more likely to descend towards the food. But perhaps there is a cat nearby. The pigeon might decide to wait in the tree until the cat has crept away. Could we predict all these possible behaviours by analysing the chemistry of the pigeon alone or even that of the pigeon and its surrounding environment? The only differences which have led to the pigeon’s altered behaviour are the pattern of light photons that fell upon its retina (carrying the images of food, cat, etc.). If we include these photons in the equations of motion that describe the pigeon and its environment, would the equations predict such widely different outcomes?
I hope to convince you that the answer to this question is no. We cannot account for life with classical science alone. In particular, we cannot account how living creatures are able to direct their actions according to their own internal agenda. For higher animals, such as ourselves, we call this ability our will. The ability to will actions is a profoundly puzzling aspect to living organisms that appears to contradict scientific determinism. There is no role for will in determinism; we do not have choices. Every action that we perform should be determined, not by any decision we make but by the precise molecular configuration of our bodies at the time preceding our action.
So can living creatures will actions? In subsequent chapters, we will explore how all actions, at a molecular level, involve the motion of fundamental particles. Different actions will involve entirely different sets of movements of these particles. For a bird to decide to soar into the air, it must change the direction of motion of billions of particles within its body. This capability to direct motion in response to an internal will appears to escape classical determinism, and is why biological systems are so unpredictable. Its influence may even be carried over into our interactions with our surroundings. The stick of dynamite would become just as unpredictable as the pigeon, if a man was standing close by, armed with a length of lighted touch-paper. Our directed actions cause the movement of particles both within our bodies and in our surroundings.
I should emphasize at the outset that I will not be invoking any mysterious forces to account for our will, only the known laws of physics and chemistry. I am not suggesting any return to vitalism. Over the coming chapters we will explore how all biological phenomena – mobility, metabolism, respiration, photosynthesis, replication and evolution – involves the motion of fundamental particles. We will examine how these dynamics are governed, not by classical physics, but by the non-deterministic laws of quantum mechanics. At its most fundamental level, life is a quantum phenomenon. We will go on to explore the implications of this realization for our understanding of life’s origin, its nature, evolution and consciousness. I hope, by the end of this book, you will have a new and exciting insight into what it means to be alive.
2 The Limits of Life
The makers of this alien spacecraft would hardly be content with one bulletin on a rock pigeon’s flying capabilities. After its first report the spacecraft would explore further – to seek out new life – in the words of Star Trek. Its next task would be to discover what the phenomenon of life on earth actually is. What does it need? Where does it thrive? What are its limits?
THE INGREDIENTS OF LIFE
Our spacecraft would soon discover that all life on Earth is carbon-based, that carbon is the key ingredient of our biomolecules. We might also describe life as water-based, since water is the substrate for our cells and tissue fluids, taking an active role in most of life’s activities. Life’s other main chemical ingredients are hydrogen, oxygen and nitrogen and small quantities of minerals such as calcium, magnesium, iron and sulfur.
These are readily available in our biosphere. Water is in the sea, in rivers, streams and lakes and, of course, rains frequently down upon us. Carbon is found in both inorganic molecules like carbon dioxide (CO2)1, methane (CH4) or calcium carbonate (CaCO3) and as organic2 forms such as the sugars, fats or proteins derived from the bodies of other living organisms. Hydrogen and oxygen are available in inorganic forms such as water (H2O) or as a component of organic compounds. Similarly, nitrogen is available in inorganic nitrogen gas (N2), ammonia (NH3), nitrates and nitrites, and in organic compounds. Animals are unable to utilize the inorganic forms of most of these, obtaining the elements they need from organic sources – the bodies of dead plants and animals.
Life would not have progressed far on our planet if all organisms were as feeble in their synthetic capabilities as animals. Fortunately plants and microbes are much more versatile. Billions of years ago, photosynthetic bacteria3 developed the trick of extracting carbon from the carbon dioxide in the air and stringing together the carbon atoms to make simple sugars. This is not easy; in fact, photosynthesis is one of the trickiest chemical reactions we know of (we will be looking at it more closely in Chapter Five). The problem is that the carbon atoms in carbon dioxide prefer to be attached to oxygen rather than tied to each other to make complex biochemicals such as sugars or proteins. To persuade carbon atoms to form complex biochemicals, bacteria and plants need a hydrogen source (plants use water) and an energy source (sunlight). Photosynthetic organisms extract carbon dioxide from the atmosphere and add hydrogen and sunlight energy to make simple sugars. The sugars are then strung together, pulled apart and reassembled to make the cell’s complex biomolecules – proteins, fats, carbohydrates and DNA.
Plants did not invent photosynthesis but stole the idea from bacteria – quite literally. Chloroplasts, the organelles performing photosynthesis inside leaves are descendants of a bacteria called cyanobacteria. Cyanobacteria are far more ancient than plants, and performed photosynthesis on Earth at least a billion years before the arrival of plants. The ancestors of modern plants were probably symbiotic partnerships between primitive fungus-like organisms and the photosynthetic cyanobacteria, perhaps resembling today’s lichens. This partnership slowly became permanent, and all today’s trees, ferns, flowers and grasses are the descendants of this marriage of convenience.
Cyanobacteria are not the only bacteria to perform photosynthesis and probably not even the first. Like plants, cyanobacteria perform oxygenic photosynthesis – they release oxygen as a product of their photosynthesis. The oxygen comes from their hydrogen source: water. Other bacteria can utilize alternative sources of hydrogen – such as hydrogen sulfide (H2S), ammonia or organic compounds – to fix carbon. These bacteria perform an anoxygenic photosynthesis which does not generate oxygen. This form of photosynthesis almost certainly preceded its oxygenic cousin.
Many bacteria and all animals are unable to fix atmospheric carbon dioxide, extracting it instead from alternative inorganic and organic chemical sources. Bacteria are the most versatile chemical feeders, able to extract carbon from a wide range of chemicals, which include organic compounds, carbon monoxide, calcium carbonate, methane, methanol, ether and formic acid. One group of bacteria using methane as both carbon and energy source is common in animals’ intestines, marshes and oxygen-deficient mud. But their most bizarre habitats were discovered on the sea-bed. In the summer of 1997, Chuck Fisher of Pennsylvania State University and Phil Santos from the Harbour Branch Oceanographic Institute were in a mini-submarine, Johnson Sea Link, exploring the sea-bed seven hundred metres below the Gulf of Mexico. They were examining the huge bubbles of methane hydrate forming when natural gas (methane) seeps up from the ocean floor, mixing with water and other hydrocarbons to form a dirty yellow methane ice. Scientists had suggested that methane-eating microbes might also feed on the hydrates, but what Fisher and Santos did not expect to find was a multitude of pink worms using oar-like paddles to crawl over, or burrow into, the ice. They were a new species of polychaete worms. It is unlikely that they eat methane directly, instead the worms probably graze on methane-eating bacteria colonizing the ice. It has even been suggested that the worms might build burrows to cultivate farms of these bacteria.
The next ingredient for life, nitrogen, should not be a problem since eighty per cent of the air we breathe is nitrogen gas. However, we cannot assimilate nitrogen gas from air – too unreactive – we breathe it in and right back out again. Instead we obtain our nitrogen from organic chemicals in food such as, for example, the protein in meat. Plants are able to assimilate inorganic forms of nitrogen such as nitrate (a compound of nitrogen and oxygen), but this does not solve the problem since the only non-biological source of nitrate is lightning strikes which generate temperatures high enough to burn atmospheric nitrogen and yield nitrate.
With only very limited supplies of fixed nitrogen available from lightning, life might have become severely nitrogen-limited billions of years ago. Fortunately, bacteria (including cyanobacteria) discovered how to fix nitrogen in the air to make the soluble compounds ammonia and nitrate. Nearly all biological nitrogen is derived from these nitrogen-fixing bacteria in soil and water. Leguminous plants (such as peas) form symbiotic partnerships with nitrogen-fixing bacteria, allowing them to grow in nitrogen-depleted soil.
The last ingredients of life – the minerals like calcium, sodium, magnesium, phosphorus and iron – are fairly readily available, usually as salts dissolved in water. Most organisms can readily assimilate inorganic sources of these elements, such as the sodium chloride (NaCl): the salt we sprinkle on our food.
Living organisms are extremely versatile in their ability to utilize a wide range of both organic and inorganic chemicals for the elements that make up their biomolecules. Animals need much of their biomass supplied as ready-made organic molecules. Bacteria have minimal requirements: some subsist on little more than a diet of air and rock.
ICE-COLD LIFE
The average temperature in London is about 13° Centigrade, rarely going above thirty degrees or dropping much below zero. Most higher plants and animals are happiest within a similar range of temperatures, so it is hardly surprising that life is particularly abundant in these latitudes. Humans do live in far more extreme environments. In Timbuktu, the Saharan temperature can rise to 50°C, whilst the inhabitants of Dawson in the Yukon valley endure nights where temperatures drop to –30°C. However, even mad dogs and Englishmen would succumb to heatstroke under a Saharan midday sun and frostbite would soon freeze anyone foolish enough to brave the winter nights of Alaska. Man survives these extremes of temperature by building shelters to provide warmth or shade, thus creating a more equable microenvironment protecting him from the heat and cold outside. The range of temperature that humans can endure (without resort to ingenuity) is actually quite narrow, lying somewhere between 5°C and 30°C.
Many animals survive more extreme environments. Often considered a barren wasteland, during its summer months the Antactic is teeming with life. Millions of seabirds and sea mammals nest on its coasts and fringe of drifting pack ice. Even the snow harbours life. Warmed by the summer sun, the interior of the pack ice becomes laced with channels of slushy brine filled with photosynthetic bacteria and algae. Antarctic mites burrow through the snow to graze upon on the microscopic bloom. The summer melt releases billions of these microbes into the ocean, to be harvested by the filter-feeding krill and channelled into the food chain supporting the seals, penguins and whales of Antarctica.
Within the interior, conditions are far harsher. The coldest temperature ever recorded was a chilly one hundred and twenty-nine degrees below zero at the Russia Vostok station in July 1983. Yet Antarctica is far from sterile. It harbours more than a thousand plant species, mostly mosses, fern and lichen. The topmost peaks of mountain ranges that rise above the ice are often colonized by lichen. Indeed, brown yellow and grey spots of lichen are ubiquitous on exposed rocks throughout the world. In Donegal, Ireland, where I was born, lichen has been scraped off rocks for centuries and used to colour wool for the cloth known as Donegal tweed. The same coloured lichen spots cling to the paving stones of disused paths and cover the crumbling ruins of ancient buildings. Lichen is actually two organisms: a fungus and an algae (or sometimes a bacterium) living in symbiosis. The photosynthetic algae provide nutrients that feed the fungus. What the fungus contributes is less clear, but it probably provides support and the ability to extract essential minerals from the rock. The success of this pairing allows lichen to colonize extreme environments barred to fungi or algae alone. However, even lichen cannot perform photosynthesis below zero. Although they survive the freezing temperatures of the Antarctic winter, they must await the warming sun to heat their rock substrate to a balmy 0–10°C before they can grow and reproduce. A similar freeze and burst strategy is followed by most of Antarctica’s flora which await rare warm spells to initiate a frantic flurry of growth and reproduction – generating millions of frost-tolerant spores or seeds – and closing down again when wintry conditions return.
The dry valleys of Antarctica are probably the most inhospitable regions on Earth. Bone-dry hurricane force winds race unimpeded across the Antarctic plateau, bringing temperatures dropping to –52°C. The winds quickly evaporate any traces of moisture from stray snow drifts. Bodies of long-dead seals and penguins lie perfectly preserved, desiccated, in conditions where even microbial decomposition is halted. A group of American scientists led by Diana Freckman of Colorado State University occupy a research station near the permanently frozen Lake Hoare in the McMurdo Dry Valley. Studying the ecology of the area, they have discovered a curiously simple ecosystem within the soil. Though the land is frozen half a mile deep, it is covered by a thin dry soil eroded from the rocks by the scouring wind. This soil harbours frost-tolerant bacteria and algae which are grazed upon by one or two species of nematode worm, themselves the prey of a third species of worm. The worms are mostly present in the soil as desiccated husks. Only when a rare trickle of snow meltwater moistens the soil do the microbes and nematodes spring into activity, hurriedly grazing, eating and reproducing before the freeze entombs them again. Nobody knows how long the worms can endure this Rip Van Winkle lifestyle, years certainly, but perhaps decades or even centuries.
When the sun sets on the Antarctic summer, the temperature plummets and everything freezes. Birds flee north and most seals seek warmer waters. An exception is the Wedell seal which remains a lonely outpost of mammalian life on the frozen pack ice (apart from the occasional naturalist). This hardy survivor winters over in the Antarctic by using its teeth to drill holes in the ice to the relative warmth of the ocean waters below (at temperatures a few degrees above freezing – far warmer than the air above) and its still plentiful food supply.
On the Antarctic landmass, covered by three miles of ice, there is no escape from the winter cold. The lichen, algae, nematodes and mites freeze within the snow, ice and soil to await the return of the sun. No living thing moves.
Except, that is, for the Emperor penguin. When all other animals flee north, the Emperor penguins head south to the freezing continental interior where they congregate in nesting sites on the central Antarctic plateau. The female lays her single egg and heads north herself, to the ocean, leaving the male penguin to perform perhaps nature’s most exemplary display of paternal duty. He gathers the egg in a pouch and, huddled together with as many as twenty-five thousand other penguins, he braves the coldest place on Earth. For three months the male penguin endures bitterly cold temperatures, searing winds and hunger before his mate finally returns with food for the newly hatched chick (but none for the stalwart male who must make his own way to the sea to find his next meal).
So do the activities of the Emperor penguin represent the lower temperature limit for active life on Earth? Not really, for the Emperors do not live at a low temperature. What the colony achieves is essentially equivalent to what the town of Dawson manages to do for its inhabitants – it maintains an equable microclimate. Each penguin shuffles continually around the colony, burning his fat store, generating heat which remains trapped within the huddled mass of feathers in the nesting site. The birds are thus able to maintain their internal body temperature close to the avian optimum of 42°C, well above the freezing temperatures outside the colony. Impressive though the penguins’ adaptation to the Antarctic winter is, their cells do not function at temperatures any lower than our own.
To find the lower limit for active life we must plunge into the waters below the ice of Antarctica. The world’s oceans occupy two thirds of the earth’s surface and approximately ninety per cent of the surface waters are colder than 5 degrees. Yet the oceans teem with life and are our most productive ecosystem. Most of the fish and invertebrates that live in the sea have body or cell temperatures that remain close to 5°C for most of their lifespan. Although the pace of life does tend to slow down at these temperatures (with concomitant longevity for many marine animals – sea turtles may live for more than two hundred years), life does thrive. Even temperatures a few degrees below the normal freezing point of water (salty water freezes below 0°C) are tolerated by Antarctic fish which incorporate a kind of antifreeze protein in their cells to prevent their tissues from freezing.
The coldest waters in the world are probably Antarctica’s brackish pools. Don Juan Lake is saturated with about forty-five per cent calcium chloride and does not freeze unless the water temperature falls below – 48°C. There is no photosynthetic activity in the lake, but live bacteria have been recovered from its waters. Whether these are true colonists or have merely drifted into the lake is unclear. They are certainly active when warmed to zero degrees. Liquid water is also found within the Antarctic Dry Valley lakes. Though the surfaces of these lakes are permanently frozen, geothermal activity can warm the deeper waters to temperatures as high as 25°C. A team of scientists drilled into these polar oases and extracted water samples that were found to contain a rich microbial flora with many unique bacterial species. The bacteria thrive just below the ice layer, where the temperatures may be as low as – 2°C, and in brackish waters may drop to – 12°C.
Freezing kills most living organisms. There are two ways ice damages living cells. Firstly, the mechanical shearing brought about by the formation of sharp ice crystals in cells, slices through the membranes, making the cells leaky, so that they die once thawed. Another problem arises because freezing expels dissolved salts that accumulate between the ice crystals, reaching concentrations toxic for living cells. There are, however, many organisms that can endure freezing. Even animals, particularly insects, frogs and lizards, can be frozen and thawed. Some species of frogs and turtles actually encourage the formation of ice crystals within their tissues. They make ice nucleation proteins which promote rapid freezing with the formation of smaller, less damaging ice crystals. Many microbes survive freezing due to the presence of cryoprotectants in their cells. Dormant life forms (seeds and spores) may survive for long periods, frozen, and have even been shown to endure temperatures close to absolute zero (–273°C, the temperature corresponding to a complete absence of heat – covered further in Chapter Six). The spores and seeds prevent ice formation by excluding free water from their cells. Many also produce large amounts of simple sugars that harden to form a glassy casing to protect the delicate enzymes and membranes inside. Even watery animal cells can also survive freezing. Human egg and sperm cells and even human embryos are routinely frozen during fertility treatments. The key to their survival seems to be freezing under carefully controlled conditions which minimize the damage to cells by promoting the formation of only very tiny ice crystals.
So although the frozen state is generally damaging to living cells, it does not necessarily destroy life. It does, however, prevent all living activity. Though ice and snow may shelter frozen seeds and spores and even frozen animals, nothing stirs in solid ice. Everything once alive is either dead or dormant. Active life is clearly incompatible with water in its solid state, ice. So the lower temperature limit for activity seems to be that experienced by marine and fresh water life in Antarctica, which may remain active down to about – 12°C.
