Darwin's Island Page 8
Often, the special food is produced only when enough ants are around to make it worthwhile. Some trees are even more parsimonious. The whistling thorn of Kenya, which gets its name because the wind howls through its hollow thorns, uses ants to keep hungry giraffes at bay. It gives shelter, but no food, to its resident army. Ants cheat just as much, for some get a meal from the honeydew made by scale insects that feed on sap (and do no good to the tree) while others have little interest in attacking herbivores. Some species are even more selfish, for they castrate their host by chewing off flower buds to ensure that it does not waste its efforts on show, but puts out new and tasty shoots, with their free food instead. The tree fights back with a chemical that keeps the ants off the flowers. Yet another insect destroys its host’s food bodies to dissuade more aggressive ants that might protect the tree but throw off the resident.
The ants gain sugars, based on carbon, from their host - but their corpses and those of their prey provide precious nitrogen to the tree. The shelters have thin walls through which the excretions of the residents, or the remains of their bodies, are picked up by the host tissue. Some acacias take, as a result, nine-tenths of their nitrogen from their insect visitors. It would not be too hard to transform an arboreal ants’ nest into a trap that soaks up nitrogen while giving nothing back.
The acacias, like the sundews, are nitrogen hunters that depend on other creatures for help and, with the entry of the ants, make that hungry clan - already diverse - even wider than before. A further look around the botanical world shows that the tactics of acacias or Venus flytraps are feeble when compared with the ingenuity shown by other species. Many plants thrive in what would otherwise be famine conditions thanks to a series of obscure but intimate associations with other creatures in the search for the essential element. They negotiate not with insects - which, as animals, are quite close to plants in evolutionary terms - but with bacteria and fungi around their roots that pull the gas from the air and receive food and shelter in return. That habit is central to the survival of life on earth. It represents a series of evolutionary convergences between minute creatures far less closely related to each other and to their hosts than are insects.
The roots of many plants secrete chemicals that attract bacteria able to transform bound nitrogen into a more digestible product. They then soak up their invaluable wastes. Peas, beans and certain trees have entered into a closer arrangement with specialised ‘nitrogen-fixing’ bacteria that combine nitrogen gas in the air with hydrogen to form ammonia and other compounds which can be soaked up by roots. Many of the insect-eaters and ant-exploiters, with their spectacular adaptations above the ground, also depend on a similar pact with tiny aliens within their roots.
Farmers take advantage of such arrangements when they rotate their cereal crops with legumes such as clover and soybeans, all of which have close relations with nitrogen-fixers. Together, such plants now generate half the nitrogen used on the world’s farms. Without them we would starve. Their bacterial allies make a special enzyme which forces the sullen molecules of the gas into a marriage with the active hydrogen ions made as food is broken down. The reaction consumes a great deal of energy and costs both the bacteria and its host a lot.
Before today’s technical developments in biology, the bacterium involved, and the protein that does the job, looked more or less the same in each of the thousands of species that indulge in the habit. They are not. Just as in the insect-eaters, many unrelated plants, and even more of their minute helpers, have taken up the pastime. DNA shows that bacteria themselves, unimpressive as they might appear, are more diverse as a group than are the two kingdoms of animals and plants put together. The nitrogen-fixers span a good part of the spectrum of bacterial life. They are joined in their helpful habits by fungi, who are themselves more related to ourselves than are bacteria, and by members of a quite distinct group of single-celled beings known as the Archaea that teem in hot springs, deep sea vents and the soil. The sea, too, is itself full of a great variety of gas-fixers, most of them little understood.
Most of the bacteria involved live for most of the time alone. When they come into contact with a root of the right kind, a certain sugar locks into a receptor on its surface. The nitrogen-fixer squeezes its way in and its host’s cells divide to produce a nodule filled with descendants of the invader - a billion or more from a single founder cell. Both parties benefit for the plant provides fuel for the hard chemical work needed to drag the crucial element from the air while the bacteria churn it out in a form that can be used by the other member of the consortium.
The association between the two emerged, in evolutionary terms, not long ago; just after the destruction of the dinosaurs. There was, at about that time, a sudden outburst of carbon dioxide and a spike in temperature, both of which favour plant growth - and meant that a sudden shortage of nitrogen made it worthwhile to enter into the arrangement. It has evolved again and again in distantly related families. Alder trees (but not their close relatives the beeches) have root nodules that contain bacteria better known as the producers of the antibiotic actinomycin. With their help the trees grow on starved soils such as those on dunes or mountains. Tropical ironwoods have the same association as do a few members of the rose and pumpkin families. Liverworts, certain ferns and the giant rhubarb of Brazil all benefit from the ability to use single-celled creatures to soak up the vital gas.
The plants that have come up with that solution are diverse indeed. The creatures that do the work are far more so. The nitrogen-fixers within the roots of beans, clover and their relatives have been widely studied because of their economic importance. Hundreds of different helpers have been pressed into service. Some are tied to a single host - or even to a particular cultivated variety of peas or beans - while others are promiscuous. Under a mask of similarity, the biochemical mechanisms involved, the molecules that signal willingness to enter into an association, the amount of food provided and the rewards paid are diverse indeed.
Like ants on acacias or insects that buzz around a pitcher, the system shows a fine balance between cooperation and conflict. Some bacteria enter their hosts through wounds as a hint that they were once agents of infection (a few are related to known pathogens). Others grow within a membrane that protects them from attack, or make poisons that suppress a host’s ability to fight back. The plants have stayed suspicious of their partners. Now and again a cheat gets in - an invader that produces little of its valuable product but demands free food and shelter. At once the host cuts off supplies, the nodule withers and the fraudster starves.
Nature’s market in nitrogen turns over billions of tons of the element each year, which passes from air to soil, from land to water and from plants to animals and back again in an endless cycle. As is true for all markets the accounts of profit and loss are checked with great care. The struggle for the element is pitiless as is that for water, air or sex, but only now and again is the truth of its dealings exposed in all its brutishness. Plants that eat animals are just one instance among many to show how competitive that business must be and how the most improbable expedients are pressed into service to squeeze the most out of what little is on offer.
Now the global trade in nitrogen has been thrown into turmoil. Farmers pour nitrogenous chemicals on to the soil. They buy it from factories that each year generate a hundred million tons of the stuff from oil, or by extracting the gas from the air. The reaction is carried out with the help of catalysts in boilers held at high temperature and extreme pressure. Without that technology, invented just a century ago, the world would starve. The industry is profligate indeed in its use of power, most of it gained from burning the remains of ancient life. Cars, chimneys and aircraft also pump nitrogen salts into the air. All this means that far more nitrogen is available in useful form than in Victorian days. The amount has doubled in the past century.
To add fertiliser to fields does increase the yield of crops but also changes the economics of their bargain with a living sour
ce of nitrogen. First, it alters the balance of profit and loss. After a dose of fertiliser, crops need less help from their tiny assistants and squeeze them out. As a result the amount of the element taken from the air by those useful creatures goes down, so that the overall gain from the added nitrogen is less than it might otherwise be. For the starved soils of Africa such opportunistic behaviour by the plants is a real problem.
In addition, excess nitrate is washed to where it is not wanted, and more is added by acid rain, itself full of salts of the element emitted by exhausts and chimneys. Insectivores, ant-shelterers and bacterial hosts all respond, for now they have a cheaper source of the crucial nutriment than they did before. The rain-fed bogs of New England were once full of pitcher plants that flourished as they sucked up nitrogen from their prey. Their competitors could not manage in such starved places. The acid marshes have been enriched. In those hardest hit - near cities or close to fertilised fields - the insect-eaters have abandoned their carnivorous habits in favour of a conventional life. Other species move in and drive the pitchers and Venus flytraps to extinction, and in Europe the sundew faces the same problem, which means that the insectivores are converging in death, as they did in life.
Carnivory, which began with shortage, may perish with excess and insects at least can breathe a sigh of relief. To an evolutionist, the shared fate of nitrogen-fixing bacteria and fungi, of the Venus flytrap and the sundew, and of trees and their ants, is further proof, as their diverse talents disappear, that under natural selection, and in both life and death, parallel lines may converge.
CHAPTER III
SHOCK AND AWE
Many American politicians have taken pleasure in gloating over the fact that Zacarias Moussaoui, the Frenchman accused of involvement in the Twin Towers disaster, will certainly go mad, held as he is in solitary confinement in the Colorado ‘Supermax’ prison. As the judge who passed sentence said: ‘You will never get a chance to speak again … and will die with a whimper.’
The eminent jurist was not quite justified in his satisfaction at his captive’s fate, for many of the tens of thousands kept in endless isolation in American prisons end their lives not with a whimper, but a scream. Some do fall into insanity in such places, but much as the religious right might celebrate their mental decay, they would be dismayed to learn that Moussaoui will lose his mind for Darwinian reasons. Guy the Gorilla, star of London Zoo in the 1950s, was admired for his solemn disposition. In fact, the animal was deeply depressed, kept as he was for years alone in a small cage (although unlike his human equivalents he had no opportunity for suicide). Homo sapiens is a social primate and - like gorillas or chimpanzees - descends from an ancestor with the same habits. Had our forefathers been solitary beasts like the orang-utan, which spends most of the year alone, the worst of all punishments would not be solitary confinement but an endless dinner party. The constant exchange of subtle emotional cues around the table would drive all those present to their wits’ end.
Science is often asked to explain what makes men different from chimpanzees or orangs but in many ways that is not a scientific issue at all. Such questions deal with the mind rather than the body or the brain; a topic that most competent biologists consider to be outside their expertise. Even so, as scientists compare man’s anatomy and behaviour with that of his relatives, biology does reveal a little about how humans became what they are. We are, says all the evidence, creatures that crave society. To satisfy that yearning we spend large parts of our time in silent and sometimes subliminal conversation with each other.
Rousseau had a different view of the origin of human nature. He saw man as in decline from a pure and animal state and modern society as a corruption of what the world should be. ‘Savage man, left by Nature to bare instinct alone … will begin with purely animal functions … His desires do not exceed his physical needs: the only goods he knows in the Universe are food, a female, and rest.’ The true life was near-solitude, on a remote island best of all, with a bare minimum of interaction with others. The French philosopher’s ideas were romantic, but wrong. Members of all communities, human or otherwise, must negotiate to maintain peace, to have sex and to reap the benefits of cooperation. They use signals both self-evident and subtle to test the mental state of their fellows and to advertise their own, and even the solitary orang hoots now and again to impress its neighbours. Civilisation is based on the ability to respond to another’s sentiments and to express a mood of one’s own.
In 1879, at the Derby, Darwin’s cousin Francis Galton noted that he could assess ‘the average tint of the complexion of the British upper classes’ as he observed the crowd through his opera-glass. Then the race started, and in a letter to Nature entitled ‘The Average Flush of Excitement’, he observed that it became ‘suffused with a strong pink tint, just as though a sun-set glow had fallen upon it’. A shared hue was a statement of a common passion and Galton could work out what it was even when he could not tell individuals apart. In the same way, someone exposed to an image of a group of people who bear a range of expressions from happy to miserable can sense their general state of mind far faster than he could by scanning each visage separately. Our brain, it seems, has a filter that picks up not just how many are in a crowd, but how, on average, they feel. The ability has its down-side. Mass hysteria can spread through society as shared sentiments feed on themselves; as Charles Mackay put it in his 1841 book Extraordinary Popular Delusions and the Madness of Crowds, in an account of the South Sea Bubble and other mass fantasies, men ‘go mad in herds, while they only recover their senses slowly, and one by one’.
In 1872, in The Expression of the Emotions in Man and Other Animals, Darwin discussed the role of signals in the herds, packs, flocks, towns and cities in which social animals come together. The book was a first attempt to understand our own sentiments in scientific terms. He was interested in how mental actions are manifest in the face and the body and realised how closely the displays of inner feeling made by men and women resemble those of animals. The book discusses instinct, learning and reflexes in creatures as different as moths and apes. Its author knew that elephants wept and hippopotami sweated with pain and when he heard a cow grind her jaws in agony he was reminded of the gnashing of teeth in hell. He saw that loneliness, fear or anger and their outer signs have all - like limbs or eyes - evolved. Kick a dog and it crouches and turns down the corners of its mouth; torture an al-Qaeda suspect and he does the same. The Expression of the Emotions makes a powerful case for the shared mental descent of humans, primates, dogs and more.
Our own sentiments have long been compared to those of other creatures. The seventeenth-century painter Charles Le Brun, who is referred to in the Emotions book as a pioneer in the study of human feelings, urged those who tried to portray their subject’s mood to scrutinise beasts first. A few hours with swine, lascivious, gluttonous and lazy as they were, would, he was sure, help depict the inner life of a debauchee. Charles Darwin’s friend George Romanes went further. He set out a scale with fifty ranks. Worms and insects came in at step 18 as they could experience surprise and fear; dogs and apes were equal at point 28 as each had ‘indefinite morality along with the capacity to experience shame, remorse, deceit and the ludicrous’. Levels 29 to 50 were reserved for men or women of greater or lesser virtue.
Psychology is still marked by such ideas. Emotions’ central theme was, as ever, a world in which all of life’s attributes, from anatomy to anguish, emerge from shared descent. Science uses that logic on elephants, cows, apes, fruit flies and bacteria in its attempts to build a shared narrative of inner feelings. Those who transmit their sentiments expect a response from those who receive them. That two-way commerce involves a need to acknowledge, to copy and to respond to the moods of others. People gasp in sympathy at a sad tale, gaze at where another person’s eyes are directed or avoid food that someone else has rejected. Such reflections of another individual’s mental state are part of what makes us human.
Charles Darw
in, a practical man, had little interest in philosophy. Even so, he realised that the biology of the mind was harder to interpret than was that of the body. He wrestled with the issue in rather the same way as modern psychologists try to come to grips with some of their own sometimes murky ideas. Can our thoughts be explained just as the ‘direct action of the excited nervous system on the body, independently of the will’ and if so, what (if anything) does that mean? Shakespeare speaks of Cardinal Wolsey when ‘Some strange commotion/ Is in his brain; he bites his lip and starts;/ Stops on a sudden, looks upon the ground …’ That, Darwin writes, came from the ‘undirected overflow of nerve-force’ - but is that phrase just an attempt to avoid deeper and less tractable questions? The task was made harder by his quarrel with the anti-evolutionist Charles Bell, author of the standard text on facial anatomy. Bell was convinced - and he was wrong - that humans had unique muscles divinely designed to express morality, spirituality or shame: a notion not of much help to someone anxious to understand the smile or the blush, but an early example of the preconceived truths that still plague many attempts to understand the human mind.
After a long stumble through the Freudian fog, the study of the mental universe has once again become a science, even if the many claims to have found the neural foundations of society do not yet deserve that status. Now, physicists and chemists busy themselves with questions once raised only by intellectuals. In institutes of psychiatry and neurology, cats, mice and dogs are used to dissect human habits. Even bacteria behave in a rational fashion when they settle down close to a source of food, or join hands with their colleagues to form a sticky film over teeth or wounds. Certain fruit-fly genes lead to homosexual behaviour and others to loss of memory, which might one day help in the study of illnesses such as Alzheimer’s disease. In mice and monkeys, experiments on brains once done with a scalpel are now carried out with machines of fantastic complexity. They are also used on people with brains damaged by strokes or accidents, while drugs help understand the mental universe of the normal, the reckless and the insane. Many of the questions raised in The Expression of the Emotions have a notably modern air and many remain unanswered.