Darwin's Island Part 2

Thomas Belt was an engineer and naturalist who spent five years in charge of a gold mine in Central America. Darwin called Belt's 1874 book A Naturalist in Nicaragua A Naturalist in Nicaragua 'the best of all natural history journals'. It notes how some trees in the Acacia family have fallen into an a.s.sociation with certain ants, who protect them against grazing insects and mammals. As Belt saw, the balance of advantage is delicate indeed. 'the best of all natural history journals'. It notes how some trees in the Acacia family have fallen into an a.s.sociation with certain ants, who protect them against grazing insects and mammals. As Belt saw, the balance of advantage is delicate indeed.

The total ma.s.s of ants in a patch of Amazon jungle is four times that of all its mammals, birds, reptiles and amphibians put together. Certain plants, there and elsewhere, have put them to work. Many tropical trees have hollow thorns which shelter the vicious insects, together with small structures filled with sweet and sticky material. Both parties benefit, for any creature that dares to browse on the tree is attacked and the ant gets a free meal. If its garrison is killed off with insecticides the tree is attacked by grazers at ten times the previous rate. The helpers also prune back branches of nearby trees that shade their host, and clean up the ground around its trunk, reducing compet.i.tion for food. Some ants even poison nearby plants as they inject formic acid into the leaves. That then allows their own host to flourish on patches of cleared ground known to the locals as 'devil's gardens' and thought to be cultivated by an evil and cloven-hoofed spirit. In return the insects feed on secretions from the acacia's leaves and feed their young from what Darwin called its 'wonderful food bodies'. They also gain protection by laying eggs inside the hollow thorns.

More than a hundred distinct groups of tropical trees, and forty families of ants, have entered into such an alliance. The habit has evolved many times and - like insectivory - has enabled natural selection to pick up a diversity of parts for use in a novel way. The shelters are based on thorns, on hollow stems or on rolled-up leaves, or on special pouches made on the surface of the leaf. Once again, evolution makes do and mends, as it must.

The details of the liaison give proof of Darwin's insistence that natural selection allows n.o.body a free lunch. At first sight, the bond between ant and trees is based on a shared dedication to a common end. In fact, each tries to get the most out of the arrangement while putting the least possible in. Their tactics hint at how the tie between the botanical carnivores and their prey may have begun.

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 a.s.sociations 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 a.s.sociation 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 a.s.sociation 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 a.s.sociation, 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 pa.s.ses 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 s.e.x, 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 compet.i.tive 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 source of nitrogen. First, it alters the balance of profit and loss. After a dose of fertiliser, crops need less help from their tiny a.s.sistants 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 compet.i.tors 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 pa.s.sed 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). h.o.m.o sapiens h.o.m.o 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. 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 s.e.x 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 a.s.sess 'the average tint of the complexion of the British upper cla.s.ses' as he observed the crowd through his opera-gla.s.s. Then the race started, and in a letter to Nature Nature ent.i.tled '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 pa.s.sion 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. Ma.s.s hysteria can spread through society as shared sentiments feed on themselves; as Charles Mackay put it in his 1841 book ent.i.tled '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 pa.s.sion 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. Ma.s.s 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 Extraordinary Popular Delusions and the Madness of Crowds, in an account of the South Sea Bubble and other ma.s.s 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 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 gnas.h.i.+ng of teeth in h.e.l.l. 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 The Expression of the Emotions makes a powerful case for the shared mental descent of humans, primates, dogs and more. 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 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. 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 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 Darwin, 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 inst.i.tutes of psychiatry and neurology, cats, mice and dogs are used to dissect human habits. Even bacteria behave in a rational fas.h.i.+on 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 h.o.m.os.e.xual 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 The Expression of the Emotions have a notably modern air and many remain unanswered. have a notably modern air and many remain unanswered.

Emotions is in some ways a less satisfactory work than are the plant, barnacle or earthworm books and an unusual note of apology creeps in: 'Our present subject is very obscure . . . and it always is advisable to perceive clearly our ignorance' (and there its author was franker than some of his successors). Charles Darwin soon found that even what looked simple - the objective description of the facial expression of a man or a dog, for example - was hard, while to represent the sentiments behind it was even harder. That problem, in spite of the wonders of electronics, still baffles students of the nervous system. He was suspicious of phrenology - the notion that particular segments of the brain are a.s.sociated with, for example, obstinacy, pride or guile - even if an admirer had claimed that the naturalist's own head had 'the b.u.mp of reverence developed enough for ten priests'. He struggled long and hard with the question as to just where felt experiences might be seated. is in some ways a less satisfactory work than are the plant, barnacle or earthworm books and an unusual note of apology creeps in: 'Our present subject is very obscure . . . and it always is advisable to perceive clearly our ignorance' (and there its author was franker than some of his successors). Charles Darwin soon found that even what looked simple - the objective description of the facial expression of a man or a dog, for example - was hard, while to represent the sentiments behind it was even harder. That problem, in spite of the wonders of electronics, still baffles students of the nervous system. He was suspicious of phrenology - the notion that particular segments of the brain are a.s.sociated with, for example, obstinacy, pride or guile - even if an admirer had claimed that the naturalist's own head had 'the b.u.mp of reverence developed enough for ten priests'. He struggled long and hard with the question as to just where felt experiences might be seated.

The student of the inner world looked first at the animals and children of his own household. As a kind-hearted man, he was careful not to disturb them too much, although his book does contain images of frightened babies that would see him accused of cruelty today. His sons, he noted, never pouted, although Francis's mouth a.s.sumed that expression when he played the flute. He did not hesitate to play the animal himself. Francis remembered that his father's body was very hairy, and that the great man would growl like a bear when his children put their hands inside his s.h.i.+rt.

Even in play the Beagle Beagle's naturalist was serious, and he soon identified some general rules about human and animal behaviour. Intimations of happiness or grief, of welcome or rejection and of other opposed sentiments often came as mirror images. Thus, a frown is the opposite of a smile and a look of surprise the converse of a greeting. Some gestures emerged from movements that once had a function of their own. To beg with open hands is related to the posture taken when holding food and, in the same way, a person who rejects an advance closes his eyes and looks away, as if from an unpalatable meal. Animals seemed to follow similar rules and the paterfamilias of Down House saw almost the same downcast looks in his household pets as those adopted by his infant son.

From such simple observations emerged the science of comparative psychology. It began with dogs.

Pets gain their status because they seem, to their owners at least, to be almost human. Darwin was no exception and kept a dog - Sappho by name - even when he was a student. He saw no problem in describing canine sentiments in the same terms as our own. His pet when in 'a humble and affectionate frame of mind' acted in a way quite different from that of a hostile animal with its bristling hair and stiff gait. The 'principle of ant.i.thesis' was hard at work, for opposed sets of muscles were set into action to express contrasting emotions. The 'piteous, hopeless dejection' of his favourite hound when it discovered that it was not about to go out for a walk but instead was to sit in on an experiment in the greenhouse was manifest in a 'hothouse face', the 'head drooping much, the whole body sinking a little and remaining motionless; the ears and tail falling suddenly down, the tail by no means wagged'. That was quite different from its expression when happy and excited, with the head raised, ears erect and tail aloft.

As well as such individual s.h.i.+fts of mood the proud pet-owner noted marked differences in personality among breeds. Descent with modification could, it appeared, change minds as easily as it could bodies. Certain kinds, such as the terrier, grinned when pleased while others did not. Spitz-dogs - huskies, elkhounds and the like - barked while the greyhound was silent. The canine universe encompa.s.ses a wide range of talents. Some varieties herd sheep and cattle (and, in the case of the Portuguese Water Dog, chivvy fish instead) while others guard, hunt, guide or annoy the general public. The various breeds when taken together show a wider range of behaviour than that found among all wild canines - wolves, foxes, coyotes and jackals - across the world. Many of the differences are innate, and The Origin The Origin tells of a cross with a greyhound which gave a family of shepherd dogs a tendency to hunt hares. So impressed was its author with the animals' divergence in habits that he suggested some of the household types had descended from distinct wild ancestors (and there he was wrong). tells of a cross with a greyhound which gave a family of shepherd dogs a tendency to hunt hares. So impressed was its author with the animals' divergence in habits that he suggested some of the household types had descended from distinct wild ancestors (and there he was wrong).

His favourite pet is back at the centre of the emotional stage. The world has four hundred million dogs and the efforts of their owners and the wonders of science have transformed the creatures into a gigantic experiment on the biology of sentiment. Even in the brief period since modern breeds began to emerge in Victorian times dogs have undergone large - and inherited - changes in temperament.

Men long ago began to use dogs in the hunt. They soon learned to choose those with their own special abilities - to track, to run, to squat into a 'point' position when prey is spotted, or to bite and tear or recover corpses - as parents for the next generation. Such remnants of the chase live on in the behaviour of Bloodhounds, Pointers, Setters, Retrievers and Bull Terriers. Herding dogs such as Border Collies stalk a sheep and do not bite it, but those used to control larger animals - like the Corgis once used with cattle - go further through the sequence and snap at their charges. Pit Bulls complete the job and are vicious creatures that will hold a bull by the nose and as a side-effect sometimes kill their owners. Guard dogs such as Pyrenean Mountain Dogs, whose job is to frighten off predators, have given up the hunt sequence altogether. They play like huge puppies and show little interest in their herds, but their conduct is odd enough to persuade wolves to stay away. Such differences emerge from inherited variation in behaviour within the common ancestors of each breed, from new genetic errors as the generations succeed each other, and from the acc.u.mulation of change by human choice.

One way to a.s.sess a dog's personality is to startle it with the appearance of a stranger. Does the animal play with the visitor, back off, sniff him or chase him out of the room? Does a sudden noise anger the beast, terrify it or leave it unmoved? Other tests include the ability to sit still, to cope with solitude without whining or panic, to run through a maze or to find hidden food.

c.o.c.ker Spaniels are calm and obey orders, while Basenjis are nervous and almost impossible to train. Crosses between the two suggest that the difference in their nature is inborn, for the offspring have a range of talents, intermediate between each parent. A survey of ten thousand German Shepherds and Rottweilers in Sweden showed, within each type, a shared inheritance of excitability, a tendency to wag the tail and a need to bark, while aggression appears to be under separate control. In an echo of Expression Expression's principle that ant.i.thetical emotions are expressed as mirror images, variation in all those capacities depends on just how shy or how bold a particular breed might be.

As the dog-fanciers' tastes became more refined, more and more specialised varieties emerged. Some began to develop habits that perturbed their owners. Mating like with like exposed rare and once-hidden genes, many of which had undesirable effects on personality. Some have parallels in the mental lives of men and women. In an echo of human obsessive-compulsive disorder, Bull Terriers chase their own tails for hours until they collapse, while Springer Spaniels may savage their masters as they fall into a sudden attack of uncontrolled rage. Certain families of Ba.s.sett Hounds suffer from a delusion reminiscent of paranoid schizophrenia and cower at the slightest noise. Some Dobermans, in contrast, fall into a heavy slumber after an unexpected snack. They have narcolepsy, a distressing and sometimes dangerous condition also found in people - and the dogs respond well to the drugs used to treat human patients.

The double helix reveals why some breeds diverge so much in personality. The first complete sequence came from a Boxer. The animal had less DNA than we do, with about twenty thousand genes altogether, several thousand fewer than ourselves. The hope is to find canine matches to our own disorders, and some have already emerged. The sleep problem in Dobermans involves damage to a certain receptor protein on the surface of brain cells - and the human equivalent is due to a fault in the same gene. No doubt our companions will help track down many more of the inherited errors behind our own mental illnesses, as they already have for conditions such as blindness. Charles Darwin would be proud.

Dogs are anomalous animals for their habits have been so subdivided by human effort that their mental universe is far from typical of a wild creature. Darwin soon moved on in his search for the roots of human emotion. He spent many hours in the company of our relatives in London Zoo. He had particular fun with the anatomy of amus.e.m.e.nt: 'Young Orangs, when tickled, likewise grin and make a chuckling sound . . . as soon as their laughter ceases, an expression may be detected pa.s.sing over their faces, which, as Mr. Wallace remarked to me, may be called a smile . . . I tickled the nose of a chimpanzee with a straw, and as it crumpled up its face, slight vertical furrows appeared between the eyebrows. I have never seen a frown on the forehead of the orang.' He was particularly taken by the attempts of a monkey to court its own image in a mirror and by the antics of Jenny the orang-utan, who when teased with an apple on the wrong side of the bars 'threw herself on her back, kicked and cried, precisely like a naughty child'.

Primates, like people, reveal their feelings on their faces. Someone who has never before seen a macaque can at once identify its mood as sad, happy or enraged, when shown the appropriate photograph. Many chimpanzee expressions have been named. They include the closed-mouth smile, its bared-teeth equivalent (which descends from an ancestor shared with our own smile), the bozo smile and the play face (a relative of human laughter), together with subtler statements of mood such as the stretched pout-whimper. Bon.o.bos have an amused expression - and noise - which is uncannily like a guffaw. A German expert has identified an o.r.g.a.s.musgesicht o.r.g.a.s.musgesicht or 'o.r.g.a.s.m countenance' in that species, although its existence in humans remains to be demonstrated. Gorillas are more impa.s.sive for they grin and make bozo faces but otherwise keep their thoughts to themselves unless they are simply furious. or 'o.r.g.a.s.m countenance' in that species, although its existence in humans remains to be demonstrated. Gorillas are more impa.s.sive for they grin and make bozo faces but otherwise keep their thoughts to themselves unless they are simply furious.

Apes and monkeys can interpret their fellows' moods to a considerable degree. Electronic avatars of chimps can have their looks manipulated to simulate pout-whimpers and the rest. When real animals are presented with their artificial comrades, they pick out different expressions at once, screaming faces best of all. They also show some insight into another's emotions. If one animal sees another grimace in fear when it hears a sound it had learned to a.s.sociate with an electrical shock, the observer will flinch when the buzzer goes off, even if it has never itself experienced the shock.

Humans are even better at sensing the moods of others. We are so aware of facial features that we often see them when they are not there (which explains the sad ape-like countenance in NASA's pictures of the mountains on Mars). Two scrunched-up newspapers look much the same although their shapes are quite different, while two faces are seen as quite distinct although their shapes are almost the same. A simple bar code, the position of six stripes of dark and light - hair, forehead, eyebrows, nose, lips and chin - stores most of the data. Most people can recognise thousands of individuals and sense dozens of emotional states. Faces are important even to infants. Darwin noted that, when they were very small, his children spent long periods gazing at their mother. Now we know that a baby responds to a human countenance - even in a photograph - within minutes of birth.

Men, like apes, speak with their faces and use more or less the same language to do so. Angry people and angry gorillas bare their teeth and a frightened chimpanzee looks rather like a frightened person. For humans, as for apes, some expressions are ambiguous. Men and apes bare their teeth when amused but do the same when filled with terror. Emotions Emotions has a picture of a Sulawesi macaque as it grins in pleasure when stroked - but in other macaques the same gesture marks submission to a threat. Not all our grimaces are shared with our relatives, for apes never signal disgust and their noses, which are more sensitive than our own, remain unwrinkled even to a repulsive smell. A wide-open mouth is a threat in many primates but conveys no more than mild surprise for humans and while elephants weep, our closest kin do not. has a picture of a Sulawesi macaque as it grins in pleasure when stroked - but in other macaques the same gesture marks submission to a threat. Not all our grimaces are shared with our relatives, for apes never signal disgust and their noses, which are more sensitive than our own, remain unwrinkled even to a repulsive smell. A wide-open mouth is a threat in many primates but conveys no more than mild surprise for humans and while elephants weep, our closest kin do not.

Monkeys and apes reflect their moods in their postures as well as their expressions, and gorillas really do slap their chests in rage. Men, orangs, chimps and gorillas share the Italianate habit of waving their hands. Bon.o.bos flap their wrists in irritation, point at themselves when they need a hug and stick out their palms when food is on offer. In a further nod at our common heritage, they prefer to signal with the right hand.

Our faces are more eloquent than are those of any other primate. Many pages of The Expression of the Emotions The Expression of the Emotions are devoted to the way they reflect their owners' inner state. Some read rather quaintly nowadays: 'the breach of the laws of etiquette, that is, any impoliteness or are devoted to the way they reflect their owners' inner state. Some read rather quaintly nowadays: 'the breach of the laws of etiquette, that is, any impoliteness or gaucherie gaucherie, any impropriety, or an inappropriate remark, though quite accidental, will cause the most intense blus.h.i.+ng of which a man is capable. Even the recollection of such an act, after an interval of many years, will make the whole body to tingle. So strong, also, is the power of sympathy that a sensitive person, as a lady has a.s.sured me, will sometimes blush at a flagrant breach of etiquette by a perfect stranger, though the act may in no way concern her.' In the interests of science, modesty gave way to the search for truth: 'Moreau gives a detailed account of the blus.h.i.+ng of a Madagascar negress-slave when forced by her brutal master to exhibit her naked bosom' and the s.e.xual nature of that expression means that 'Circa.s.sian women who are capable of blus.h.i.+ng, invariably fetch a higher price in the seraglio of the Sultan.' Mark Twain, himself an ardent evolutionist, put it well: 'Man is the Animal that Blushes. He is the only one that does it-or has occasion to.'

Darwin was keen to discover whether signals such as the blush were the same in every human culture, or whether, like skin colour, they changed from place to place. He rejected the popular notion that different races had evolved from higher or lower forms of primate and that their mental lives and expressions of mood reflected this. Soon he began to acc.u.mulate a ma.s.s of anecdotes that made the case for the universal nature of facial cues. People also wrote to him about their dogs frowning in concentration or showing moral courage when their teeth were pulled. His correspondents included William Ewart Gladstone, who commented on statements about the Greek visage found in Homer, but also 'Captain Speedy who long resided with the Abyssinians; Mr Bridges, a catechist residing with the Fuegians and Mr Archibald O. Lang of Coranderik, Victoria, a teacher at a school where aborigines, old and young, are collected from all parts of the colony'. One letter told of a Bengali boy with 'a thoroughly canine snarl'. Its recipient fired off a series of questions to those servants of the Queen, sometimes to ludicrous effect ('Mr B.F. Hartshorne . . . states in the most positive manner that the Weddas of Ceylon never laugh. Every conceivable incitive to laughter was used in vain. When asked whether they ever laughed, they replied: "No, what is there to laugh at?"').

In spite of the Weddas, Darwin became certain that such signs were more or less universal across the globe: 'The young and the old of widely different races, both with man and animals, express the same state of mind by the same movements.' Hard as it is to believe, that observation was forgotten and for many years students of humankind a.s.sumed that expressions were determined by culture and were not coded into DNA (even if n.o.body found a place where people laughed in pain or screamed in welcome). Looks of anger, disgust, contempt, fear, joy, sadness and surprise all are universal. One tribe in New Guinea cannot separate expressions of fear from those of surprise - but in that society any intruder is a threat. People from different cultures do find it harder to identify each other's guilty or shamefaced looks than they do a smile or an expression of terror, so that such subtle statements of mood may in part be learned. Even smiles are equivocal, for the beam, grin, smirk, sn.i.g.g.e.r, simper and leer each convey a different message while people who smile too often come across as nervous rather than contented. Darwin, too, saw some ambiguities. The expression in a photograph of a man almost in tears was recognised by some as a 'cunning leer', a 'jocund' frame of mind or even as someone 'looking at a distant object'.

Once he had established that most such signs were common to all mankind, Darwin set out to describe them. Measurement, he knew, is the first step in science (a lesson still ignored on the wilder sh.o.r.es of psychology) and he tried hard to give an impartial description of human features ('The contraction of this muscle draws downwards and outwards the corners of the mouth, including the outer part of the upper lip . . . the commissure or line of junction of the two lips forms a curved line with the concavity downwards and the lips themselves are generally somewhat protruded').

In today's world of fraud, terrorism and ident.i.ty cards such attempts to put facts on to faces have become an industry. Remarkable claims are made about the ability to identify people and to sense their states of mind. Some enthusiasts recognise thirty indications of anger and eight of sadness, with additional criteria based on how the subject holds his head. George W. Bush's countenance was more or less blank whatever message he tried to put across, but his Department of Homeland Security has spent millions on machines that claim to detect when a terrorist is about to attack by the look on his countenance. n.o.body denies that the expression of a Scotsman with a grievance is easy to distinguish from a ray of suns.h.i.+ne but such claims go too far.

The face says a lot about how we feel, but - as in apes - the body adds information to the stream of emotional cues: a man with raised fists is not about to make a visitor welcome. Psychologists tend, for practical reasons, to use pictures of faces alone. That can be a mistake. An image of a man with a disgusted expression taken from a modern catalogue of facial poses is interpreted as manifesting revulsion when superimposed on to a body holding a pair of dirty underpants - but as a look of anger when added to a torso with fists raised, or of triumph when stuck on to the beefy frame of a body-builder. The same photograph shown against the background of a cemetery is interpreted in a different way than when seen against a neutral surface. For students of the emotions, the a.s.sumption of simplicity can confuse results taken from the most complicated machines.

The face is a real mirror to the soul. Even a brief glimpse reveals the presence of another person, identifies who it might be and gives a strong hint as to what its bearer will do next. Most westerners interpret a set of features with a quick triangular scan of both eyes and the mouth, each of which say a lot about ident.i.ty and state of mind - but the Chinese tend to concentrate their attentions instead in a fixed look at the nose and pick up the general expression of the whole visage in the background. Scans show that when someone flashes into view, the brain first notes his or her presence, then identifies who it might be and last of all of tests their mood: this is a face, it belongs to Fred, Fred is furious. It processes a portrait twice as fast as a picture of other objects. A certain part lights up about a tenth of a second after a face is first seen, notes its ident.i.ty about a fifteenth of a second later and takes even longer to interpret what humour the person might be in.

Some expressions are easier to identify than are others. The smile is coded deep within the skull and everyone has an inborn ability to a.s.sume it. As Darwin noted, babies born blind smile without difficulty and (as he did not) blind athletes raise their arms in the air in a chimp-like gesture of triumph when they win. Children find it easier to pick out expressions of good cheer than they do those of fear or disgust. Women smile more at strangers than do men, while men are worse at working out mood from a slight movement of the lips. A lopsided grin to the right is seen by most of us as more joyful than is its equivalent on the left. Even sheep, when given a choice of a smirking or a sombre shepherd from whom to take food, prefer the cheery individual. We smile or raise our arms not to rea.s.sure ourselves that we are happy or proud, but to tell others how we feel. Context is all; when Chelsea score, fans respond with roars of triumph rather than smiles of delight, but gold medal winners as they stand on the Olympic podium have wider grins than do those who have gained bronze.

Signs of delight or terror seem simple enough, but there are real differences in the ability to decode them. I have a talent that ill.u.s.trates that fact, for I can waggle my eyebrows. It began in school when I was rebuked for glowering. I then tried dumb insolence with a one-brow grimace rather than the full two-brow scowl and in time it became easy to alternate. It is still my occasional habit to amuse small children with the trick - and almost always they smile back. Unfortunately, an occasional infant screams instead. The signal is clear but the response uncertain.

Both steps can go wrong. Some people cannot tell individuals apart from their faces and use clues from voice or clothes instead. In one instance, a litigant wandered into court and discussed his case with a barrister - not his own, but his opponent's. The context was right; a lawyer, with a gown, in a courtroom. The face alone did not fit. Needless to say, he lost. Face-blindness may be caused by a stroke, but a certain form runs in families with perhaps just a single gene involved.

Other unfortunates lose the ability to broadcast their emotions. For some reason - injury, infection, cancer or brain haemorrhage - the facial nerve no longer works and the patients cannot express their feelings. They find it hard to a.s.sume looks of happiness, fear or surprise, and their wives, husbands and friends soon notice the problem. The condition might appear to be trivial, but in fact causes real distress and sometimes even suicide, most of all when an attempt to smile emerges as a grimace or a leer because the eyebrows - usually lifted at a happy moment - refuse to obey instructions. Some people have their brows surgically moved upwards (which gives them a permanent look of surprise), while others grow a long fringe that hides the offending forehead. The readiness to take such steps shows how much a signal of mood is a pa.s.sport to society.

Emotions marked the first real attempt by science to infer the action of the mind from its external signs. Scientists now study the activity of brain cells rather than of facial muscles as they try to understand our inner feelings. The use of electricity - and of the sophisticated electronic devices that depend on it - in psychology has become a science of its own. It was first expounded in Charles Darwin's book. marked the first real attempt by science to infer the action of the mind from its external signs. Scientists now study the activity of brain cells rather than of facial muscles as they try to understand our inner feelings. The use of electricity - and of the sophisticated electronic devices that depend on it - in psychology has become a science of its own. It was first expounded in Charles Darwin's book.

The ancient Greeks had used electric fish to treat headaches but for many years the galvanic fluid was no more than an entertainment. An entire community of monks was once connected by a mile-long iron wire and made to jump for the amus.e.m.e.nt of the King of France (castrati were tested to see if they acted as insulators, but they did not). Emotions Emotions contains several pictures of faces stimulated by shocks to give expressions that resemble the natural look of horror, rage and the like. They came from the French physician Guillaume-Benjamin-Armand d.u.c.h.enne de Boulogne. d.u.c.h.enne is best remembered for the muscle disease named after him but he also studied the expression of what he called the 'pa.s.sions', using electrodes touched to different parts of a countenance to stimulate the muscles. He was the first to notice that a genuine smile involved raised eyebrows, and his machine could easily activate those 'sweet muscles of the soul' to simulate a happy beam. He even made the visage of a decapitated criminal a.s.sume a simulacrum of pleasure with a probe upon its cheek. d.u.c.h.enne chose as his main subject an aged man of feeble intellect, for he 'wanted to prove that, despite defects of shape and lack of plastic beauty, every human face can become spiritually beautiful through the accurate rendering of emotions'. His pictures first came to public attention when they were published in contains several pictures of faces stimulated by shocks to give expressions that resemble the natural look of horror, rage and the like. They came from the French physician Guillaume-Benjamin-Armand d.u.c.h.enne de Boulogne. d.u.c.h.enne is best remembered for the muscle disease named after him but he also studied the expression of what he called the 'pa.s.sions', using electrodes touched to different parts of a countenance to stimulate the muscles. He was the first to notice that a genuine smile involved raised eyebrows, and his machine could easily activate those 'sweet muscles of the soul' to simulate a happy beam. He even made the visage of a decapitated criminal a.s.sume a simulacrum of pleasure with a probe upon its cheek. d.u.c.h.enne chose as his main subject an aged man of feeble intellect, for he 'wanted to prove that, despite defects of shape and lack of plastic beauty, every human face can become spiritually beautiful through the accurate rendering of emotions'. His pictures first came to public attention when they were published in The Expression of the Emotions. The Expression of the Emotions. They played an important part in Darwin's attempts to give an objective account of expressions of pleasure or pain They played an important part in Darwin's attempts to give an objective account of expressions of pleasure or pain.

The machines have marched on. Where d.u.c.h.enne used a battery, a metal rod and a plate-camera, scientists in search of the springs of sentiment now depend on electro-encephalograms, positron emission tomography or functional magnetic resonance imaging (FMRI). Tiny electrodes are used to activate single nerve cells, while the EEG and its relative the magneto-encephalograph pick up electrical activity within the skull. PET scanners use a sugar marked with a radioactive label which is taken up by active parts of the brain and then detect its decay products. The fMRI machine, in contrast, senses tiny changes of blood flow through the grey matter from a s.h.i.+ft in the magnetic properties of the red pigment haemoglobin as it gains or loses oxygen.

Marvellous as such techniques are, they run into many of the problems that plagued Charles Darwin. He had found it hard to decide just where the jaw ends and the cheek begins or to identify the precise arrangement of facial muscles. Today's arguments about the boundaries between areas of the brain as defined by electronic scans - confidently coloured and labelled as the images might be - reflect his own doubts about the anatomy of the human countenance. Some claim that particular emotions can be mapped to a definite part of that organ. Others see the brain - as he saw the face - as a connected structure, with most sections contributing to most of its functions. Any attempt to pinpoint centres of anger, joy or despair might be of its nature a mistake.

Another problem for both the nineteenth and the twenty-first centuries comes from the need to describe broad sentiments in narrow terms. Darwin was happy to talk about dogs in a 'humble and affectionate frame' of mind - but how is it possible to put figures on humility or affection? Objective fact soon slides into mere interpretation and Expression Expression was itself not immune from that temptation. Its photographic plates are not originals but engravings, some touched up to make a point. A mad lady with tousled hair was given a furrowed brow by the engraver and a screaming infant was made to look even more miserable than before by copying the portrait and re-photographing the sketch (the picture sold hundreds of thousands of copies to a gullible public). The stream of lurid images of centres for pain, pa.s.sion and pleasure that decorate the scientific literature and leak into the press are also in some senses fakes. Digital information is processed in a complicated and sometimes subjective way to make a picture which is often rather more than the sum of its parts. was itself not immune from that temptation. Its photographic plates are not originals but engravings, some touched up to make a point. A mad lady with tousled hair was given a furrowed brow by the engraver and a screaming infant was made to look even more miserable than before by copying the portrait and re-photographing the sketch (the picture sold hundreds of thousands of copies to a gullible public). The stream of lurid images of centres for pain, pa.s.sion and pleasure that decorate the scientific literature and leak into the press are also in some senses fakes. Digital information is processed in a complicated and sometimes subjective way to make a picture which is often rather more than the sum of its parts.

A final difficulty for both the Victorians and their descendants was to find subjects who were willing to display their emotions to the world. d.u.c.h.enne set up a theatre in which the public could be delighted by actors galvanically activated to produce an air of grief or delight. Many of Expression Expression's pictures are also based on members of that profession. Among them, a bearded thespian looks remarkably implausible as he strikes his att.i.tudes. Actors still play an important part in neuroscience. Their photographs are taken as they simulate a mood and are then shown to subjects whose brains are scanned to see which bits light up. Many of the images look just as posed as do those of Darwin's theatrical friend. The artistes overdo the job, often to a bizarre degree. People shown pictures of frightened or unhappy people taken from real life have far less of a nervous response than they do to images of those who simulate a mood as they strut and fret upon the laboratory floor. Most of us find it harder to interpret the sentiments (apart from laughter) in silent clips of Hollywood stars from Dustin Hoffman to Meryl Streep than we do the simulated joy or terror of a ham actor - and yet the hams are used as raw material for experiments of huge technical sophistication and expense.

Many claims have been made that particular parts of the brain respond to the sight of a happy or miserable countenance and that they prepare the nervous system to beam back or look sympathetic in return, but they have been hard to replicate. Because light comedy is a subtler form of entertainment than is Greek tragedy, the scientists who study that great theatre of emotion, the face, often focus instead not on mild signs of contentment or sadness but on expressions of horror and dread that might provoke an unambiguous response in those who see them.

A blank stare is a signal of terror and Shakespeare knew as much. A furious Oth.e.l.lo says to his supposedly unfaithful (and frightened) wife Desdemona before he kills her: 'Let me see your eyes.' We have larger eye-whites than any other primate and take more notice of them, for the mouth is far more important than the eyes as a chimpanzee emotional signal. We process eyes quicker than any other feature and fearful, stretched-open eyes even faster - and women do the job better than men. One woman could not recognise a picture of a terrified individual because she did not look at the eyes. When instructed to do so she at once understood the subject's frame of mind.

The brain's main activity in response to a frightened look takes place in a pair of structures called the amygdalae. They are almond-shaped groups of nerve cells deep within the temporal lobes, the side sections of the brain, one on each side, embedded into what is sometimes seen as the organ's most primitive parts. Each is connected to other brain centres, to the hypothalamus - that hormonal bridge between the nervous system and the bloodstream - to nerves that feed from pain receptors and from the eyes, and, in primates more than other mammals, to nerves to and from the face itself.

Animals in which the structures have been damaged find it hard to pa.s.s the cla.s.sic test in which fear of an electric shock becomes a.s.sociated with the sound of a bell. Experiments on monkeys in which those parts of the brain were cut out showed that the unfortunate creatures in addition lost their ability to recognise familiar objects, together with their nervousness about humans, and a mother's affection for her infant. Human patients with damaged amygdalae have similar problems with emotionally draining tasks. The amygdala is also involved in memory. People recall where they were on 11 September 2001 with its help, but those in whom the structure is damaged remember the Twin Towers disaster no better than what they had for breakfast.

The amygdalae are busiest when a frightened gaze is directed straight at its target - which fits Darwin's idea that a countenance stricken by terror is an immediate signal of danger. A few people have such severe brain damage that they perceive themselves as blind - but show them a scared person and the amygdala lights up. We are slower to notice the racial origin of an angry than of a happy face, so that fear has priority over familiarity. In the United States, images of black people shown to whites stir up more activity than do those of individuals of their own skin colour.

The case for the amygdala looks persuasive but, as usual when it comes to the contents of the skull, real life is not simple. Other parts of the brain are also involved in the response to a terrified countenance. The amygdala lights up in response to a whole face rather than just the eyes, and does so to some degree whether or not the subject shows signs of alarm. Its main role might be to notice new events, whatever they might be, rather than to make a specific response to a particular emotion.

The structure helps to process a nerve-transmitter called serotonin (which is also involved in temperature control, sleep, hunger, l.u.s.t, response to injury, liver repair and more). Many antidepressants work because they change the way in which serotonin is broken down, or taken into cells. Variation in the ability to respond to or to make the substance might be behind individual responses to fear. Some people are terrified even by the simplest problems of society. Darwin writes of a dinner party given for a man who, in response, 'did not utter a single word; but he acted as if he were speaking with much emphasis. His friends, perceiving how the case stood, loudly applauded the imaginary bursts of eloquence, whenever his gestures indicated a pause, and the man never discovered that he had remained the whole time completely silent.' The unfortunate fellow could now be comforted with the information that he may have a more active amygdala than normal and that his nervousness might be treated with drugs that alter his body chemistry.

Inborn errors in the ability to synthesise serotonin make some people sad, angry or suicidal. A gene whose product helps remove the chemical from the junctions between nerve cells comes in two common forms, one better at the job than the other. The less active type is more frequent among people who are anxious, neurotic or depressed - and its bearers are less able to decode expressions of fear or sadness than are their fellows. The orang-utan - the most solitary of our primate kin - has a version of the gene that is even less busy than that of the most socially isolated human. Whether its feeble serotonin pump has much to do with its lonely life and presumed dislike of dinner parties remains to be proved.

People with severe depression often find it hard to sense the emotions of others. Drugs that affect serotonin can help the illness - and their immediate effect, sometimes within hours of the first pill, is to improve a patient's ability to interpret their fellow citizens' feelings from their faces. That simple talent turns the key that restores them to society.

Nowhere is the importance of signals better seen than in children. When very young their insights are limited and self-centred, but soon they begin to understand and to respond to the moods of those around them. Darwin wrote a Biographical Sketch of an Infant Biographical Sketch of an Infant, an account of child development based on his son William: 'When 110 days old he was exceedingly amused by a pinafore being thrown