Darwin's Island Part 4

All mammals, smelly or not, carry inherited ident.i.ty cards on the surface of every cell. We cannot accept kidney transplants because our immune system compares the donor's genes with our own, recognises the tissue as foreign and rejects it. The less related the source of the organ, the fewer the genes in common and the lower the chance of success, which is why brothers and sisters are better donors than are pairs of strangers. The ident.i.ty system is based on a set of genes that sit close together on the DNA. They live in a section that codes for the many functions of the immune system, our prime defence against infectious disease, and, as an incidental, against the novel challenges presented by tissue transplantation. Each comes in many different forms, which means that vast numbers of combinations are possible.

Disease is a potent agent of natural selection. Individuals with the most diverse set of immune-system genes, and those with large numbers of rare variants, tend to fight off infection better than others. Mice and even fish prefer to mate with those least similar to themselves in immune ident.i.ty, as a hint that the tie between s.e.xual choice and disease resistance is ancient indeed. In the fight against infection, such diversity pays, for the next generation will have, thanks to s.e.x, a new mix of defensive genes, confusing the parasites' ability to evolve fast enough to evade our immune system. It hence pays to choose someone as different as oneself as possible.

As Darwin discovered, cowslips and other plants are very careful when deciding which pollen is acceptable, with a variety of devices to ensure that their reproductive parts stay free of cells from males physically similar to themselves. The erotic stink of mice does the same job and humans, too, may learn to avoid familiar kin. In truth, the s.e.xual examination goes on well after the male cells arrive. Plants choose what pollen tubes are allowed to grow, and female insects may store the sperm of many males before deciding which should be allowed to travel further. Even after fertilisation, plants and mice are happy to abort a high proportion of their embryos, most of all those that arise from the attentions of a male relative.

The female reproductive system is a difficult and dangerous place for a sperm to find itself. Promiscuous mammals have longer v.a.g.i.n.as than do those who stick to a few mates and make the male cells work harder to reach their goal. The v.a.g.i.n.al tract is acid, too, and sperm do not much like that. In humans, of the millions implanted by a successful man, no more than a few hundred reach the neighbourhood of the egg, twenty or so make it to the point where they might be able to fertilise it and just a single cell gets in.

The smell of success lingers on after the s.e.x act is over. Human sperm pick up and move towards chemical signals from the egg with the help of a gene that sits right inside the group that codes for smell perception. The complicated pore in the nose or the sperm cell membrane that picks up a single scent molecule, or a signal from the egg, each do more or less the same job and the two look remarkably alike - and, in a nod to their common heritage, some of the genes used by mice as they sniff the air to a.s.sess kins.h.i.+p by smell are also active in sperm. In an unexpected link between two s.e.xual worlds, the sperm receptor also responds to the scent of lily of the valley and, given the choice, will swim towards it. Whether human eggs prefer to attract, or to allow entry to, sperm genetically different from themselves, we do not yet know.

Darwin's work on the s.e.x lives of plants has strayed into fields that would have shocked his contemporaries. His interest in their reproductive habits grew from his concerns about the effects of inbreeding in humans and on his own family in particular. Its influence is real, albeit less severe than he had predicted, and both plants and humans have evolved mechanisms that limit its effects. Faced with the same set of challenges, natural selection has come up with similar solutions in both kingdoms of life, which would not have surprised him (although he would, perhaps, be startled to discover that human sperm are attracted by the scent of a flower).

The great man's concern about the possible damage done by cousin marriage to his own children was not justified. Of his sons, William became a banker and Leonard an army major. George was elected Professor of Astronomy and Francis Reader in Botany at Cambridge, while Horace set up as a scientific-instrument maker and was for a time mayor of that fair city. The naturalist's offspring married into several eminent clans including those of Keynes and Huxley and - in spite of their progenitor's concerns about inherited feebleness - have produced dozens of descendants eminent in science, medicine and the professions. They stand as living proof that intellectual aristocrats, unlike their botanical and blue-blooded equivalents, need not pay the price of keeping their biological heritage in the family.

CHAPTER V.

THE DOMESTIC APE.

'Let them eat cake!' said the Queen, and they did. Two centuries after the demise of Marie Antoinette, the poor are fat and the rich thin. Across the globe death from excess has, for the first time in history, overtaken that from deficiency. Eight hundred million people are hungry while a billion are overweight. The problem comes from evolution, as manipulated by man.

Darwin saw how farmers had bred from the best to produce new forms of life and used that notion to introduce the idea of natural selection. His argument is set out in the first chapter of The Origin of Species The Origin of Species. Given time, and with conscious or unconscious selection of the best by breeders, new and modified versions of creatures from pigs to pigeons will soon emerge. Were they to be found in nature rather than in sties or lofts many would be recognised by naturalists as distinct species of their own.

In The Variation of Animals and Plants under Domestication, The Variation of Animals and Plants under Domestication, published in 1868, Darwin went further in exploring the tame as the key to the wild. The book speaks of ancient times, when 'a wild and unusually good variety of a native plant might attract the attention of some wise old savage; and he would transplant it, or sow its seed'. That interesting event - the choice of favoured parents to form the next generation - was a microcosm of what had moulded life since it began. The variety of breeds seen on the farm was, he wrote, 'an experiment on a gigantic scale', both a test of his theory and a proof of its power. published in 1868, Darwin went further in exploring the tame as the key to the wild. The book speaks of ancient times, when 'a wild and unusually good variety of a native plant might attract the attention of some wise old savage; and he would transplant it, or sow its seed'. That interesting event - the choice of favoured parents to form the next generation - was a microcosm of what had moulded life since it began. The variety of breeds seen on the farm was, he wrote, 'an experiment on a gigantic scale', both a test of his theory and a proof of its power.

Savages have been replaced by scientists. Their work has produced many new varieties of plants and animals and, on the way, has revealed the eccentric history of the food on our plates. Modern biology has transformed farming. Planned breeding - directed evolution - has led to an enormous drop in the effort needed to feed ourselves. The British spend a sixth of their income on breakfast, lunch and dinner, a proportion down by half in the past five decades and by far more in the past five centuries. For most people, shortage has given way to glut and for many citizens of the developed world food is in effect free.

The blessings so brought are equivocal. The real price of sugar, starch and fat - high energy but low-quality comestibles - has plummeted. Famine disguised as feast has spread across the globe. Evolution on the farm transformed society ten millennia ago and is doing the same today. Farmers have been powerful agents of selection on wheat, maize, cows, pigs, chickens and more, but the influence of those domestic creatures on the biology of the farmers themselves has been almost as great. Diet began to act as an agent of natural selection as soon as the wild was domesticated ten thousand years ago and caused people to evolve the ability to deal with new kinds of food. Today's s.h.i.+ft in what we eat will have equally powerful effects on the genes of our descendants.

A new global power - and a new agent of natural selection - is on the move. The empire of obesity began to flex its stomach in the 1980s and shows no sign of retreat. Twenty years before that dubious decade there was, in spite of a collapse in the real price of food, little sign of the coming wave of lard. Then, thanks to technology, came the industrialisation of diet; the last step in the scientific exploitation of the Darwinian machine. Now, a tsunami of fat has struck the world and its inhabitants are paying the price.

It does not take much to alter a nation's waistline. The rise in American obesity over the past thirty years can be blamed on an increase in calories equivalent to no more than an extra bottle of fizzy drink for each person each day. At the present rate two-thirds of Americans and half of all Britons will be overweight by 2025 and Britain will be the fattest nation in Europe. Among industrial powers, only China and its neighbours are insulated from the scourge.

The twenty-first-century plague is a side-effect of the triumph of scientific agriculture. Many of those worst afflicted suffer because they bear genes that make it hard for them to deal with the new diet. Many of the obese will die young or fail to find a mate. As a result obesity will soon be - as farming itself was when it began - a potent cause of evolutionary change.

The people who laid out the first fields lived above the rivers that snaked across a green and leafy Levant. For millennia they hunted game and gathered seeds as man had done for the whole of history. Just after the peak of the last ice age the Middle Eastern weather became wetter and warmer and the gra.s.ses flourished. The gatherers prospered. Thirteen thousand years ago came a nasty shock, for the climate turned cold and dry for several centuries. The chill persuaded people to plant grains, rather than just to collect them. Soon the thermometer went up once more, the crops flourished and agriculture made its presence felt. Within a few centuries, the Fertile Crescent was filled with tillers of the soil.

A similar way of life, based on maize and rice rather than on wheat and chickpeas, soon got under way in South America and China and, in time, even in Papua New Guinea, where banana and sugar-cane cultivation emerged six and a half thousand years ago. The habit spread fast. Farming reached Britain some four thousand years ago. The s.h.i.+ft to the new economy was quite rapid, and the pursuit of wild game was more or less replaced by agriculture within just a couple of centuries, although people still ate plenty of seafood (and that remnant of the chase persists today). As new crops emerged the locals began to husband animals that could feed on them. Soon a hundred people could live on the s.p.a.ce that had previously supported but one.

The new economic system led to a grand simplification of diet. h.o.m.o sapiens h.o.m.o sapiens has eaten some eighty thousand kinds of food since he first appeared on Earth. A dig in Syria of the homes of hunters who lived just before the new economy emerged revealed a hundred and fifty varieties of edible fruit, grain and leaf in that single society. Even in the nineteenth century, Queensland aborigines feasted on two hundred and forty different kinds of plant. As the new way of life spread, the cuisine became simpler. Within a few years, the Middle East had just eight crops: emmer and einkorn (antecedents of wheat), barley, peas, lentils, bitter vetch and chickpeas. Quite soon the people of the whole world considered together ate no more than half the number of plants once used by a single hunter-gatherer band. In most places just a couple of crops - rice, maize or wheat included - became the staple food. They kept that status for ten millennia. has eaten some eighty thousand kinds of food since he first appeared on Earth. A dig in Syria of the homes of hunters who lived just before the new economy emerged revealed a hundred and fifty varieties of edible fruit, grain and leaf in that single society. Even in the nineteenth century, Queensland aborigines feasted on two hundred and forty different kinds of plant. As the new way of life spread, the cuisine became simpler. Within a few years, the Middle East had just eight crops: emmer and einkorn (antecedents of wheat), barley, peas, lentils, bitter vetch and chickpeas. Quite soon the people of the whole world considered together ate no more than half the number of plants once used by a single hunter-gatherer band. In most places just a couple of crops - rice, maize or wheat included - became the staple food. They kept that status for ten millennia.

Now, things have changed once more. Some lucky citizens have taken a great leap backwards. The middle cla.s.ses have returned to the hunter-gatherer diet. They forage in pricey supermarkets for an eclectic range of edibles, from avocado to zucchini, imported from across the globe. The revolution of the rich began soon after Columbus, when exotic delicacies such as potatoes, peanuts and tomatoes were brought from the New World. Other delicacies went the other way, albeit sometimes after a long delay; broccoli, for example, was almost unknown in the United States until the 1920s. On both sides of the Atlantic, those who can afford it have put ten thousand years of dietary history into reverse.

The advocates of avocado are still in a minority. Many of their fellow Americans or Europeans have meals almost as dull as those of the first peasants, without the privilege of growing the raw materials themselves. Just as at the dawn of agriculture, their choices are narrower than were those of their parents and grandparents. Cheeseburgers, chips and sweet drinks are full of cheap energy and the poor have seized upon them. Nowadays, the British obtain twice as many calories from fats as did their immediate forebears and on average the intake of sodium has gone up by ten times and that of calcium down by half compared with earlier times.

The junk food revolution tells the tale of artificial Darwinism in all its details. The taming of the hamburger also shows how man, the most domestic creature of all, has paid a high price as he tests the biological limits set by his own evolution.

The first farmers, like the modern poor, became less healthy as their dietary options shrank. The symptoms were different from those of today, but the causes - an abundant but inadequate cuisine - were the same. Their bones show signs of deficiency disease and the average height of adults dropped by fifteen centimetres as the new way of life spread. The loss was not regained for several thousand years. In North America, where maize became the basis of almost every meal and where it was wors.h.i.+pped as a G.o.d, another problem was a shortage of iron, for maize lacks the mineral and also interferes with the ability to absorb it from meat. Many people became anaemic. No doubt they were tired, weak and depressed as they pursued their wretched lives as tillers of the soil. Deficiency and its diseases - la.s.situde, infirmity and sadness included - have returned, but disguised as excess.

Thirty thousand premature deaths a year in the United Kingdom are due to an expanded waistline and ten times that number in the United States, where, in 2005, obesity overtook smoking as the main preventable cause of mortality. It is more than a coincidence that as America's spending on food as a proportion of national income went down by almost half, that on health care was multiplied by three times. In Central and Eastern Europe, even more healthy years of life are lost per head than in Britain. The present generation of men and women - those who grew up before the new age of edible trash - may be the longest-lived in history.

The problem for their children is fat. Medicine has long known how dangerous the blight can be; in Hippocrates' words: 'Corpulence is not only a disease itself, but the harbinger of others.' Thousands have died before their time of heart disease, stroke, cancer and diabetes, the four hors.e.m.e.n of the obese. Many others suffer from gout, arthritis, bladder problems, reduced fertility and the other conditions that affect the fat far more than the thin. The most dangerous effect of gluttony is to grow to resemble an apple rather than a pear, for extra inches on the waist are much more harmful than the same on the backside - and the apples are taking over from the pears even among women, who used to put more on the bottom than the belly as their weight went up. The apple brigade store fat around their livers, where it is readier to spring into action and to release fat itself, hormones and agents of inflammation into the bloodstream.

In the modern United States, as in the New World at the dawn of agriculture, Native Americans have paid a particularly high price for the change in diet. A century ago, many kept to their traditional cuisine. The Pima Indians of Arizona - the Corn People as they called themselves - were thin as they ate their hearty meals of tortillas or porridge, based on maize. Now, they gorge on burgers, chicken and sweet drinks instead. In some ways, however, their food input has not changed for the Pima eat just as much maize as did their grandparents. The difference is that today it has been through a cow, a chicken or a soft-drinks factory first.

Cheap corn gave birth to fast food. One American meal in five is eaten in the car and the maize needed to feed its four pa.s.sengers with a cheeseburger each would more than fill its tank. A Chicken McNugget has thirty-eight ingredients - and thirteen come from maize. The fizzy beverage that washes it down is based on corn syrup and the raw material of the post-prandial milkshake comes from a cow fed in a yard, on maize, rather than in a field, on gra.s.s. The 'natural strawberry flavour' that adds its dubious tang is natural only because it is made from corn and not synthesised from chemicals. A quarter of the food items in American supermarkets now contain maize, and their rows of cheap packaged products - and thousands are introduced each year - bear witness to the second agricultural revolution that has taken place in the lifetime of most readers of these pages.

Seed crops - maize most of all - transform suns.h.i.+ne into food. Even better, they are easy to store and to move. Cows evolved to eat gra.s.s in fields, but now it makes more economic sense to feed them on grain on giant lots. More than half the maize and soy grown in the world is eaten by animals. As a result, global meat production has gone up four times since the 1960s, and the amount of flesh available per head has doubled.

Scientific farmers have done in a few decades what took peasants centuries to achieve with no science at all, but the early farmers' approach was, in its essence, identical to that of modern technologists. They understood little of what they were up to and may not even have made the tie between s.e.x and reproduction. By the Middle Ages, the idea that attributes ran in families was accepted; as the 1566 book The Fower Cheifyst Offices Belongyng to Horsemans.h.i.+ppe The Fower Cheifyst Offices Belongyng to Horsemans.h.i.+ppe put it: 'it is naturally geven to every beast for the moste parte to engender hys lyke'. Soon artificial selection, conscious breeding from the best, was under way (even if the horse-racing and dog-fancying fraternities clung to the odd idea that qualities were pa.s.sed only down the male line). In the eighteenth century, English improvers became aware of the need to mate animals of equal 'beauty' and agricultural science was born. Robert Bakewell, chief among the breeders, was frank about his motives. He called his barrel-chested New Leicester sheep 'machines for turning herbage . . . into money' and hired out his rams for stud at 1000 a season - a huge sum for those days. put it: 'it is naturally geven to every beast for the moste parte to engender hys lyke'. Soon artificial selection, conscious breeding from the best, was under way (even if the horse-racing and dog-fancying fraternities clung to the odd idea that qualities were pa.s.sed only down the male line). In the eighteenth century, English improvers became aware of the need to mate animals of equal 'beauty' and agricultural science was born. Robert Bakewell, chief among the breeders, was frank about his motives. He called his barrel-chested New Leicester sheep 'machines for turning herbage . . . into money' and hired out his rams for stud at 1000 a season - a huge sum for those days.

Now animal breeding has become a ma.s.sive business. Champion bulls and stallions can sire thousands of offspring, and new statistical methods allow their young to be compared over hundreds of farms to see which have done best. Often, the actual genes that lie behind their talents are not known. Milk yield in cows has doubled since the 1940s, but the sections of DNA that did the job stayed hidden for sixty years. Molecular biology is beginning to change that, with the DNA sequence of most domestic animals now complete, together with maps of hidden diversity that can track down where the most productive variants might be. The annual gain in meat or milk production brought by genetics is, in the developed world, around 1.5 per cent a year, well over a billion pounds' worth in Europe alone. Artificial aids - mechanical cows into which bulls can e.j.a.c.u.l.a.t.e and have their s.e.m.e.n smeared across the globe, cloned sheep, engineered crops and more - promise wondrous things. Even so, with the consumption of meat expected to double in the next decade that will not keep up with demand. Plant technology has been even more successful and many genes for high yield or disease resistance have been tracked down, with many brought in from the wild relatives of our domestic species. Agriculture now works with foresight, a talent quite unknown to evolution but used, at least subconsciously, by the first farmers of all.

What did it take to become domestic? The basic demand is for a creature able to coexist with man. Men can choose, often without much thought, the most favoured individuals to found the next generation. For both plants and animals, improvement becomes inevitable.

Darwin knew little about the origin of fruits, grains and vegetables: 'Botanists have generally neglected cultivated varieties, as beneath their notice. In several cases the wild prototype is unknown or doubtfully known; . . . Not a few botanists believe that several of our anciently cultivated plants have become so profoundly modified that it is not possible now to recognise their aboriginal parent-forms.' Now the aboriginals have been found, hidden in their descendants' DNA.

All crops have a lot in common. From tomatoes to barley and from chickpeas to plums, the domestics are less diverse than their wild predecessors, grow taller and less branched and have fewer but larger fruits or grains that taste less bitter than before. They flower at different times of year and their seeds spring into life at once when planted rather than (as do those of many of their wild fellows) demanding a long rest.

The tale of all those changes is hidden in the DNA. The story of maize - the raw material of junk food - shows how biology can reveal the past. Maize descends from a wild plant moulded not long ago into a dietary staple so different in appearance from its ancestor that for many years its origin was unknown.

Darwin himself knew that maize was ancient, for on the Beagle Beagle voyage he found cobs embedded in a beach raised by slow upheaval many metres above the sea. Its story began in southern Mexico around eight thousand years ago when people began to harvest, and then to grow, a wild gra.s.s called teosinte, the 'grain of the G.o.ds'. The tale of maize is that of the New World. Teosinte is still abundant over large parts of South America, even if several of its dozen or so species are under threat. Male and female organs are held in different places on the same individual, with a 'ta.s.sel' that bears pollen, and a number of small spikes that carry the female parts. voyage he found cobs embedded in a beach raised by slow upheaval many metres above the sea. Its story began in southern Mexico around eight thousand years ago when people began to harvest, and then to grow, a wild gra.s.s called teosinte, the 'grain of the G.o.ds'. The tale of maize is that of the New World. Teosinte is still abundant over large parts of South America, even if several of its dozen or so species are under threat. Male and female organs are held in different places on the same individual, with a 'ta.s.sel' that bears pollen, and a number of small spikes that carry the female parts.

At first sight, the wild version looks quite unlike the familiar corn on the cob of today and was once a.s.sumed to be a relative of rice instead. A teosinte cob - a family of seeds held together on the same structure - is little more than twenty-five millimetres long, compared with thirty centimetres or more for its cultivated equivalent. When mature, the teosinte seeds, each within its own hard coat, form an 'ear' with half a dozen or so separate segments. The coat protects them from the digestive juices of the animals and birds that eat the cob. Each seed breaks off when ripe and, with luck, pa.s.ses through the gut, falls on fertile ground and germinates to form the next generation.

The maize cob, in contrast, has five hundred or more kernels. The seeds are larger than before, come in a variety of colours and contain far more starch. They do not fall off without help and lack a protective outer sheath. They are, as a result, digested, rather then excreted, should they be eaten. If, at the end of the season, the whole cob is not harvested but falls to the ground, it bears so many seeds that almost none survive the intense compet.i.tion for light and food. Maize is, as a result, entirely dependent on its human masters for reproduction. The plant has changed to such a degree that it looks quite unlike its ancestor.

Even so, the kins.h.i.+p of maize and teosinte is still close enough to allow certain wild strains to hybridise with their tamed descendants (farmers hate the idea for it degrades their crop, but scientists use it to rescue valuable genes before the natives disappear). The DNA of the modern crop is closest to that of the teosinte that grows in the hills around the Balsas river basin in south-west Mexico. There, McDonald's finds its roots. The oldest known cobs, six thousand three hundred years old, come from a cave in the valley of Oaxaca, four hundred kilometres away. At about that time, the people of South America began to thrive on their tamed gra.s.s. They soon learned to treat it with lime to release its essential vitamins - a talent forgotten until the mid-twentieth century, when the deficiency disease pellagra was tracked down to a diet of untreated maize.

At least a thousand genes in modern maize differ from those of teosinte. Fossil DNA from seeds four and a half thousand years old shows that, even by then, the farmers had already selected genes to improve grain quality and size. Just five genes, or groups of genes, were responsible for most of the s.h.i.+ft towards the domestic. Many more play a smaller part. The move from gra.s.s to food involved mutations that change the slim side-branches of the gra.s.s into stout maize ears, others that remove the hard case around each seed, while yet others ensure that the grains stick to the cob and do not shatter when touched. Long stretches of DNA on either side of those points scarcely vary at all, as a hint that large blocks of inherited material were dragged through the population by breeders as soon as the new attribute was noticed.

Maize improvement has become an industry. The plant is the most widely cultivated crop in the world, with three hundred million tons grown each year in the United States alone. It has been mutated, selected and hybridised to give hundreds of distinct strains. Some are tall - seven metres high - and some short, some large, coa.r.s.e and used as cattle fodder, with others selected to have tiny ears, the size of those of teosinte itself, and just right for a c.o.c.ktail snack. Sweet-corn is full of sugar. The starch itself, in some kinds, bursts apart when heated, to give popcorn. The plant now flourishes from the far north to the tropics and is far more productive than its ancestors of even fifty years ago. The science of maize has changed the global economy as much, or more, than has nuclear power.

The maize genome has a bizarre and unexpected structure. It contains almost as much DNA as our own and can boast of twice as many genes. Most consists of bits of mobile DNA that invaded long ago. Some of those molecular parasites can no longer copy themselves and sit sullenly in place, while others wake up now and again and move to a new site. They can cause mutations as they go or capture a functional gene, altering its effects as they do. Many of the mutations involved in the improvement of maize emerged from this constant flux. Maize DNA still changes fast. Some inbred lines descend from a shared ancestor that lived just a few decades ago, but are already as distinct from each other as are humans and chimpanzees. The mobile elements have been so active that, when two inbred lines are compared, on average a fifth of all genes differ in where they sit on the chromosome. Maize, plain food as it is, has a complicated biology.

Other crops have a less chequered history. Apples are easy. Fifty years ago, Almaty, in Kazakhstan, was - like Norwich in Tudor times - 'either a city in an orchard or an orchard in a city'. Its name means 'father of apples', but the place is now more notable for its Porsche and Mercedes dealers.h.i.+ps. The city and its surrounds were the site of a vast domestication. The genes of the chloroplast - the green structure found in leaves - show that the apples we eat today are almost all the descendants of just two ancient Kazakh trees. Those mothers of all the world's apples grew not far from Almaty. Wild trees, some as big as an oak, are still scattered through the Tien Shan Mountains nearby. They are part of what was once a vast fruit forest, the home of the snow leopard, filled with walnuts, grapes and apricots as well as apples. Today's varieties, from the insipid Golden Delicious to rare strains such as Zuccalmaglio, have emerged through mutations and selective breeding in the lines that trace their ancestry from those two progenitors. They are maintained with grafts and cuttings.

Unlike the small and bitter crabs borne by most wild apples, the fruits of the Tien Shan are large and sweet. They became luscious when the trees changed their reproductive partners. The seeds of most wild apples are moved by birds that peck at the fruit, but in the Tien Shan, the Mountains of Heaven, bears do the job instead. Both animals eat fruit and both scatter seeds in their excrement. A bird is happy with a small reward but a hefty mammal demands a more substantial bait. The trees grew sweet apples to oblige. Eight thousand years ago, people and their horses moved into the fruit forest and developed a taste for the ursine delicacy. Kazakh apple seeds travelled in horse and human guts down the silk roads that skirt the mountains. Now, their descendants fill supermarkets across the globe. The peach also traces its origin to a wild mountain landscape in western China and reached Europe only in Greek times. It has diverged, like many other fruits with stones, into a variety of forms since then.

The potato has a more restricted history. It finds its home in a small patch of land, in Peru, north of Lake t.i.ticaca. It has been cultivated for five thousand years and has diverged into a large variety of forms (to underline its importance, the United Nations Food and Agriculture Organisation defined 2008 as the International Year of the Potato). Lentils and chickpeas, too, each descend from just a single wild ancestor, as do peas. The various strains of rice, in contrast, emerged from two or three distinct species of wild gra.s.s cultivated in China. Wheat is different again, for the modern crop emerged as a result of crosses between several species of gra.s.s, some still around today, which came together to generate a plant with many more chromosomes than before.

Not long after wheat, chickpeas and the rest appeared on the plate, wild beasts were invited into the household. No more than a few accepted the offer and most of them had done so before the time of Christ. Quite soon society was transformed. Cows, pigs, horses and sheep became every farmer's treasured possessions, and a lot of effort was devoted to keeping them happy. As soon as they abandoned the wild, the animals began to change, and all in more or less the same way.

Farm animals, of whatever kind, tend - like their botanical equivalents - to follow some general rules. They are smaller and more lightly built than their unbroken counterparts, with shorter faces and smaller jaws. Often, they vary more in colour and shape than before, and many develop spotted coats. They are fatter, with longer intestines, breed through the year and make more milk. Most show less of a difference between males and females than in the wild. In many of their ancestors - wild cattle, horses and pigs - a large part of the force of natural selection involves differences in s.e.xual success. Battles among males lead to the evolution of expensive horns or tusks, with days spent locked in combat. Once s.e.x is under human control that wasteful effort can be directed to the production of milk, meat or wool instead, which is why domestic bulls or rams are less infuriated by their rivals than are their untamed relatives. In their lazy lives they tend towards promiscuity rather than the faithful bonds some of their ancestors preferred - which is useful for farmers when they wish to choose particular animals as parents.

One species in particular was quick to abandon its ancestral habits. It was the first to accept servitude and has used its own personality to manipulate mankind. It reveals, more than any other, quite what it takes to become tame.

Darwin was a dog-lover. He devotes the first chapter of The Variation of Animals and Plants under Domestication The Variation of Animals and Plants under Domestication to the history of those creatures. So great was their diversity even in his day that he was uncertain whether dogs had descended (as in fact they do) from a solitary ancestor, the wolf, or from several, with the fox and jackal as additional candidates (although he did dismiss the widespread view that each breed had descended from a separate wild ancestor, now extinct). As he points out in to the history of those creatures. So great was their diversity even in his day that he was uncertain whether dogs had descended (as in fact they do) from a solitary ancestor, the wolf, or from several, with the fox and jackal as additional candidates (although he did dismiss the widespread view that each breed had descended from a separate wild ancestor, now extinct). As he points out in Variation under Domestication Variation under Domestication, even barbarians attend to the qualities of their pets, to such a degree that the dogs of Tierra del Fuego have gained the instinctive ability to knock limpets off rocks. The breeders were often ruthless: the book tells of Lord Rivers, who, when asked why he always had first-rate greyhounds, answered, 'I breed many, and hang many.'

As Darwin noted, the dog is now the most varied of all mammals, both in mind and body. Some breeds were ancient. On a visit to the British Museum Darwin identified images of a Mastiff on a.s.syrian monuments from the sixth century BC. Others were more recent and had diverged much more from their wild ancestor. Some attributes - such as the shape of the head and the receding jaw of the Bulldog and Pug - might, he suggested, have arisen as sudden 'monstrosities' (or mutations, as we would call them), but the majority came from the slow acc.u.mulation of favoured forms. The dog was a marvellous model of how flesh can be moulded by human choice.

DNA shows that all dogs are the descendants of wolves, which will still cross with them when they get the chance (Domestication tells of 'the manner in which Fochabers, in Scotland, was stocked with a mult.i.tude of curs of a most wolfish aspect, from a single hybrid-wolf brought into that district'). The earliest bones found with those of humans are in a German dig some fifteen thousand years old, and the animals probably loitered around camp-fires long before that - which means that they entered the household well before any other creature. Even in their first days they changed, with shorter legs than their vulpine ancestors as a hint that they no longer roamed the countryside. tells of 'the manner in which Fochabers, in Scotland, was stocked with a mult.i.tude of curs of a most wolfish aspect, from a single hybrid-wolf brought into that district'). The earliest bones found with those of humans are in a German dig some fifteen thousand years old, and the animals probably loitered around camp-fires long before that - which means that they entered the household well before any other creature. Even in their first days they changed, with shorter legs than their vulpine ancestors as a hint that they no longer roamed the countryside.

Since then, the animals have been subdivided into a wide variety of forms. Four hundred breeds are recognised and a hundred and fifty have official pedigree societies. Such organisations keep a close eye on their pets' s.e.x lives and their rules often insist that both parents must belong to a rigidly defined type and that any dubious bloodline must be thrown out. Such exclusivity can lead to rapid change.

Certain breeds such as Mastiffs, Chows and Salukis have been distinct for centuries (even if the Pharaoh Hound, with its pointed ears and short coat that resemble those of the images on Egyptian tombs, is in fact a fake; a modern copy of an extinct breed). Most, however, are less than four hundred years old, and many are even younger. Their vast diversity is witness to what human choice can do to a once-wild animal.

In 1815, there were no more than about fifteen designated dog breeds in Britain. The first formal dog show was held in 1859, the year of The Origin The Origin. By then the numbers of breeds had risen to around fifty. Many of the most popular of today's hundreds of varieties - terriers, spaniels, retrievers and so on - trace their origin as distinct breeds no further than the past century, which means that they have gained an ident.i.ty in no more than fifty or so canine generations. The genes show that almost all were founded by fewer males than females, evidence that - in the ancient tradition that quality pa.s.ses only through fathers - a popular sire was mated with many b.i.t.c.hes. Some males still have over a hundred litters, a pattern at variance with the monogamous s.e.x life of the wolf. The breeders hold to their eccentric belief in the power of sperm over egg and by choosing only the very best as fathers much reduce the size of the available population.

Sometimes, a single mutation can spark off a new variety. The largest dog, the Irish Wolfhound, stands a metre high at the shoulder and tips the scales at well over fifty kilograms. Sixty Chihuahuas would fit into a single Wolfhound - but the difference in size is due to a single gene, which comes in one form in the big animal and another in the small. The Whippet is a racing dog. It too owes some of its ident.i.ty to a simple genetic change. Now and again a heavily muscled individual - a 'Bully Whippet' - appears in a litter. It bears two copies of an altered gene for a muscle protein and is much misshapen. Most such pups are killed at birth. Many other members of the breed carry just a single copy of that gene. They are faster than average and the gene was unwittingly selected for as the animals were bred for speed (and it has now revealed itself in beef cattle and even in a young German, whose mother was a champion sprinter). Perhaps the most repellent of dogs is the Mexican Hairless, or Xoloitzcuintli, first bred for food and also used as a bed-warmer by the Aztecs. As its name suggests, the creature is entirely bald. A mutation in a gene which in humans leads to loss of hair and sweat glands is responsible (and, unusually enough, ancient statues show that the error has been around for three thousand years). Darwin himself identified a family with the condition - and almost worked out the pattern of inheritance, for he noted that it was pa.s.sed through daughters but expressed only in sons, which is exactly what is expected from its position on the X chromosome. It was, many years later, the first human gene to be precisely located on that structure.

A few other dogs, Dachshunds included, also owe their ident.i.ty to simple inherited errors. For most of the named forms, in contrast, divergence involves many genes. As they build up each type gains its unique appearance. Canine diversity is arranged in a way quite distinct from our own. People, wherever they come from, are more notable for similarities than for differences, but a large part of the variability among dogs as a whole emerges from divergence among breeds. The pedigree clubs have, the double helix proves, been real barriers to the movement of DNA. That in turn has led to intense inbreeding within particular lines. The three hundred thousand Golden Retrievers in Britain trace their descent in the past thirty years to no more than seven thousand males. Other kinds have lost nine-tenths of their total variation in just the dozen or so generations since the 1970s. The dogs have paid a high price. Determined - or deranged - insistence on forcing each line towards an arbitrary standard has led to King Charles Spaniels whose brains are too big for their skulls and Pugs whose eyes pop out so far that they are scratched whenever the animal b.u.mps into something. Pugs are so inbred that the ten thousand in Britain share recent ancestry with only around fifty animals.

In spite of the genetic damage they suffer, dogs have exploited humans very effectively. They do not pay with their lives, or the products of their bodies, for food and shelter. No other creature is so tied to its master and no other domestic animal has been so subdivided. Most farm animals joined the family far later than did the dog, and some walked into the fields on several occasions and in different places.

A hamburger has a chequered history. As the Domestication Domestication book notes, the cow was tamed on two continents, in Africa and in the Middle East. Cattle were precious long before they were farmed. In the caves around Lascaux, in southern France, are images of more than a hundred aurochs, its gigantic ancestor. That impressive beast roamed wild in Europe, North Africa and parts of Asia and lasted until the 1620s. Sumerians had a cow G.o.ddess, the 'Midwife of the Land and Mother of the G.o.ds'. Later came the Semitic deity Ishtar, whose bull-G.o.d partner made enough s.e.m.e.n to fill the Tigris. Egypt, too, had a bovine obsession. The Pharaoh was called the Mighty Bull, with a tail on his kilt and bull legs on his throne. The Israelites for a time wors.h.i.+pped a golden calf, and suffered divine displeasure for their ways. The cults of the Minotaur, the toreador and the Western show how the animal retains its emotional power - and some golfers, it is said, still use a desiccated bull p.e.n.i.s as a lucky putter. book notes, the cow was tamed on two continents, in Africa and in the Middle East. Cattle were precious long before they were farmed. In the caves around Lascaux, in southern France, are images of more than a hundred aurochs, its gigantic ancestor. That impressive beast roamed wild in Europe, North Africa and parts of Asia and lasted until the 1620s. Sumerians had a cow G.o.ddess, the 'Midwife of the Land and Mother of the G.o.ds'. Later came the Semitic deity Ishtar, whose bull-G.o.d partner made enough s.e.m.e.n to fill the Tigris. Egypt, too, had a bovine obsession. The Pharaoh was called the Mighty Bull, with a tail on his kilt and bull legs on his throne. The Israelites for a time wors.h.i.+pped a golden calf, and suffered divine displeasure for their ways. The cults of the Minotaur, the toreador and the Western show how the animal retains its emotional power - and some golfers, it is said, still use a desiccated bull p.e.n.i.s as a lucky putter.

The bones of domesticated cattle appeared in the Middle East around nine thousand years ago and in Europe around 5500 BC. Cows continued to mate with wild bulls for thousands of years. Ancient DNA shows that the female, mitochondrial, lineages of today's European cattle are quite distinct from those of their aurochs ancestors, while their Y chromosomes, the indicators of male ancestry, resemble the male chromosomes of the huge and extinct bovine. The wild bulls must have continued to impose their desires upon the domestic cows, with or without man's consent.

Bacon sandwiches tell a different tale. Pigs came into the household on several occasions, in different parts of Europe and the Near East, with some later input from Asia. Fossil DNA reveals a wave of Near Eastern pigs that moved into Europe and was then replaced by a taming of European wild boar.

Horses, too, have several origins, one of which is close to the home of the apples in Kazakhstan. Traces of mare's milk (still popular in that country) on five-and-a-half-thousand-year-old pottery fragments suggest that they were tamed by then. The s.e.x bias by the horse-breeders has been extreme. Ninety-five per cent of the three hundred thousand racehorses alive today bear the same Y chromosome as evidence of descent from a single stallion. His name was the Darley Arabian, who was brought to England from Syria in 1704 by the then British Consul, Thomas Darley (two others, the Byerley Turk and the G.o.dolphin Barb, provided almost all the other chromosomes). Europe has no native sheep or goats and the domestic forms had a simple and single origin in the Middle East.

The chicken, whose rendered flesh is a staple of the junk-food diet, descends from two or more species of Asian jungle fowl - and is now, with a population of almost thirty billion, the commonest bird in the world. It spread from its home in Thailand to fill Europe and the far Pacific and from there to reach South America before Columbus. In spite of their long years on the farm the birds retain a lot of diversity, perhaps because, like cattle, they continued to cross with their wild relatives. Their reproductive lives have altered more than those of any other creature, for some lay ten times as many eggs a year as do jungle fowl, each egg twice as heavy as those of their ancestors.

All these animals changed their minds as much, or more, as their bodies as they left the wild. Zoo animals submit to man, but are not tame, let alone affectionate, as many keepers know to their cost. Only animals willing to bow to human desires have even a chance of becoming domestic. As Darwin put it, 'Complete subjugation generally depends on an animal being social in its habits, and on receiving man as the chief of the herd or family.' A hundred and fifty million Indian Water Buffalo live in harmony with their owners, but that creature's close cousin the African Water Buffalo has a strict hierarchy within its herds. The African Water Buffalo is among the most dangerous of all mammals. Its pecking order does not give entry to men and the animal has never been broken in. Elephants are semi-domestic at best and kill many people each year, and plenty of breeds of cattle are happy to attack their masters. Sheep, too, show elements of the fragile personality of their ancestors, for they panic if disturbed. An aggressive animal is no help to man or beast and farms need good behaviour. Men bred from the most submissive, which led farm animals to become less alert, less active and less angry than their ancient fellows.

Their good nature is coded for within the skull. The brains of domestic animals are, without exception, smaller than those of their ancestors - the pig by a third and the horse by a sixth. The hardest job for the first farmers was to persuade the wild to become tame, but once that behavioural Rubicon was crossed the domestics could afford to put aside their need to outwit Nature and pa.s.s the job to ourselves. Many now pa.s.s their lives in a sort of calm and extended youth in which the trials of life are dealt with by their masters. The price of tolerating human company was a much diminished intellect.

Nowhere is the importance of behaviour better seen than by the fireside. The wolf is aloof, suspicious and avoids humans as much as possible. Dogs live in a mental universe quite different from that of their ancestors. Given a choice of dog or man, a wolf cub will run to the dog but a puppy to the human. Wolves hunt in packs, while feral dogs live in chaotic and quarrelsome groups that soon split up. The canine mind has been modified in many ways. Men and women follow another person's gaze. Point at an object, and all eyes will turn towards it. Dogs share that talent and if its owner indicates where a bone is hidden with a glance or with his hand, the animal runs to the right place. Wolves are baffled by the exercise. So attuned are they to their master's moods that dogs will yawn when their owner does the same.

Men become fond of their canine companions - and they return the sentiment, for when left with a stranger the dog plays less than when with its master. The bond between man and pet can last for years, and Charles Darwin himself noted how his own favourite responded to him at once after his long absence on the Beagle Beagle. Owners often delude themselves that their pet understands everything they say. That is not true but the animals have without doubt gained an insight into the human mind. They became fully domestic as soon as they could tolerate the other members of their household.

To gain that talent, the dog brain has been modified. Regions a.s.sociated with aggression shrink and the hypothalamus, the structure that links the nervous system to the world of the hormones, is much altered. As a result, our favourite pets, unlike their wolfish ancestors, breed all year round. The hypothalamus is also a centre of activity for the nerve-transmitter serotonin, a chemical that, when in short supply in humans, is a.s.sociated with aggression, impulsive behaviour, depression and anxiety. A deficiency of the stuff is also behind the attacks of rage that affect Springer Spaniels. Other fierce breeds also have low levels. High serotonin may hence be a key to the animal's calm personality. Drugs that influence its levels in humans (Prozac included) do reduce aggression in dogs as a further hint that the chemical was a key to a place around the fire.

A remarkable experiment in Russia has begun to disentangle the chemistry of calm. In the 1950s, Dmitry Belyaev became interested in the inheritance of social skills in both animals and man (a risky pastime at the time, for genetics was under attack from Stalin). He began to work on silver foxes, a coat-colour mutant of the red fox, which had first been bred in captivity on Prince Edward Island in Canada in the nineteenth century. Their elegant fur was a valuable commodity in icy Russia, but the caged foxes were almost impossible to control as they were so aggressive and fearful. After a few years he moved his experiment to Siberia, where it continues today.

At first his stock was suspicious and agitated in the presence of a keeper, albeit far less so than the animals first captured in Canada. Each generation, Belyaev chose as parents those best able to withstand the sight of a human without an attack of frenzy. The rules were strict, for just one male in thirty, and one female in ten, was allowed to pa.s.s the test and breed. Within a few generations he saw a great change. The creatures became calmer and friendlier. They wagged their tails and learned to bark. Soon they revelled in human company, and for a time were sold as pets to raise funds. Even their appearance s.h.i.+fted, to embrace piebald coats, curly hair, floppy ears and blue eyes. Like dogs, but not like foxes, they have s.e.x all year round - and like dogs they are good at the hidden food test, while the unselected foxes fail. Thirty generations on, almost all the animals are tame.

The new and tranquil beasts were cubs that never grew up. Selection for lack of fear led over the generations to an increase in the nerve-transmitter serotonin. Among the few other genes that changed were some involved in the synthesis of the red blood protein haemoglobin, which were less active in the tamed animals. That seems odd, but those proteins also help soak up certain chemicals involved in the serotonin response to stress.

In fact, the real revolution in the life of the silver fox had taken place long before the Siberian experiment began. Their ancestors in Canada, when first captured, had found it almost impossible to cope with humans. On nineteenth-century Prince Edward Island, where the 'silver' mutation was discovered, it took years before any fox could be persuaded to reproduce in captivity, let alone to tolerate a keeper. Once that s.e.xual barrier was breached, artificial selection could begin and the rest of the emotional agenda followed. Like dogs, farmed foxes - of both the amiable and the aggressive stock - diverge from their wild ancestors in the activity of a whole series of brain genes, some of which alter transmitter levels. However, there are almost no discernible differences in the activity of a sample of brain genes between Belyaev's newly serene silver foxes and their unselected and agitated relatives in their cages. Friendliness, it appears, demands fewer mental adjustments than does the simple but formidable task of coping with human company. Most farm animals show few signs of amiability towards man, but for them, and their owners, acceptance is quite enough. To cross that barrier rescued them from the wild. The simulacrum of comrades.h.i.+p, as seen in tamed foxes and domestic dogs, came from later changes in other genes.

The first farmers modified their charges in both body and mind and their modern descendants do the same in a more rational fas.h.i.+on - but the new way of life altered the ancient farmers' minds and bodies as well. The new domestics, both plant and animal, were agents of selection upon those who had tamed them. The past ten millennia have been an era of exceptional change for humankind, for h.o.m.o sapiens h.o.m.o sapiens has evolved fast since agriculture began. has evolved fast since agriculture began.

Our own genetic equivalent of the silver fox mutation, the blonde, is a creature of the fields. Just one person in fifty, worldwide, has fair hair. Before Columbus confused matters, almost all of them lived within two thousand kilometres of Copenhagen (and their close relatives the redheads abounded in Scotland, Wales and Ireland). The three or four genes responsible have become common in recent times. Farming is to blame.

North-west Europe is (or was, before the development of modern varieties) the only place on Earth where grain can be cultivated north of a line that pa.s.ses, more or less, through Birmingham. Wheat, barley, rye and the rest need warmth to grow. In the Middle East, where those crops began, the sun s.h.i.+nes upon both the fields and those who cultivate them. Our own landscape is dreary for many months of the year, and the peasants till their fields in gloom. Even so, the Gulf Stream imports energy from the tropics and heats up the ground at the end of winter, when Europe is short of sunbeams but seeds need warmth. As a result, the crops can flourish. For the five thousand years since they arrived in northern parts, a grain-based economy depended on Neptune's help.

I once spent a decade in Edinburgh and saw the sun for a few days. My present home in London has, by comparison, the equivalent of an extra whole month of full suns.h.i.+ne each year. Scotland has the worst health in western Europe and Glasgow, its cloudiest city, has levels of chronic illness higher than any other British town. Perhaps its climate is as much to blame as its much-discussed fondness for alcohol, tobacco and deep-fried Mars Bars. Vitamin D deficiency is twice as frequent in Scotland as in England and any gene that reduces skin pigment, improves the ability to soak up suns.h.i.+ne and make the crucial substance would be favoured. The nation has, as a result, plenty of blondes, and the incidence of the gene for red hair, with the almost translucent skin that often goes with it, rises to nearly one in three.

A cereal diet (even when its ingredients are transformed into sticky sweets) is all well and good when supplemented by other foods, but is risky when life is just one grain after another, which for our peasant ancestors it often was. Cereals are low on vitamins, vitamin D, the anti-rickets substance, most of all. A move out of Africa had, many years earlier, led to the evolution of white skin to help the northern hunter-gatherers to make the vitamin in sunlight. The arrival of farmers in the English Midlands marked a new challenge, for north of that earthly paradise the winter sun is so weak that a typical Greek, Spaniard or Italian with their dark skin and hair cannot make enough of the vitamin to stay healthy. Any child born to such a dreary diet and dank climate who inherits a new mutation for fair hair and skin is at a real advantage, for the sun can penetrate further into their flesh. The infant can make more of the crucial chemical for more months of the year and is safe from rickets. The Age of the Blonde began with the first harvest. Quite soon the homeland of the flaxen-haired expanded to overlap that of the northern cereal-growers almost precisely, with its high point in Scandinavia and north Germany, where more than half the population has fair hair, and where muesli is still a central item of diet.

Natural selection by plants acted upon the peasants in other ways. A muesli-eater digests a lot of breakfast before he swallows it, for enzymes in the spit break down starch into sugars that can be absorbed. People from places with a high-starch diet, those from northern Europe included, have up to fifteen copies of a gene for the crucial enzyme compared with just four or five in peoples who eat wild fruit, meat or fish instead. The farmers found another marvellous way to get goodness out of grain (and as an added bonus to avoid winter gloom) when they invented beer. That was bad for their brains but good for their guts, for bacteria do not like alcohol and ale was safer to drink than was the polluted water of ancient villages. Since brewing began natural selection has done its job well, for almost all Anglo-Saxons can swill the stuff down. Most Asians cannot, for they lack the bibulous West's new and potent version of the enzyme that breaks down the poison.

Animals, too, changed the fate of their keepers. Most people across the world (and most adult mammals) cannot digest milk once they have left their mother's knee because they lack the enzyme needed to do so. It works in the small intestine to break an indigestible milk sugar, lactose, into two simpler sugars, each of which can then be absorbed. In many animals - and most humans - the gene responsible is switched off not long after birth. If they drink milk as adults they feel bloated or suffer from diarrhoea. Fortunately, milk products such as b.u.t.ter, yoghurt or cheese do not have such an effect.

For many northern Europeans, in contrast, milk stays nutritious throughout life. Once again, natural selection has done its job: a mutation that appeared soon after cattle were tamed allows the lactose-cutting enzyme to persist and those who have it to digest milk when they grow up. Nineteen out of twenty Swedes but no more than one in ten Sicilians have that talent. The map of its distribution fits with that of genetic diversity in the local cattle breeds (which is itself a hint as to how long the animals have been on farms). Milk tolerance is most common in the homeland of the blonde, perhaps because its calcium also helps build healthy bones. Eight-thousand-year-old remains of fat from cheese or yoghurt caked on to pottery fragments from Anatolia indicate that cows were milked there at that time, although in Greece they were used only as meat. Even so, the locals probably did not drink raw milk, for DNA in the bones of Europeans from around three thousand years later still show no signs of the variant for tolerance of the stuff. Domestication led to human genetic change, rather than the other way around.

Natural selection leaves its footprints on the double helix in many ways. Long stretches of h.o.m.ogeneous DNA on either side of the European genes for blonde hair and milk digestion show that the new and beneficial variants dragged their neighbours along as they swept through the population over the past few thousand years. Such stretches of h.o.m.ogeneity are a hint of the action of selection - even if in most cases we have no idea of what gene was involved or why. In time, the segments are broken up by the reshuffling that accompanies s.e.x. A search through the DNA of people from Africa, Asia and Europe reveals many such segments, each a relic of a sudden attack of selection - often since the origin of agriculture. The change in lifestyle and diet in the ten millennia that followed caused much more evolution than in an equivalent period during the millions of years that humans lived as hunters since the split from chimpanzees. Man, like his animals, has changed a lot since he moved to the farm.

For mankind, domesticity itself began long before he began to till the soil. The notion of h.o.m.o sapiens h.o.m.o sapiens as a house-trained ape has a long history. Darwin himself saw the parallels between the farmyard and the parlour: 'We might, therefore, expect that civilized men, who in one sense are highly domesticated, would be more prolific than wild men . . . The as a house-trained ape has a long history. Darwin himself saw the parallels between the farmyard and the parlour: 'We might, therefore, expect that civilized men, who in one sense are highly domesticated, would be more prolific than wild men . . . The increased fertility increased fertility of civilized nations would become, as with our domestic animals, an inherited character. ' He had hoped to add a whole chapter on humans to his work on the origin of farm animals, but he saw that the book was already 'horridly, disgustingly, big' and abandoned the idea. That chapter has now been written. of civilized nations would become, as with our domestic animals, an inherited character. ' He had hoped to add a whole chapter on humans to his work on the origin of farm animals, but he saw that the book was already 'horridly, disgustingly, big' and abandoned the idea. That chapter has now been written.

As in the famous Siberian foxes, the real revolution in the human line took place when an ape became human. There have been further and more minor adjustments as hunters settled into their new life in the fields. Many of the physical changes in the human line since it emerged resemble those found in domestic animals. Compared with our ancestors, we have a lighter build, thinner skull, shorter jaw and smaller teeth, and with less marked differences between males and females. We quarrel less about s.e.x and are less enthused by it than are our closest relatives and, like dogs, men and women copulate all year round, rather than in a short season as do wolves. Our b.r.e.a.s.t.s - like the cow's udders - are larger and milkier than theirs. Like pigs, we store fat more readily than do our great ape kin and are less keen on physical activity. As is the case for dogs, sheep and cattle, various odd physical mutations (blonde hair, light skin and blue eyes included) have emerged in some populations, although we have not yet gained a patchy coat. Our brains, alone, have not diminished.