Darwin's Island (22 page)

 

The first farmers modified their charges in both body and mind and their modern descendants do the same in a more rational fashion - 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
Homo sapiens
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 passes, 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 shines 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 sunshine 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 sunshine 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 butter, 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 homogeneous 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 homogeneity 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 sex. 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
Homo 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
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.

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 sex 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 breasts - 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.

Such changes in physical structure, together with those towards baldness, an upright gait and an agile mind which marked the transition from ape to man, involved a large cost, the death or sexual failure of millions who could not cope with the new way of life. The more recent changes in skin colour, the ability to drink milk or beer, or to digest grains demanded just the same sacrifice. The speed at which advantageous genes have spread since the origin of farming suggests that the price was high indeed. The same process is at work today. We face a new abundance, quite different from anything in the evolutionary past, and have not yet evolved to deal with it. We may do so, but the process will not be cheap. For most of history, humans have had to cope with shortage rather than excess and have evolved mechanisms to guard against excessive weight loss when food is short. Our bodies deal less well with today’s glut. Starvation disguised as surfeit means that evolution’s inexorable machine has cranked up again, with natural selection by diet as active as it was ten thousand years ago.

Our individual response to excess depends strongly on our DNA. A study of twin boys and girls suggests that around seven-tenths of the variation in body weight within a population is due to genetic variation. Identical twin children also resemble each other in how much they will eat if offered a huge meal. Adult twins paid to gorge themselves, or to starve, for several weeks also tend to gain, or lose, weight - to resist, or to surrender to, the challenges of the new diet to the same extent.

Dozens, perhaps hundreds, of genes have been blamed for the new wave of obesity. Fatness runs in families but so do frying pans - and fat cat-owners tend to have fat cats. Their pets share their diet but not their DNA. Nature and nurture work together and the inheritance of pot bellies is - like that of almost everything else - not simple. The notion that fat people can blame the way they are born and ignore what they choose to eat is wrong. Instead, like alcoholics they are more at risk of a certain kind of diet than are others and must struggle harder against temptation. Many people fat today would have been thin a century ago, whatever the nature of their DNA.

Even so, genes have a real influence on waistline. Most of those that predispose to obesity have a small effect, but a certain variant, when inherited in double dose (as it is by ten million Britons), increases body mass by three kilograms above average. Even a single copy, as borne by almost half the inhabitants of these islands, adds a kilogram. The DNA variant concerned changes appetite and is active in parts of the brain involved in hunger or satiety. The gene is harder at work in starved individuals than those who have just eaten. It makes no difference to their weight at birth but babies with two copies begin to pile on the kilograms within just two weeks. Other genes that dispose to obesity alter the efficiency with which food is soaked up or the rate at which the body burns its fuel.

The environment itself has effects that stretch over the generations. Just as alcoholic mothers have damaged babies, women who eat fast food have fat children not only because they pass on their genes, but because they were themselves overweight while pregnant. About a third of all pregnant Americans - and half of pregnant African-Americans - are obese. Their internal economy shifts to deal with the problem, as does that of their unborn child. The evidence is clear, for those who lose weight, perhaps through stomach surgery, between one child and the next, tend to have lighter babies than before. The foetuses of overweight mothers respond to the high levels of insulin (the hormone that controls blood sugar) in their mother’s bloodstream and are born attuned to lay down fat. Genes that dispose to diseases of the obese are then put under further pressure.

The biggest threat to the overweight is diabetes; not the rare variety that affects a few infants and can be treated with insulin but a related illness that comes on later, defies treatment and has a strong tie with an expanded waist. The problem arises from a resistance to insulin. Its symptoms include heart disease, kidney failure, blindness, nerve damage and even gangrene. It was once a disease of the elderly, but is seen more and more in children and adolescents. Just one extra notch on the belt adds a lot to the danger and those in the top tenth of trouser size have twelve times the risk of diabetes than do the slim.

Half a billion people will soon suffer from adult-onset diabetes. Unless matters improve, a baby born in the USA today has a one in three chance of the condition when it grows up and the illness already takes up a sixth of the country’s entire health budget. Even in Britain, two million people show signs of it. In some places the figures are dreadful, with half the adult population of the island of Nauru, in the Pacific, afflicted. Genes that increase body weight are most to blame, although others do increase the risk in both the fat and the thin.

Obesity is in part inherited, and is a target of natural selection because it kills many people before their time. Its effects on the evolutionary future are made worse because those who suffer from it face not just premature death but sexual failure. Fat people tend to have fewer children than average. Apart from the romantic problems involved, obese men find it harder to sustain an erection, and obese couples copulate less often, than do the fashionably slim. Even worse, a fat man’s sperm count drops by around a quarter, perhaps because his over-insulated testicles are too warm. Female fertility, too, drops with every extra kilo. Excess fat interferes with the menstrual cycle and has other harmful effects. Among women anxious to become pregnant even a slight weight excess increases the time before a favourable outcome by a month, and it takes nine months longer for an obese woman to have a good chance of becoming pregnant than for a person of normal size. In addition, overweight women are more liable to miscarry and their children are at higher risk of birth defects.