Darwin's Island (20 page)

 

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
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 grass called teosinte, the ‘grain of the gods’. 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 ‘tassel’ 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 assumed 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, passes 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 competition 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 kinship 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 grass. 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 shift towards the domestic. Many more play a smaller part. The move from grass to food involved mutations that change the slim side-branches of the grass 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, coarse and used as cattle fodder, with others selected to have tiny ears, the size of those of teosinte itself, and just right for a cocktail 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 dealerships. 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 Titicaca. 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 grass cultivated in China. Wheat is different again, for the modern crop emerged as a result of crosses between several species of grass, 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 sexual success. Battles among males lead to the evolution of expensive horns or tusks, with days spent locked in combat. Once sex 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
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
, 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 Assyrian 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 accumulation 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 multitude 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’ sex 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.