The oat is the great grain of the cold and wet, and it came to the table by an unusual road: it was not sought out and tamed like wheat or barley but arrived as a stowaway, a weed of other people's fields that was promoted to a crop only when it followed farming into country too cold and too rainy for the grains it had shadowed. Botanically it belongs to the genus Avena, and unlike the wheats and barleys, whose seeds sit tight in a spike, the oat carries its grain in an open, drooping panicle, each grain wrapped in a tough husk that must be removed before eating. Its wild ancestors, Avena sterilis and Avena fatua, grew across the Fertile Crescent and the Mediterranean as vigorous weeds amongst the earliest sown wheat and barley, and for several thousand years that is all they were: a nuisance pulled from the crop, not a crop themselves.

The promotion happened in the north. As Neolithic and Bronze Age farming spread out of the Near East into the cool, damp temperate zone of Central and Northern Europe, wheat and barley began to struggle, whilst the weedy oats amongst them, better suited to short summers, acid soils, and heavy rain, flourished and set more seed. Farmers who had once weeded oats out began, in effect, to let them take over, and by the Bronze Age the common oat, Avena sativa, had become a crop in its own right across temperate Europe, its earliest cultivated remains found at Bronze Age sites in Switzerland, Germany, and Central Europe. This is why the oat has no single ancient cradle in the manner of wheat: it is a secondary domesticate, born of the movement of agriculture into the European north rather than of any deliberate taming in the south. The related red or Mediterranean oat, Avena byzantina, followed a parallel path in the warmer south and stands behind many modern cultivars.

A quite separate oat was taken up at the other end of Eurasia. In the cold, dry highlands of northern China and Inner Mongolia, farmers cultivated the naked or hulless oat, Avena nuda, whose grain threshes free of its husk like wheat and needs no hulling; called youmai, it became a hardy staple of Shanxi, Hebei, and the Mongolian steppe, ground into a flour with a character all its own and worked into a family of steamed noodle and dumpling dishes found nowhere else. Because the naked oat is a distinct cultivated species with its own homeland and its own cuisine, it is shown here as a separate origin from the common oat of Europe.

For most of its history the oat did not so much travel as settle, sinking deep roots wherever the climate was too cold, too wet, or too poor for better-regarded grains, and rising there to the status of a staple whilst remaining, in the warm and fertile south, mere fodder. From its Central European hearth the common oat spread across the whole of the cool Atlantic and northern fringe of Europe, and it was in these lands, where wheat ripened badly if at all, that it became the bread grain of the poor and then the pride of whole nations. It reached the Atlantic islands and the far north early, and by the early Middle Ages oats were the dominant grain of Scotland, of Ireland, of the uplands of northern England and Wales, and of much of Scandinavia, eaten as porridge, as flat oatcakes baked hard on a girdle, and as the thickening of every pot. Dr Johnson could sneer in his Dictionary that the oat was a grain given in England to horses but in Scotland to men; the Scots reply, that England had fine horses and Scotland fine men, records exactly the divide between the oat lands and the wheat lands of Britain.

Because the oat travels as people travel, its great voyages followed the emigrations of the northern peoples. The Scots, Ulster-Scots, and English carried it across the Atlantic to North America from the early seventeenth century, where it flourished in the cool northern states and the Canadian prairies and became, in the industrial age, the breakfast grain of a continent: rolled flat and boxed by the Quaker Oats Company from the eighteen-seventies, baked into the oatmeal cookie, toasted into granola, and folded into the fruit crisps of the American autumn. British settlers took it south to the cool highlands of the Antipodes, where it became the oat of the ANZAC biscuit and the muesli bowl. And in the twentieth century the oat was reinvented once more, in Switzerland, where the physician Maximilian Bircher-Benner devised, around 1900, the raw soaked-oat and grated-apple breakfast he called muesli, and in Scandinavia, where oats are grown on a vast scale and where Swedish food science gave the world, at the century's end, the oat drink that has carried the grain into the coffee of half the world.

The naked oat of China kept its own separate country. It never became a world traveller in the manner of the European oat, but within its highland homeland of Shanxi and Inner Mongolia it remained a beloved staple, and the youmian dishes built upon it, the hand-rolled tubes and steamed honeycombs of oat dough, are eaten there today with a devotion the West reserves for its own daily bread.

The oat occupies an unusual place in the modern world: a grain that spent most of history as the food of the poor and the horse, and that has become, in the last century, one of the most fashionable foods on earth. It is grown chiefly in the cool temperate zone, with Russia, Canada, Poland, Finland, Australia, and the northern United States amongst the largest producers, and the great bulk of the crop is eaten not as bread but as the grain itself: hulled and steamed and rolled into the flakes that make porridge and muesli, cut into the coarse pinhead and steel-cut grades prized for their texture, or milled into oatmeal and oat flour. The reasons for the oat's modern rise are partly medical, for the soluble fibre beta-glucan, abundant in oats, lowers blood cholesterol, a finding that turned the humble porridge grain into a health food and put the oat on the front of the cereal packet; and partly cultural, for the oat drink developed by Swedish food scientists in the nineteen-nineties has become the default plant milk of the world's coffee bars, carrying the grain into places it had never reached.

In the kitchen the oat is defined by its grades, which are not interchangeable. Whole hulled grains, or groats, are chewy and slow to cook; steel-cut or pinhead oatmeal, the groats chopped into pieces, makes the nubbly, substantial porridge of Scotland and Ireland; rolled oats, the groats steamed soft and pressed flat, cook quickly into a creamy porridge and bind the flapjack, the crisp, and the biscuit; and fine oatmeal and oat flour go into oatcakes, breads, and the coating of a herring. Oats lack the gluten to raise a loaf, so the grain belongs less to the baker's oven than to the porridge pot, the girdle, and the biscuit tin.

The oat sits at the heart of an unusually rich body of national tradition for so long-despised a grain. It is the porridge and the oatcake of Scotland, the brose and the skirlie and the cranachan; the white pudding and the stirabout of Ireland; the parkin, the flapjack, and the Staffordshire oatcake of northern England; the havregrøt of the Nordic breakfast; the Bircher muesli of Switzerland; the oatmeal cookie, the granola, and the fruit crisp of North America; the ANZAC biscuit of Australia and New Zealand; and, in its naked-oated eastern form, the youmian of the Chinese highlands. From a weed pulled out of the wheat, the oat has become both the most homely and the most modish of grains.