The almond is, despite its long career as a nut, the seed of a stone fruit. Prunus dulcis belongs to the rose family and to the same genus as the peach, the plum, the apricot, and the cherry; what the tree bears is a leathery green drupe, and the almond of commerce is the kernel drawn from the woody stone at its heart, the counterpart of the peach pit, eaten before the surrounding flesh has had the chance to swell and sweeten. It is one species, not a clutch of separate domestications, and its central drama lies not in different kinds of tree but in a single, decisive division within the one species: the difference between the sweet almond and the bitter.

Wild almonds are bitter, and bitterness here is not a matter of taste alone but of poison. The wild kernel is charged with the compound amygdalin, which on crushing and contact with the saliva releases hydrogen cyanide; a few dozen wild almonds hold a lethal dose, and the bitterness is the plant's chemical defence of its seed. That such a tree was ever domesticated at all is one of the quiet marvels of the Neolithic, and the answer lies in a single non-bitter mutation. Now and again a wild almond carries a dominant gene that switches off the amygdalin, yielding a sweet, harmless kernel; the first cultivators, finding such trees, propagated them by seed and later by grafting, and from that selected sweet form descends every eating almond grown today. The bitter almond did not vanish, however. In small, carefully rationed quantities its intense, heady aroma (the very scent the word 'almond' calls to mind) is prized as a flavouring, in the Italian amaretto and the French ratafia, in the bitter heart of true marzipan, and in the almond extract of the baker, where the toxin is driven off or diluted to safety.

The sweet almond is itself grown in a great range of named varieties, selected over centuries for the qualities that each tradition prizes. Spain raised the round, flat, sweet Marcona and the long Largueta; Italy the Avola almonds of south-eastern Sicily, the Pizzuta and the Fascionello, prized above all others by the confectioner; California, latterly the giant of the trade, the Nonpareil, soft-shelled and uniform, bred for the orchard and the machine. The almond tree is the earliest of all the orchard trees to flower, breaking into white and pale-pink blossom while the winter is barely over, and this precocious, vulnerable flowering, beautiful and easily killed by a late frost, has shaped both its cultivation and its long symbolic life.

From its Near Eastern cradle the almond travelled in two great ages, the first ancient and the second medieval, and the second mattered more for the kitchen than the first. In antiquity the cultivated sweet almond spread westward out of the Levant: into Greece, where Theophrastus described its cultivation and the Greeks pounded it into honeyed sweets; and on to Rome, which knew it as the nux graeca, the 'Greek nut', planted it across Italy, and carried it to the edges of the empire. Almonds lay among the offerings in the tomb of Tutankhamun; they are named in the Hebrew Bible as a gift fit for a ruler. Yet the Roman almond remained, for the most part, a nut to be cracked and eaten, a thickener and a garnish; the transformation of the almond into the foundation of an entire confectionery was the work of a later world.

That work was Arab. The cooks and physicians of the Abbasid Caliphate, drawing on the Persian and Levantine traditions they had inherited, made the almond central to their kitchen in two enduring forms. The first was almond milk, the pale liquor pressed from ground blanched almonds and water, which kept where dairy spoiled and which carried no meat, so that it could be used on fast days and in the heat of the East alike. The second was the marriage of pounded almond and sugar, the lauzinaj of the Baghdad cookbooks, a paste of almond and sugar wrapped in the thinnest pastry that is the direct ancestor of both baklava and marzipan. From the Arab heartland these techniques spread along the whole arc of Islam: west across North Africa to Morocco, whose almond-and-sugar pastries are amongst the finest in the world; into Sicily, conquered by the Arabs in the ninth century, where the almond became latte di mandorla and the moulded marzipan fruits of the Martorana; and into Al-Andalus, Moorish Spain, whence the ground almond entered the cold soup ajo blanco, the Galician tart of Saint James, and the great Christmas confection turrón.

It was from Moorish Spain and Arab Sicily that medieval Christian Europe received the almond in its transformed state. Almond milk became the indispensable ingredient of the medieval European kitchen, used on the Church's many fast days in place of the forbidden milk and butter; the blancmange, a dish of almond milk thickened with rice and shredded capon, stood at the centre of the aristocratic table from England to Italy. Marzipan, carried north by the Hanseatic almond trade, took root in the German lands, above all at Lübeck, and in the marzipan ring cakes of Scandinavia. In France the almond matured into the bedrock of the pâtisserie, the frangipane of the galette des rois, the calisson of Aix, the macaron and the financier. Eastward, the Persianate Mughals carried the badam, the Persian almond, into India, where it enriched the milk sweets and the celebration rice of the imperial kitchen. And at the last the almond crossed the Atlantic: Spanish Franciscan missionaries planted it in Alta California in the eighteenth century, and from those mission trees grew the orchards of the Central Valley that now bear the overwhelming majority of all the almonds in the world.

The almond is today both an industrial commodity of staggering scale and a luxury of the confectioner, and the gap between its two lives is wide. California alone grows roughly four-fifths of the world's almonds, a million and more acres of orchard in the Central Valley pollinated each spring by the largest managed migration of honey-bees on earth; Spain, Australia, and the lands of the Mediterranean and the Near East make up most of the rest. The bulk of this vast crop is sold shelled and blanched, roasted and salted as a snack, slivered and flaked for the baker, milled into almond flour, ground into almond butter, and, in the most striking recent development, pressed and strained into almond milk, the plant-based drink that has carried the ancient Arab technique into the refrigerator of every supermarket in the Western world.

In the kitchen the almond keeps a double character that no other nut quite shares. It is the great nut of the cold table and the sweet: ground into marzipan and the almond pastes of a dozen national confectioneries, baked into the Spanish tarta de Santiago, the Sicilian and Lübeck marzipan fruits, the Norwegian kransekake, the French galette des rois, the Moroccan kaab el ghazal, and the Indian badam halwa; pounded into the cold soups of Andalusia; steeped into the latte di mandorla and the orgeat of the Mediterranean summer. Yet it is equally at home in the savoury kitchen, fried golden and scattered over the festive rice and tagines of Persia, Morocco, and Mughal India, pounded into the picada that thickens a Catalan stew, and crusted over a fillet of trout in the French amandine. The bitter almond, rationed to a few drops or a single kernel, lends its haunting note to the amaretto of Saronno, the ratafias and noyaux of France, and the marzipan that would taste of nothing without it. From a poisonous wild seed tamed by a single fortunate mutation, the almond has become one of the most versatile and most universally grown of all the foods carried out of the ancient Near East.