The hazelnut is one of the oldest wild foods of the temperate world, and one of the very first that human beings gathered, roasted, and stored in bulk. It is the fruit of the hazel, a shrub or small tree of the birch family, and unlike the almond or the walnut it is a true nut in the botanical sense: a single hard-shelled seed that does not split to release its kernel, cradled in a leafy green husk (the involucre) that gives the filbert and the cobnut their distinctive ragged collar. The genus Corylus is spread right across the Northern Hemisphere, and it was taken up not once but in several places at once, so that the hazelnut, like the raspberry and the blackberry, has no single homeland but a scattering of them.

The dominant nut of the kitchen and of world commerce is the common hazel, Corylus avellana, native across the whole of Europe and into Anatolia and the Caucasus; it is joined in cultivation by the filbert, Corylus maxima, whose longer nut is wholly enclosed by a husk longer than itself, and by the great Turkish tree hazel, Corylus colurna, valued as a timber and as the hardy rootstock upon which orchard hazels are grown. In North America the woodland nations gathered their own species, the American hazel Corylus americana and the beaked hazel Corylus cornuta, and in the forests of Northeast China and Siberia grew a fourth lineage, Corylus heterophylla, the Asian hazel, one of the 'four famous nuts' of the Chinese table. These separate wild stocks are the reason the hazelnut belongs to Europe, Anatolia, East Asia, and North America all at once.

What sets the hazelnut apart from almost every other orchard fruit is the sheer antiquity and scale of its wild gathering. Long before any tree was planted in a row, the peoples of Mesolithic Europe were harvesting hazelnuts by the tonne: at Staosnaig on the Scottish island of Colonsay, archaeologists found a great roasting pit filled with the charred shells of hundreds of thousands of hazelnuts, processed in a single intensive season around 7000 BCE, and at Star Carr in Yorkshire and across the whole of post-glacial northern Europe the hazelnut was a staple of the forager's autumn, roasted to preserve it and stored against the winter. True cultivation came late and gradually: the classical world prized the fine, large nuts of Pontus and of Avella above the wild fruit of the woods, and it is from these cultivated stocks, improved over two thousand years, that the orchards of Turkey, Italy, and the wider world descend.

For most of its history the hazelnut did not travel, because the hazel grew wild across the whole temperate breadth of the Northern Hemisphere and every region gathered the nut at its own door; its story is less one of migration than of the slow refinement of a fruit that was already everywhere. The decisive early distinction was one of quality, not of place. The classical Mediterranean drew the finest nuts from two cultivated sources, and both left their names on the fruit. From the southern shore of the Black Sea, the ancient land of Pontus, came the large, sweet nut the Greeks called the 'Pontic nut' and the 'Herakleotic nut', after the city of Heraclea Pontica; Theophrastus described it in the fourth century BCE, and it passed westward into Greece and then to Rome. In Campania, at the town of Avella, the Romans grew the nut they called nux avellana, the Avellan nut, and it is this name, not the Greek one, that the botanists made permanent in Corylus avellana and that survives in the Italian avellana and the Spanish avellana to this day.

Through the Middle Ages and the early modern centuries the hazelnut spread as a cultivated tree across the kitchen gardens and orchards of Europe, each country making its own of it. In Kent the English raised the cobnut and the filbert as an orchard crop, eaten fresh and green in the autumn; in Catalonia the nut became the backbone of the pounded sauces of the table; in the German-speaking lands and in Austria it filled the great nut tortes of the Kaffeehaus; and in France it was ground with sugar into the praline paste that became one of the foundations of the pâtisserie. The most consequential of all these transformations came in Piedmont, where in the early nineteenth century, with cocoa made scarce and dear by the Napoleonic blockade of the Continent, the confectioners of Turin stretched their chocolate with the abundant local hazelnut of the Langhe hills and created gianduja, the hazelnut-and-chocolate paste that would, a century and a half later, conquer the world as Nutella.

The hazelnut's true crossing of the oceans is recent and largely a matter of the cultivated European nut following European people. In the eighteen-seventies the French nurseryman Felix Gillet carried European filbert varieties to the Pacific coast of North America, and in the cool, damp Willamette Valley of Oregon they found a second home so perfect that Oregon now grows very nearly the whole of the United States crop. Yet all of this is dwarfed by the modern dominance of the nut's oldest cradle: the Black Sea coast of Turkey, the Pontus of the ancients, today produces the overwhelming majority of the world's hazelnuts, the terraced groves of Giresun, Ordu, and Trabzon supplying the great chocolate and confectionery industries of Europe with the raw material of their pralines and their gianduja. From a wild nut of the forager's autumn, gathered on four continents, the hazelnut has become one of the most heavily traded and industrially important of all the tree nuts.

The hazelnut today is, above all other things, the nut of the confectioner and the chocolatier. Although it is eaten whole and roasted, and pressed for a fragrant oil, the overwhelming bulk of the world crop is destined for the chocolate and confectionery trade, which absorbs the great majority of the harvest: a single manufacturer of chocolate-hazelnut spread is reckoned to consume a quarter of all the hazelnuts grown on earth. The nut's affinity for chocolate, discovered in the gianduja of Napoleonic Turin, has made it the most industrially valuable of the tree nuts after the almond, and the price of a jar of spread in a European supermarket is tied directly to the fortunes of the harvest on the Black Sea coast.

Production is extraordinarily concentrated. Turkey alone grows the majority of the world's hazelnuts, its crop drawn from the steep, terraced groves that run along the whole Black Sea littoral from Ordu and Giresun to Trabzon, the ancient Pontus; Italy is the second grower, its finest culinary nut the Tonda Gentile of the Piedmontese Langhe and the Tonda di Giffoni of Campania; and the United States, meaning very nearly Oregon alone, is third. Beyond these, Azerbaijan, Georgia, and, increasingly, Chile have become significant suppliers to the same insatiable confectionery market. The cultivated nut is overwhelmingly the European hazel, Corylus avellana, in a range of named varieties; the American species survive chiefly in the breeding of blight-resistant hybrids for the North American climate, and the Asian hazel, Corylus heterophylla, is grown and gathered as a roasted snack across Northeast China.

Around this industrial nut there survives an unusually rich body of regional tradition. The hazelnut is the gianduja, the Nutella, the torta di nocciole, and the baci di dama of Piedmont; the Haselnusstorte and the Nusskipferl of the Austrian coffee house; the praliné, the Paris-Brest, and the dacquoise of the French pastry kitchen; the romesco of Catalonia, in which the pounded nut thickens and enriches a sauce of dried peppers and tomato; the fındık baklavası and the fındık helvası of the Turkish Black Sea coast; the fresh cobnut of the Kentish autumn; and the roasted hazelnut torte and the hazelnut-crusted salmon of Oregon. From the Mesolithic roasting pits of Colonsay to the chocolate factories of Alba, the hazelnut has been, for nine thousand years, one of the most useful and best-loved nuts of the temperate world.