Kale is the oldest of the cabbages and the closest of all the cultivated Brassica oleracea to its wild ancestor. The wild cabbage is a sprawling, bitter, blue-grey perennial of the sea cliffs and limestone headlands of the Mediterranean and the Atlantic coasts of Europe, and from this single unpromising plant human selection drew out the most astonishing range of vegetables of any species on earth: the cabbage by swelling the terminal bud into a head, broccoli and cauliflower by arresting the flower, kohlrabi by swelling the stem, the Brussels sprout by multiplying the side buds. Kale is what the wild cabbage becomes when none of these things is done to it. It is, quite simply, a leaf cabbage that never learned to form a head, and the Latin name of its principal group, var. acephala, says exactly that: 'without a head'.

Because it is the least altered, kale is almost certainly the most ancient cultivated form, the leafy cole that the Greeks called krambe and the Romans brassica, grown and eaten for centuries before the heading cabbages of the medieval north existed. From that loose-leafed ancestor the cultivators of Europe drew their kales: the frilled and ruffled curly kale (var. sabellica) of the cold north; the long, dark, blistered leaves of the Tuscan cavolo nero (var. palmifolia); and the broad, flat, smooth-leafed collard (var. viridis). At the eastern edge of the plant's range the Chinese selected their own leafy and flowering cole, the kai-lan (var. alboglabra), the Chinese kale.

Kale is also one of the rare vegetable names that gathers three different species under a single word. Besides the many kales of Brassica oleracea, the cold-hardy Russian and Siberian kales belong to Brassica napus, the swede and oilseed-rape lineage, and the gomen of the Ethiopian table is Brassica carinata, the Abyssinian mustard, domesticated in the Horn of Africa entirely apart from the European cole. Three species, one humble idea: the green leaf of a cabbage that was never made to close.

Kale travelled as the hardy green of the poor, the plant that fed northern Europe through the winters when little else would grow, and only in our own age became the darling of the health-food shelf.

From the leafy cole of the Greek and Roman Mediterranean, kale spread north with the legions and the monasteries into a belt of cold-winter countries that would make it a true staple. It became the kale of Scotland, where the word itself is Scots and the kitchen garden was the kaleyard; the colcannon of Ireland; the boerenkool of the Low Countries; the Grünkohl of north Germany, eaten after the first frost had sweetened it; and the grønlangkål of the Danish Christmas. In the wet green northwest of Iberia it became the couve of the Portuguese caldo verde and the berza of the Galician caldo gallego. Carried east along the Silk Road, Brassica oleracea reached Tang China and was selected there into the kai-lan. And carried by Europeans across the Atlantic, the broad-leafed collard took root in the American South, where the enslaved cooks of West African descent made it, simmered long with smoked pork and its pot likker, one of the defining dishes of the region; the same couve went south with the Portuguese to Brazil, finely shredded into the couve à mineira that sits beside every feijoada.

Two further streams complete the picture. With British colonisation, Brassica oleracea kale and collards reached the highlands of East Africa and became the dominant sukuma wiki, the everyday green of Kenya and Tanzania, where they met the older Ethiopian kale, Brassica carinata, which had itself long since spread south down the highlands from the Horn; for the Ethiopian highlands had domesticated that native kale, the gomen of the Ethiopian table, entirely apart from the European cole. And in the late twentieth century the Tuscan cavolo nero, the curly kale, and the Russian kale (Brassica napus) all converged in California, where a Mediterranean peasant green was reinvented as the emblem of the modern superfood, massaged raw into salads, baked into crisps, and blended into the smoothie.

Kale occupies two quite different places in the world's kitchens at once. Across much of Africa it is a daily staple of the first importance: sukuma wiki, the braised collard-kale of onion and tomato, is eaten with the maize porridge ugali by tens of millions in Kenya and Tanzania, and the spiced gomen of Brassica carinata is fundamental to the Ethiopian and Eritrean table. Across the cold north of Europe it remains the traditional winter green of national comfort dishes: the Dutch boerenkool stamppot, the German Grünkohl mit Pinkel, the Danish grønlangkål, the Irish colcannon, the Portuguese caldo verde, and the Galician caldo gallego. In southern China the kai-lan, dressed with oyster sauce, is one of the most-ordered greens of the Cantonese table and the dim sum trolley. And in Brazil the finely shredded couve à mineira is the inseparable companion of feijoada.

Then, from around 2010, kale underwent one of the most sudden reinventions in the history of any vegetable. Long dismissed in the English-speaking world as cattle fodder or a garnish for buffet tables, it was taken up by the Californian and wider Western health-food movement as the archetypal 'superfood', prized for its density of vitamins and antioxidants. The Tuscan cavolo nero, rebranded as 'dinosaur' or 'lacinato' kale, and the tender Red Russian kale became fashionable restaurant greens; the massaged kale salad, the kale chip, and the kale smoothie became ubiquitous. A peasant survival crop of the European winter and the African dry season had become, within a few years, a global symbol of wellness, and one of the few vegetables ever to be celebrated with its own annual day.