'Millet' is not a plant but a category: a loose, convenient name for a whole scattering of small-seeded cereal grasses, drawn from several different genera and domesticated quite independently on three continents. What they share is a habit rather than a lineage. They are the grains of the dryland and the difficult season: small, hardy, quick-growing grasses that ripen in a matter of weeks, ask little of the soil, endure heat and drought that would kill a rice plant or a wheat field, and so became, across the hot and arid belts of the Old World, the staff of life where nothing more demanding would grow.

The major millets belong to distinct species and homelands. Foxtail millet (Setaria italica) and broomcorn or proso millet (Panicum miliaceum) were the founding grains of Chinese civilisation, domesticated in the Yellow River valley some eight thousand years ago. Pearl millet (Pennisetum glaucum), the most widely grown of all, was tamed in the West African Sahel around forty-five hundred years ago, and beside it grows fonio (Digitaria exilis), the tiny 'hungry rice' that is among the oldest of African cereals. Finger millet (Eleusine coracana), the ragi of the Indian kitchen, was domesticated in the highlands of East Africa. To these the Indian subcontinent, the world's great secondary home of the millets and today their largest producer, adds a clutch of minor millets, the kodo, the little, the barnyard, and the browntop, grown together as the 'nutri-cereals' or Siri Dhanya.

Two near neighbours are best set apart. Sorghum, the 'great millet', is large-grained and catalogued in its own right; and teff, the grain of the Ethiopian injera, is a small cereal of the genus Eragrostis often grouped with the millets but botanically distinct from them. The true millets are gluten-free, rich in minerals, and, after a long age of being dismissed as poverty grains and famine food, have been rehabilitated in the present century as nourishing, climate-resilient crops, a revival crowned by the United Nations' International Year of Millets in 2023.

The millets travelled three separate roads, one from each of their homelands. From the Yellow River, foxtail and broomcorn millet spread outward as the original grain of the East Asian world: east to Korea and to Japan, where they were eaten long before rice came to dominate, and, most remarkably, far to the west, for broomcorn millet proved so hardy and so quick that it ran the whole length of the Eurasian steppe in prehistory, reaching the Caucasus and Europe and becoming, in time, the millet porridge, the pshennaya kasha, that was a staple of Russia and Eastern Europe for centuries before the potato.

From the West African Sahel, pearl millet spread across the whole dryland belt of Africa as the bread grain of the savannah, and, carried anciently across the Arabian Sea by the monsoon trade, reached India well before 1500 BCE, where as bajra it became the staple bread grain of the hot, dry north-west, of Rajasthan and Gujarat. From the East African highlands, finger millet made a parallel crossing to India, where as ragi it became a staple of the southern Deccan, of Karnataka above all, ground for the daily ragi ball and the weaning porridge. India thus became the second great heartland of the millets, gathering grains from two African homelands and several of its own into the most diverse millet cuisine on earth.

For much of the twentieth century the millets were in retreat, displaced almost everywhere by the high-yielding rice, wheat, and maize of the green revolution and dismissed as the coarse food of the poor. In the twenty-first they have returned, prized anew as drought-proof crops for a warming world and as gluten-free 'ancient grains' for the health-conscious kitchens of the West, their rehabilitation marked by the United Nations' International Year of Millets in 2023, championed above all by India.

The millets remain, as they have always been, the staple grains of the world's drylands. Pearl millet is the daily bread of the West African Sahel, pounded and fermented into the millet balls and soured-milk drinks of the Hausa and Fulani and steamed into the millet couscous of Senegal, and it is the bajra of north-western India, ground into the thick, nutty flatbread (rotla or roti) that is the winter bread of Rajasthan and Gujarat. Finger millet is the ragi of southern India, steamed into the stiff ragi ball (ragi mudde or ragi sangati) that is the staple of rural Karnataka and stirred into the malted porridge given to children and the elderly, and it is the wimbi of East Africa, the finger-millet flour of the Kenyan and Ugandan ugali and the fermented porridge uji. Foxtail millet is still simmered into the comforting millet congee of northern China, the everyday food of the convalescent and the new mother, and broomcorn millet into the buttery millet kasha of Russia.

Millet runs, too, through the festive and the everyday across this dryland world: the kibi dango of Japan, the fermented uji of East Africa, the fonio of the West African feast, and the bajra khichdi of the Indian winter. After a long eclipse, the grain is enjoying a genuine renaissance, grown as a climate-resilient crop in a drying world and sold, gluten-free and mineral-rich, on the health-food shelves of the West; India, the largest producer, has led a global campaign to restore the millets to the table. From the Sahel to the Deccan to the loess plains of China, the small hardy grain that fed the dry country through every lean season is once more a grain with a future.