Tamarind is one of the great paradoxes of the plant world: a tree whose name proclaims it Indian, whose cooking the world associates above all with India and South-East Asia, and which is in fact a native of Africa. It is the single species of its genus, Tamarindus indica, a large, long-lived, evergreen tree of the pea and bean family, and what the kitchen uses is neither leaf nor seed but the pulp: the sticky, fibrous, intensely sour brown flesh that packs the curved, brittle-shelled pod around its hard, glossy seeds. That sourness is unusual in its chemistry, for where most fruits owe their acidity to citric or malic acid, the tamarind is exceptionally rich in tartaric acid, the same acid that dominates the grape, and this gives its sourness a particular depth and roundness that no lemon or lime can imitate.

The tree is native to the dry tropical savannas of Africa, most likely the belt running from Sudan and the East African drylands across the Sahel to Senegal, where it grows wild and is gathered still. But its domestication is really a story of prehistoric transport. Carried across the Arabian Sea to the Indian subcontinent in the second millennium BCE, probably by the same seafaring trade that linked East Africa to India across the monsoon-driven Indian Ocean, the tamarind found in the Deccan and the Dravidian south a second homeland so perfect that it naturalised utterly, seeded itself along every roadside, and was woven so deeply into the cooking and the culture of India that Sanskrit named it, the Ayurvedic physicians prescribed it, and the world came to believe it Indian.

Because it is one species, the tamarind has no scattering of separate wild origins in the manner of the raspberry or the passion fruit. What it has instead is a division of cultivars, and one of these matters greatly in the modern kitchen: alongside the common sour tamarind, the workhorse souring agent of half the tropical world, there is the sweet tamarind, a low-acid selection whose pulp is mild and date-like enough to be eaten fresh out of the hand, cultivated above all in Thailand as makham wan and sold as a dessert fruit. The two are the same tree put to opposite ends, the one a sour seasoning, the other a sweet fruit.

The tamarind's travels divide cleanly into two great ages. The first was ancient and eastward. From its African cradle the tree reached India in prehistory and became, within the subcontinent, the defining souring agent of an entire culinary world; and from India its use radiated outward along the trade routes of the Indian Ocean and the Bay of Bengal. Arab merchants carried it, and it is to them that the tamarind owes its universal name, for encountering the sticky brown pulp in India they called it tamr hindī, the 'date of India', and from that Arabic phrase, through mediaeval Latin tamarindus, descends the name in every European tongue. Eastward the same maritime trade carried the tree into South-East Asia, where it became the makham of Thailand, the asam jawa of the Malay archipelago, the me of Vietnam, and the sampalok of the Philippines, the indispensable sour note of the region's soups, curries, and sambals. Along the Swahili coast of East Africa the Indian Ocean trade brought the Indian manner of cooking the fruit home again to the continent that had given it, and the tamarind entered the coconut-rich cuisine of Zanzibar and the coast as ukwaju.

The second age was the age of European empire and of the Columbian exchange, which carried the tamarind at last across the oceans it had never crossed. The Spanish, who had met the tree in their Asian possessions, carried it eastward across the Pacific on the Manila galleon from the Philippines to Acapulco, and tamarindo took such deep root in Mexico that its cooling drink and its sweet-sour candies are now amongst the most characteristic flavours of the country. Across the Atlantic the tree travelled a different road, carried in the terrible traffic of the slave trade, for the enslaved peoples of Africa knew the tamarind of their homeland and planted it wherever they were taken; it became a fixture of the Caribbean, reinforced in the nineteenth century by the Indian indentured labourers who brought their own tamarind chutneys to Trinidad and Jamaica.

The Dutch East India Company carried the tamarind-cooking of the Malay world to the Cape of Good Hope with the people it enslaved and exiled there, and it became a pillar of the Cape Malay kitchen of South Africa. And the British, who had learned to love the sour fruit across their Indian empire, carried its flavour home and hid it in a bottle: the Worcestershire sauce compounded at Worcester in 1837, said to derive from a recipe brought back from Bengal, rests on tamarind for its dark, sour depth, and through it, and through the brown sauces that followed, the tamarind entered the Western table by the back door, tasted daily by millions who have never seen the pod.

Tamarind today is one of the most widely used and least conspicuous of all the world's flavourings: a souring agent of the first importance across a swathe of the tropics from West Africa to the Philippines, and, in the West, a hidden ingredient in some of the most familiar bottled sauces on the shelf. India is by a wide margin the largest producer and consumer, and the fruit is sold there, and across most of its range, as a sticky pressed block of de-seeded pulp, or as a ready-made concentrate, to be soaked, squeezed, and strained into the sour 'tamarind water' that is its usual working form. Thailand is the other great producer, and the centre of the trade in the sweet dessert tamarind eaten fresh.

The fruit's culinary role is remarkably consistent across the many cuisines that use it: it supplies sourness, but a sourness rounder, darker, and more lingering than citrus, and one that stands up to long cooking, so that it is the acid of choice for slow-simmered dishes where a squeeze of lime would be lost. It is the sour heart of the South Indian rasam, sambar, and tamarind rice, and the tang in the North Indian chutneys of the chaat stall; the souring of the Thai pad thai and sour curry, the Malay and Indonesian asam dishes, the Vietnamese and Filipino sour soups; the tartness in the Mexican agua fresca and the Latin American candy; the acid in the Cape Malay curry; and the depth in Worcestershire and brown sauce. It is also, across its whole range, a medicine, mildly laxative, prescribed in the Ayurvedic and Unani traditions and drunk as a cooling remedy in the heat, and a household chemical, its tartaric acid used across India to scour the tarnish from brass and copper.

From a wild tree of the African savanna, the tamarind has become the invisible sourness beneath a thousand dishes on every inhabited continent, so thoroughly naturalised into the cooking of India and South-East Asia that it is thought native there, and so thoroughly hidden in the sauces of the West that most who eat it daily do not know its name.