Dioscorea spp.: D. rotundata and D. cayenensis, the white and yellow Guinea yams of West Africa; D. alata, the greater or water yam of Asia and the Pacific, which the Philippines calls ube; D. polystachya, the Chinese yam; and D. trifida, the cush-cush yam of tropical South America
The yam was domesticated at least three times, on three continents, from three different species, and the entry that follows is really three plants travelling under one English name.
West Africa took the largest of them. Dioscorea rotundata, the white Guinea yam, has two close wild relatives, the savanna D. abyssinica and the forest D. praehensilis, and the question of which gave rise to the crop was settled only recently. Nora Scarcelli and colleagues, resequencing 167 wild and cultivated yams in 2019, concluded that the crop was domesticated from the forest species and that yam farming expanded from the basin of the Niger, a river along which African rice and pearl millet were also taken into cultivation; a study published the following year read the same genomes as showing a hybrid origin, with genes from both wild species in the crop. Because tubers rot rather than carbonise, the archaeology is poor and the date is an estimate: the usual range runs between about 5000 and 3000 BCE.
Island Southeast Asia and New Guinea took D. alata, the greater or water yam, which is not known in the wild at all. The botanical consensus places its cradle in the New Guinea region; a genome-wide study of 2020 read the evidence as two domestications, one on the Southeast Asian mainland and one in the Pacific. The use of wild yams in the region is far older: starch grains on stone tools from the Ivane valley in Papua New Guinea carry it back more than 44,000 years, and charred purple yam and taro from Ille Cave in Palawan have been dated between about 8250 and 3050 BCE.
China took D. polystachya, the slender Chinese yam, into its kitchen and its pharmacopoeia together. Tropical South America domesticated a fourth, D. trifida, the cush-cush or yampi, on the ground where Brazil, Guyana, Suriname, and French Guiana meet; it is still grown by family farmers there, but it has lost ground to the African and Asian yams and is now the least of the four.
Yams moved long before the Atlantic trade. The greater yam travelled west out of Asia with the same maritime traffic that carried the banana, the taro, and the water buffalo across the Indian Ocean, reaching Madagascar and the African coast in antiquity, so that by the time Europeans arrived in West Africa the Asian water yam was already growing alongside the African white yam. The Ijebu dish ikokore, which is made with the Asian yam and no other, is what that ancient exchange looks like on a plate. Eastward, the Lapita voyagers carried the greater yam out of the Bismarck Archipelago into the Pacific more than three thousand years ago, and its tubers became the wealth of whole societies: the Trobriand Islanders store them in decorated yam houses that dwarf the dwellings behind them, the Abelam of the Sepik grow ceremonial yams three metres long, masked and exchanged between partners in rival villages, and Vanuatu bakes yam, taro, or cassava into laplap in an earth oven.
The Atlantic crossing was a different matter. Yams were loaded aboard slave ships in West Africa because they kept for months without rotting, and they crossed as the food of the Middle Passage; in the Caribbean and on the Spanish Main they were planted on the provision grounds that the enslaved were allotted to feed themselves, and they became staples of Jamaica, Panama, Colombia, and the Guianas. The word crossed with the plant: English yam comes through Portuguese inhame and Spanish ñame from a West African verb meaning to eat.
The last journey was industrial rather than culinary. In 1938 the American chemist Russell Marker worked out how to convert diosgenin, a compound in the tubers of Mexican yams, into progesterone, and in 1944 he founded Syntex in Mexico City to make steroid hormones from Dioscorea mexicana and the barbasco vine, D. composita. The price of progesterone fell from eighty dollars a gram to two, Syntex went on to synthesise the compound behind the first oral contraceptive, and a genus that had fed West Africa for five thousand years changed the twentieth century through two species that nobody eats.
Yam is a regional staple of extraordinary concentration. About ninety-two per cent of the world's crop is grown in West Africa, and Nigeria alone accounts for around two-thirds of it, some fifty million tonnes a year; Ghana, which grows far less, exports more than anyone. Almost none of it is traded as a global commodity, and almost all of it is eaten within a day's journey of where it was dug.
It is expensive food by African standards, and it is prestige food everywhere it grows. The tuber is heavy, the vine needs staking, the crop needs eight or nine months and a great deal of labour, and the harvest is counted rather than weighed: a barn of yams is a visible fortune. That is why the yam, and not cassava, carries the festivals, the titles, and the offerings, from the New Yam Festival of the Igbo to the Ga rites at which eto is served and the yam houses of the Trobriands.
The Asian yams have had a different century. The Philippine purple yam has become an international flavour, sold as ube in ice cream and pastry far beyond the islands; the Chinese yam is eaten as a tonic in soups and sold dried in every herbal pharmacy in East Asia; and Japan grates its mountain yams raw over rice. A last warning about names: in the United States, soft orange sweet potatoes are sold as yams, which they are not. The yam is a monocot from an entirely different family, and nothing in an American tin of candied yams has ever been near Dioscorea.
Historical Journey of Yam
The Niger Basin & the West African Yam Belt — c. 3000 BCE
The yam belt runs from the Bandama in Côte d'Ivoire east through Ghana, Togo, and Benin to the Cameroon border, and it grows more yams than the rest of the world put together. Its yam is Dioscorea rotundata, the white Guinea yam, and where it came from has only recently been settled. Nora Scarcelli and her colleagues, resequencing the genomes of 167 wild and cultivated yams in 2019, found the crop derived from the forest species D. praehensilis rather than from the savanna D. abyssinica, and traced the expansion of yam farming to the basin of the Niger, the same river along whose flood plains African rice and pearl millet were taken into cultivation. A team publishing in 2020 read the same relationships as a hybrid origin, the cultivated yam carrying genes from both wild species. Tubers rot rather than carbonise, so the archaeology is thin and the date is a placement; estimates of when cultivation began run between about 5000 and 3000 BCE.
What the belt built on the plant is an agriculture of stakes and barns. The vine is trained up poles, the crop takes eight or nine months, and the tubers keep for months more in an open-sided barn, which is why yam and not cassava became the crop a man could store, count, and be judged by. The dish that the whole belt measures itself against is pounded yam, iyan in Yoruba, boiled white yam beaten in a mortar until it is smooth and elastic, and eaten by tearing, dipping, and swallowing. The same food is foutou igname in Côte d'Ivoire and yam fufu in Ghana, Togo, and Benin.
Palawan, New Guinea & Island Southeast Asia — c. 3000 BCE
The second yam of the world is Dioscorea alata, the greater or water yam, and it is Asian and Pacific. It is not known in the wild, which is the usual mark of a long-domesticated clone, and the botanical consensus places its cradle in the New Guinea region, from which its cultivars spread by cuttings; a genome-wide study of 2020 read the evidence as two separate domestications, one in mainland Southeast Asia and one in the Pacific. People were eating wild yams in the region far earlier still: starch grains on stone tools from the Ivane valley in Papua New Guinea take the use of yams back more than 44,000 years.
The oldest direct evidence of the plant as food comes from the other end of the island world. Charred tissue of purple yam and taro from Ille Cave in Palawan has been dated between about 8250 and 3050 BCE. The Philippines has the greatest variety of the purple-fleshed forms, and calls the plant ube; boiled, grated, and cooked down with coconut and condensed milk it becomes ube halaya, the purple jam that fills the fiesta and Christmas table and tops a glass of halo-halo. From the Bismarck Archipelago the Lapita voyagers carried the greater yam east into the Pacific more than three thousand years ago, and it travelled west as well, reaching Madagascar and the African coast long before any European ship did.
Efate, Espiritu Santo & Vanuatu — c. 1000 BCE
The Lapita voyagers who sailed east out of the Bismarck Archipelago carried the greater yam with them, and they reached the islands of Vanuatu about three thousand years ago; the date of this point follows that settlement. Yam has been a staple and a ceremonial crop in the archipelago ever since, planted to a calendar, harvested at a festival, and piled up as a measure of a household's standing, as it is across Melanesia.
Laplap, the national dish of the eighty-three islands, is made by grating a root to a paste, wrapping it in banana leaves with coconut cream and sometimes fish or meat, and baking the parcel in an earth oven. Which root it is depends on the island, the season, and the household: yam, taro, or cassava, with banana as a filling. That interchangeability is the dish, which is why laplap sits on this journey and on the taro and cassava journeys as well, and why none of the three can claim it alone.
Jiaozuo, Huaiqing & the Yellow River Plain — c. 200 CE
China has a yam of its own, Dioscorea polystachya, and treats it as much as a medicine as a vegetable. Shan yao, mountain medicine, is a slender white tuber, crisp when raw and slippery when grated, and the Shennong Bencao Jing, the Han pharmacopoeia compiled around the second century CE, already ranks it among the superior drugs; the date of this point follows that first appearance in the written record, and the plant is older in the fields than in the books.
The yam of Huaiqing prefecture, the country around Jiaozuo in northern Henan, has been the prized one for centuries and is counted among the four Huai medicines, along with rehmannia, chrysanthemum, and achyranthes. The kitchen uses it in tonic soups with pork ribs, steams it, and, in the restaurants of the north, fries it and turns it in molten sugar as basi shanyao, candied yam that pulls into long glassy threads and is dipped in cold water at the table.
The Mekong Delta & Southern Vietnam — c. 1000 CE
Mainland Southeast Asia grows the greater yam in quantity, and the genome-wide work of 2020 reads the mainland as one of the two places where the species was domesticated, separately from the Pacific. When it reached the Mekong delta is not recorded, and the date of this point is a placement.
Vietnamese calls the purple water yam khoai mỡ, the fatty tuber, for the slippery flesh, and southern kitchens cook it into canh khoai mỡ: the yam scraped from its skin in short strokes and dropped into a light prawn or pork broth, where it dissolves into a thick violet soup finished with rice paddy herb and fried shallots. It is family food, eaten as part of a rice meal rather than on its own.
Thiruvananthapuram, Kochi & Kerala — c. 1000 CE
South India grows the greater yam in every second garden, and Kerala eats it as a household staple rather than an occasional vegetable. Malayalam calls it kaachil or kaavuth; Tamil has perumvalli kizhangu and the purple rasavalli kizhangu, which Jaffna Tamils cook into a porridge; and Sanskrit medicine knew yams as ālukas. When the plant reached the coast is not recorded, and the date of this point is a placement.
Kaachil puzhukku is the everyday treatment: the yam steamed and then crushed with a coarse paste of coconut, cumin, and green chilli, finished with raw coconut oil and curry leaves, and eaten with rice gruel. At Thiruvathira in December a whole company of tubers is cooked the same way, kaachil with chembu, koorka, sweet potato, and red cowpeas. The elephant foot yam that Kerala calls chena and Tamil senai kizhangu, which carries a large body of dishes of its own, is not a yam at all but an aroid, Amorphophallus paeoniifolius, a relative of taro; the atlas keeps it out of this entry for that reason.
Enugu, Nsukka & Igboland — c. 1000 CE
Igboland made the yam the measure of a man. Barns were built to display the harvest, titles were taken by those who could fill them, and the word for yam, ji, runs through the language of status and of farming; Chinua Achebe, writing of his grandfather's world in Things Fall Apart, called the yam the king of crops. The date of this point is a placement, since the crop is far older in the region than any record of it.
The year still turns on it. Iri ji or iwa ji, the New Yam Festival, is held between August and October at the end of the rains, and no one may eat the new crop until the oldest man or the priest has offered the first yams to the ancestors and cut them. The dish most particular to the Igbo kitchen is ji mmiri ọkụ, yam in hot water: yam simmered in a fierce pepper soup of dried fish, crayfish, and calabash nutmeg, which is the first food given to a woman after childbirth, during the month of omugwo when her mother comes to care for her, and which is eaten by everyone else at the New Yam Festival.
Ibadan, Ijebu & Yorubaland — c. 1200 CE
Yorubaland grows the white yam and eats it in three registers. Pounded, it is iyan, the food of ceremony. Dried and milled, it is elubo, and stirred into boiling water it becomes àmàlà, the dark, glossy swallow that Lagos and Ibadan argue over and that is served as abula, with ewedu, gbegiri, and pepper stew side by side on one plate. Cut into chunks and cooked in palm oil with blended peppers, it is àsáró, the everyday porridge. The date of this point is a placement.
The Ijebu, in the south-west, cook the other yam. Ikokore is made with ewura, the water yam, Dioscorea alata, which cannot be pounded because its flesh is wet; it is grated raw and dropped in lumps of two sizes into a peppery palm oil broth, where the small lumps melt into the sauce and the large ones set. That an Asian yam should be the foundation of a Yoruba festival dish is a reminder of how early the greater yam reached West Africa.
Accra, Kumasi & Ghana — c. 1400 CE
Ghana is the world's largest exporter of yams, and the variety that commands the price is pona, the white yam of the Brong and Techiman country, which boils dry and floury rather than wet. The date of this point is a placement, made at the rise of the Akan forest states; the crop is older.
Two dishes carry it. Ampesi is the plain boil, thick pieces of yam eaten with kontomire, the dense green stew of cocoyam leaves ground with melon seed, smoked fish, and palm oil, and it is what a Ghanaian means by a heavy lunch. Eto, properly oto, is boiled yam mashed with red palm oil until it turns orange and served with hard-boiled eggs pressed into it, and it is not ordinary food at all: the Ga cook it for the twins' festival, for the outdooring at which a child is named, and for puberty rites, and the Akan for naming ceremonies and marriages. The eggs belong to the rite, not to the plate.
Surat & Gujarat — c. 1500 CE
Gujarat has made the greater yam a vegetable of the cold months. It is ratalu or kand, purple-fleshed, and it arrives in the markets of Surat and Ahmedabad with the winter roots. When it reached India is not recorded, and the date of this point is a placement.
Cut into batons and fried with cumin, green chilli, crushed peanuts, and lemon, it is ratalu nu shaak, eaten with rotli; because it is a root and not a grain, it is also farali food, permitted on fasting days. Its grander appearance is in undhiyu, the winter casserole of Surat in which a whole season of vegetables is cooked together, and where the purple yam sits beside the young pigeon peas, the surti papdi, and the baby aubergines. The elephant foot yam of southern and eastern India, which shares the English name, is a different plant altogether.
Mariko, the Tōkaidō & Japan — c. 1600 CE
Japan eats yam raw, which almost nowhere else does. Nagaimo, the long yam that came from China, and the native yamatoimo and wild jinenjo are grated to a pale, viscous cream called tororo, loosened with dashi and miso, and poured over hot rice, or over barley rice as mugitoro. The Chinese yam is thought to have reached Japan by the seventeenth century or earlier, and the date of this point marks the moment the dish enters the record.
That record is unusually exact. At Mariko, a post station on the Tōkaidō in what is now Shizuoka, the teahouse Chōjiya has served tororo-jiru since 1596 to travellers about to cross the pass. Bashō wrote a verse on the tororo soup of Mariko, Jippensha Ikku sent the heroes of Tōkaidōchū Hizakurige through it, and Hiroshige drew the teahouse in The Fifty-Three Stations of the Tōkaidō. Few dishes are tied so firmly to one bend in one road.
Trelawny & Jamaica — c. 1700 CE
Yams crossed the Atlantic as ship's provisions and stayed as a crop. They were loaded aboard slave ships in West Africa because they kept for months, and in the Caribbean the enslaved grew them on the provision grounds they were allotted to feed themselves; the English word for the plant came over the same way, from the Portuguese inhame and Spanish ñame, from a West African verb meaning to eat. The date of this point is a placement for the plantation century in which the yam became a Jamaican staple.
Jamaica's yam is the yellow yam, and the parish that grows it is Trelawny, which raises the greater part of the island's crop and has celebrated it since 1997 with a yam festival every Easter Monday. Along the roads of the yam country, vendors roast whole yams in their skins over coal fires and sell them hot with a piece of saltfish, the yam smoky and dry-floury and the salt fish its seasoning.
Panama City & the Isthmus — c. 1700 CE
On the Spanish Main the yam kept the name the Portuguese and Spanish had given it, ñame, and it went into the pot that the whole isthmus cooks. The date of this point is a placement for the colonial centuries in which African yams became one of the standing ground provisions of Panama, Colombia, and the Caribbean coast.
Sancocho de gallina is Panama's national dish, and it is not a mixed stew of many roots but a plain and particular one: an old hen simmered with a great weight of ñame and a great deal of culantro, until the yam breaks down and thickens the broth to something between a soup and a porridge. It is eaten with white rice, and it is the recognised remedy for a hangover, a cold, and a funeral.
