How the Potato Changed the World: From Andean Tuber to Half of Europe's Dinner

Wild potatoes grow from the American southwest to Chile, and the cultivated potato was domesticated around Lake Titicaca on the border of Peru and Bolivia between 8,000 and 10,000 years ago, from a wild species that had to be bred out of its bitterness. The Andean farmers made it the basis of a civilisation at altitudes where maize will not grow, cultivating thousands of varieties in every colour and shape, matched to particular fields and frosts, and they invented freeze-drying: chuno, potatoes left out to freeze at night, trodden to squeeze the water out, and dried in the day, keeps for years and fed the Inca armies and storehouses. The Spanish, who arrived in 1532, took the tuber to Europe in the 1570s as a curiosity, and it was grown for a century in botanic gardens and by a few farmers before anyone else would eat it.

The potato had everything against it. It was a member of the nightshade family, and its green parts and its sprouts are indeed poisonous; it grew from lumps rather than seed, which struck people as unnatural; it was ugly and it was unmentioned in scripture; and it was associated with the Irish and the poor. Governments pushed it because they could see what it would do. Frederick the Great of Prussia ordered his peasants to plant it in 1756 and, in a story that may be legend, posted guards around a royal potato field to make it seem worth stealing; Antoine-Augustin Parmentier, who had eaten little else as a prisoner in Prussia, persuaded Louis XVI to wear potato flowers and served potato dinners to Paris society, and French dishes with his name still carry the tuber. By 1800 it was a field crop across northern Europe, and the reasons were arithmetic:

• •A hectare of potatoes yields two to four times the food energy of a hectare of wheat
• •It grows in poor, wet, cold soil where grain fails, and on plots too small for a plough
• •It supplies nearly everything a person needs: energy, protein, vitamin C, potassium and B vitamins; a diet of potatoes and milk is nutritionally complete
• •It cannot be taxed or requisitioned easily, since it stays in the ground until dug, and armies marching over a field of it could not burn the crop

The historian William McNeill argued that the potato, more than any machine, made industrial Europe possible: it fed the growing population that filled the factories, released land from grain, and let a family live on a garden plot while its members worked for wages. The population of Europe roughly doubled between 1750 and 1850, and studies attribute a substantial part of the growth, and a measurable part of the urbanisation, to the tuber. Ireland was the extreme case. By 1840 about a third of the Irish population lived almost entirely on potatoes, an adult man eating five kilograms a day, and the population had risen to over eight million on land that grain could not have fed.

In 1845 a water mould, Phytophthora infestans, arrived from the Americas and destroyed the crop in a week, turning the tubers to black slime in the ground, and it returned in 1846 and 1848. Ireland had one main variety, the Lumper, cloned from cuttings and genetically identical, and no resistance anywhere. About a million people died of hunger and disease and a million and a half emigrated, and the island's population has never returned to its 1841 level; the British government's response, which continued grain exports and insisted on relief through workhouses and public works, is the reason the famine remains a political wound. Blight is still the potato's chief enemy, controlled by spraying and by breeding, and the same mould caused the potato crisis in the Netherlands and Germany in the 1840s that helped set off the revolutions of 1848.

The potato is the world's fourth food crop after maize, wheat and rice, at around 375 million tonnes a year, and China grows the most. It is a mainstay of the diet from Peru to Poland, it has been to space, and a genebank in Lima holds some 4,500 Andean varieties against the day another blight arrives. The chip, invented in Belgium or France in the eighteenth century, and the crisp, first described in an English cookbook in 1817, have turned it into the most processed vegetable on Earth, and a plant that was once accused of causing leprosy is now more often accused of causing obesity, which is a measure of how completely it won.

For a long time the exact strain of mould that caused the Irish famine was unknown, since nobody had kept the organism alive. The answer came from herbarium collections, where botanists in the 1840s had pressed and dried blighted potato leaves as specimens. In the 2010s researchers extracted DNA from those leaves and sequenced the mould's genome, and found that the famine was caused by a lineage different from the strains that dominate potato fields today. It had apparently spread around the world in the nineteenth century and was later displaced, possibly once new, more resistant potato varieties were bred.

Breeding the potato is harder than breeding most crops, which is part of why blight has remained such a problem. The cultivated potato carries four copies of each chromosome rather than the usual two, so desirable traits combine and separate unpredictably between generations, and most varieties are propagated as clones from tubers rather than grown from seed, which keeps a good variety fixed but fixes its weaknesses too. Wild relatives in Mexico and the Andes still carry genes for resistance to blight, pests and frost, which is one reason collections of wild and traditional potatoes are guarded so carefully.

A potato is usually called a root vegetable, but botanically it is a swollen stem. The plant sends out underground shoots called stolons, and their tips swell with stored starch to form tubers. The giveaway is the eyes: each one is a cluster of buds sitting beside a tiny scale leaf, arranged in a spiral around the tuber just as buds are arranged along a stem above ground, and each can grow into a new plant. That is why farmers plant seed potatoes, whole tubers or chunks carrying an eye or two, rather than true seeds.

The stem nature shows itself when a potato is left in the light. Its skin turns green as it makes chlorophyll, exactly as any stem would, and the green itself is harmless, though it is a sign that the bitter natural toxins the plant uses for defence are building up at the same time. The sweet potato, despite its name, belongs to a different family altogether, related to morning glories, and its edible part really is a swollen root. Yams are different again, from a separate group of tropical plants, although in North American shops the name is often given to orange sweet potatoes.

Supermarkets in Europe sell a handful of potato varieties, but in the Andes, where the potato was domesticated, farmers grow an astonishing range. A single family plot may contain dozens of varieties with different colours, shapes and tastes, chosen because they suit different altitudes, soils and uses, and because a mixture protects against losing everything to one disease or bad season.

The International Potato Center in Lima, Peru, keeps a gene bank holding thousands of samples of cultivated and wild potatoes, and Andean communities run a park where traditional varieties are grown and conserved in the fields. This diversity matters because a narrow range of commercial varieties is vulnerable, the lesson of the Irish famine, and breeders turn to these old varieties for resistance to pests and diseases.

The potato was domesticated near Lake Titicaca some 8,000 years ago, reached Europe with the Spanish in the 1570s, and was distrusted for two centuries as poisonous, unnatural and unbiblical until kings and reformers pushed it as a crop that yielded several times the food of grain on poor land. It fed the doubling population of industrial Europe and above all Ireland, where dependence on a single clone met the blight of 1845 and a million died, and it is now the world's fourth food crop.