You are crying. Not because you are sad, and not because you are in pain, but because you cut an onion. The moment your knife pierced its layers, the plant attacked you, releasing a volatile gas that drifted up from the cutting board, crossed the air to your face, and met the moisture on your eyes, where it transformed into sulfuric acid. Microscopic droplets of acid on your eyeballs, and your body did exactly what it was designed to do.
It flushed them out. You stood in your kitchen, tears streaming down your cheeks, because a plant that was dead the moment it was pulled from the earth had just deployed a chemical weapon against you. And within ten minutes, you would eat it anyway. Humans have been doing this for at least five thousand years, and possibly much longer.
The onion makes you cry, burns your mouth, and leaves your breath foul for hours, yet it is the second most consumed vegetable on Earth, with more than 100 million tons grown every year in nearly every country with soil. The story of how that happened begins in Central Asia, with a plant so ancient, so soft, and so quick to rot that it left almost no archaeological trace. The onion belongs to a genus of more than 800 species. Wild onions, wild garlics, wild leeks, and wild chives are scattered across the Northern Hemisphere.
The closest wild relatives to the cultivated onion, Allium vavilovii and Allium oschaninii, still grow today in the mountains and valleys of Iran, Afghanistan, Turkmenistan, Pakistan, and western China. The Russian botanist Nikolai Vavilov, who spent the early twentieth century mapping the geographic origins of crops, identified Central Asia as the center of the onion’s origin, spanning from modern Iran through Afghanistan to the western Himalayas. The highest diversity of wild Allium species grows there. The earliest human encounters with wild onions almost certainly happened there.
But here is what makes the onion’s domestication story different from nearly every other major crop. No one knows exactly when it happened. No one knows precisely where, and no one has ever identified the exact wild ancestor that became the cultivated onion. The precise ancestor species is unknown.
It may be extinct. It may still be growing on a hillside in Afghanistan. For wheat, we have wild emmer. For corn, we have teosinte.
For the onion, we have a void. That void exists for a simple physical reason. Onions are soft. They are composed almost entirely of water and sugar.
When an onion dies, it rots. When it rots, nothing remains. No seed coat, no hard pit, no shell, no silica, no lignin. A grain of wheat can survive in dry sediment for 10,000 years.
An onion bulb disappears in a single season. The earliest crops on Earth left mountains of seeds and charred fragments. The onion left behind almost nothing. Everything we know about its early history comes from words and images, not from the plant itself.
The words begin in Mesopotamia. The Yale culinary tablets, a collection of Babylonian cuneiform tablets dating to around 1750 BC, contain some of the oldest known recipes in the world. The French Assyriologist Jean Bottéro translated and analyzed about 35 of them in the 1990s. They describe stews and broths and meat dishes, and onions appear in nearly every single recipe, not as a decoration but as a foundational ingredient.
The Babylonians were cooking with onions as a base flavor 3,700 years ago, just as a French chef does today. While Babylon was writing down its onion recipes on clay tablets, Egypt was burying onions with its dead. Onions appear in tomb paintings from the Old Kingdom, around 2600 BC, and by the New Kingdom, between approximately 1550 and 1070 BC, archaeologists have found whole onion bulbs placed inside mummies, in the eye sockets, in the chest cavity, and wrapped in the bandages. Ramesses, who died around 1149 BC, had onions placed in his eye sockets.
The Egyptians connected the onion’s concentric rings with eternity. Circles within circles, endless layers. The same geometry they saw in the universe, they saw when they cut an onion in half. The Greek historian Herodotus, writing around 450 BC, claimed that an inscription on the Great Pyramid of Giza recorded enormous sums spent on onions, garlic, and radishes for the workers who built it.
Modern Egyptologists have questioned the accuracy of his reading, and no such text has been independently verified on the pyramid, but the underlying fact is not disputed. The workers at Giza ate onions. Archaeological evidence from the workers’ villages, including Deir el-Medina, confirms that onions were part of the daily diet of the men who built the most famous structures on Earth. The pyramids were built on bread, beer, and onions.
The Ebers Papyrus, one of the oldest surviving medical texts, written around 1550 BC, lists onion as a remedy for ailments ranging from heart problems to eye diseases. The Egyptians did not just eat onions. They prescribed them. The plant was food, medicine, ritual object, and funerary offering all at once, in the same civilization, at the same time.
The Greeks inherited this. Theophrastus, the student of Aristotle who lived from roughly 371 to 287 BC, described several cultivated varieties of onion in his Enquiry into Plants. He distinguished them by origin, varieties from Sardis, from Cnidos, from Samos, each named for the city or island where they were grown. He noted differences in size, color, flavor, and storage quality.
Some were sweet enough to eat raw, while others were intensely pungent. That level of diversity, documented 2,300 years ago, tells you that onion cultivation was already ancient when Theophrastus wrote about it. The Romans went further. Pliny the Elder, in his Natural History, described the onions of Pompeii as particularly prized, noting round varieties, flat varieties, and oblong ones, ranging from mild to tear-inducingly strong.
Columella, writing his agricultural manual De re rustica around the same era, gave detailed instructions for growing onions: when to plant, how to prepare the soil, which varieties to grow for fresh eating versus long-term storage. The Roman legions carried onions across the empire, as both food and medicine. Gladiators were said to rub their bodies with onions before combat, believing it would firm their muscles. Whether it worked does not matter.
The belief tells you how seriously the Romans took this plant. Alexander the Great fed onions to his soldiers as they marched through Persia and into Central Asia, returning to the very region where the plant had originally been domesticated thousands of years before. Now comes the part most people never think about. Why does the onion taste the way it does?
Why is it pungent raw and sweet cooked? And why does it make you cry? The answer begins with sulfur. Onions absorb sulfur from the soil and incorporate it into a class of molecules called S-alkenyl cysteine sulfoxides.
The dominant one in onions is called isoalliin. These molecules sit inside the onion’s cells and do nothing. They are inert, harmless, stored in the cell cytoplasm, waiting. In a separate compartment of the same cell, stored in the vacuole, is an enzyme called alliinase.
The two never meet. As long as the cell is intact, the reactant and the enzyme are separated by a membrane. It is a bomb with a physical safety valve. The moment you cut the bulb, you break the cells.
The membrane tears. The alliinase spreads and collides with the isoalliin. The reaction produces sulfenic acids, unstable intermediates that last for fractions of a second. Then, in 2002, a Japanese researcher named Shinsuke Imai at House Foods Corporation discovered something no one expected.
There is a second enzyme, a dedicated one, whose sole purpose is to take the sulfenic acid intermediate and convert it into a specific volatile molecule called syn-propanethial S-oxide. Imai named this enzyme the lachrymatory factor synthase, or LFS. He published the discovery in Nature on October 17, 2002. It was a single page, one of the shortest research papers in the journal’s history, and it overturned decades of assumptions about onion chemistry.
Syn-propanethial S-oxide is the molecule that makes you cry. It is volatile, so it evaporates instantly and drifts upward from the cutting board into the air around your face. When it contacts the moisture in your eyes, it hydrolyzes into sulfuric acid, propanol, and hydrogen sulfide. The sulfuric acid stimulates your tear reflex.
Your eyes flood with tears to flush out the irritant, and you stand there weeping over your dinner because a plant developed a two-stage chemical weapon system to punish anything that damages its cells. The key insight from Imai’s discovery is that the LFS enzyme exists only in onions. Garlic has alliinase. Garlic has the same sulfur-containing precursors, but it does not have LFS.
That is why cutting garlic does not make you cry. The tear factor is not a general byproduct of sulfur metabolism. It is a specialized weapon. The onion evolved a dedicated enzyme for the sole purpose of making eyes burn.
The plant did not stumble into this defense. It invested in it genetically, specifically, over millions of years of natural selection favoring variants slightly better at deterring herbivores. In 2008, a team led by Colin Eady at the Crop and Food Research Centre in New Zealand used RNA interference technology to silence the LFS gene in onions. The results were published in Plant Physiology.
The onions grew normally. They tasted like onions. They just did not make you cry. The chemical defense system had been switched off at the genetic level.
Separately, Bayer Crop Science developed a variety called Sunion, released commercially in the United States around 2018, bred through years of traditional selection to produce very low levels of the tear factor. A non-tearing onion, produced without genetic modification, just patient selection. Now here is why cooking transforms the onion from a chemical weapon into one of the most important flavor agents in human cuisine. When you cook onions slowly, heat destroys the alliinase enzyme.
The LFS enzyme is destroyed too. The volatile sulfur compounds that cause the sting and the tears break down and evaporate. What remains is sugar. Onions are 3 to 10 percent sugar by weight, depending on variety.
Vidalia onions from Georgia, which grow in low-sulfur soil, are at the high end of that range. When the pungent sulfur compounds disappear, the sweetness that was always there, hidden behind the chemical warfare, becomes the dominant flavor. Then the Maillard reaction begins. Amino acids react with reducing sugars at temperatures above about 110 degrees Celsius.
Hundreds of new flavor molecules form. Brown pigments develop. The flavor shifts from sharp and sweet to deep, rich, and savory. The same group of atoms, rearranged by heat into something entirely different.
The French built an entire cooking technique around this transformation. Mirepoix, onions, carrots, and celery slowly cooked in fat, forms the flavor base for hundreds of dishes. The onion is the cornerstone. The same story repeats in nearly every cuisine on the planet.
In India, most curries begin with onions cooked in oil until golden or deeply caramelized. Indian cuisine uses more onions than almost any other national cuisine, and the relationship between onion prices and political stability in India is not a joke. In 1998, a sharp spike in onion prices contributed to the BJP losing state elections in Delhi and Rajasthan. In a country of over a billion people, onion availability is not just a market statistic.
It is a political crisis. The reason is not just flavor. Onions store in a way almost no other vegetable can match. A properly cured onion, dried in the sun or in a well-ventilated shed for a few weeks after harvest, can stay good for three to six months.
Some long-day storage varieties, in cool dry conditions, last up to a year. No refrigeration, no canning, no fermentation. In a pre-refrigeration world, that storage time meant survival. Harvest in autumn, hang your onions in a dry place, eat them all winter.
They do not rot. They do not freeze. They wait for you. Onions store energy as fructans, polymers of fructose, rather than starch.
Fructans are water-soluble and act as a natural antifreeze, lowering the freezing point of cell contents and protecting the bulb from frost damage. That is why onions hold up so well in cold storage and cold climates. The same chemistry that makes onions sweet when cooked is the chemistry that keeps the bulb alive in a root cellar through a northern winter. The onion’s genome is one of the largest of any cultivated plant on Earth, about 16.
3 gigabases. That is five times the size of the human genome and forty times the size of the rice genome. The genome is bloated with repeated DNA sequences and transposable elements, ancient genetic parasites that copied themselves over and over, inflating the genome far beyond what the plant needs. For decades, this enormous genome made the onion practically impossible to sequence.
That changed in 2023, when a consortium of researchers published a chromosome-level genome reference for the onion in Nature. It was the culmination of years of work by teams from institutions including Kew Royal Botanic Gardens and Wageningen University in the Netherlands. The assembly revealed extensive gene duplication, expansions in gene families linked to sulfur metabolism, and evidence of at least two whole-genome duplication events deep in the onion lineage’s evolutionary history. But compared to crops like tomato, corn, or rice, the specific genes that changed during onion domestication remain poorly understood.
We know the traits. The bulb increased dramatically in size. Wild Allium species produce small bulbs that form clusters. The domesticated onion produces one large, solitary bulb.
Pungency has been modified, with some varieties bred to be mild and others strong. Early flowering has been suppressed, because the onion diverts energy away from the bulb and ruins the crop. Dormancy has been enhanced so the bulbs remain viable for months after harvest. And day-length sensitivity has been modified to allow different varieties to grow at different latitudes.
This last trait, photoperiod sensitivity, is one of the most important and least understood aspects of onion biology. Short-day varieties, grown in tropical and subtropical regions, begin forming bulbs when they receive 10 to 12 hours of daylight. Long-day varieties, grown in northern Europe, Russia, and the northern United States, need 14 to 16 hours to trigger bulb formation. A long-day variety grown near the equator will never form a bulb.
A short-day variety grown in Scotland will form a bulb before the plant has grown enough to produce anything worth harvesting. Farmers around the world, over thousands of years, selected varieties adapted to their specific latitudes without understanding the hormonal mechanism controlling the response. They just noticed which onions worked in their fields and saved those bulbs for the next year. The spread of the onion across the planet follows the spread of human civilization itself.
From Central Asia to Mesopotamia by at least 3000 BC, then to Egypt, where it became a staple, a medicine, a religious offering, and a funerary artifact. Across the Greek world, where Theophrastus categorized its varieties 2,300 years ago, and across the Roman Empire, whose legions carried it and planted it in military gardens from Britain to North Africa. Into India via overland trade routes, where it became the base of one of the most complex and diverse culinary traditions on Earth. Along the Silk Road to China, though China had already domesticated its own Allium species independently, including the Welsh onion and Chinese chives, different species entirely that fill similar culinary roles.
In medieval Europe, the onion was one of the three pillars of the peasant diet, along with cabbage and beans. Charlemagne’s Capitulare de villis, issued around 800 AD, listed onions among the plants required in every imperial garden across the Frankish Empire. In some parts of medieval Europe, onions were used as currency. Rents were paid in onions.
Wedding gifts included bundles of onions. The plant was cheap, abundant, easy to grow, and durable enough to serve as a medium of exchange. When the Spanish colonized the Americas, they brought onions with them. Columbus is said to have brought onion seeds or seedlings to the Caribbean on his second voyage in 1493.
English colonists planted onions in New England. The plant adapted instantly. Within a generation, onions were growing throughout the Americas as if they had always been there. By the time of the American Civil War, onions were considered essential military provisions.
General Ulysses S. Grant reportedly told the War Department, “I will not move my army without onions. ” Three trainloads of onions arrived the next day. Whether the exact wording is apocryphal or genuine, the substance of the story is true.
Armies marched on onions for the same reason the Egyptian pyramid builders ate them: a combination of calories, flavor, storability, and vitamin C made the onion indispensable. The sweet Vidalia onion of Georgia reveals something about this plant no one would have expected. It is not a separate variety, as most people assume. The Vidalia onion derives its sweetness primarily from its soil, not its genes.
It is grown in the low-sulfur, sandy soil surrounding the town of Vidalia, Georgia. Since onion pungency comes from sulfur-containing compounds, and since the soil provides less sulfur, the plant produces less of the compounds that make you cry and burn. The same onion variety planted in high-sulfur soil elsewhere produces an ordinary, sharp-tasting bulb. The Georgia legislature designated the Vidalia onion the official state vegetable in 1990, and defined its growing region legally.
You cannot call any onion a Vidalia unless it is grown in specific counties in Georgia. Today, global onion production exceeds 105 million tons annually. India leads the world with about 31 million tons, followed by China with about 25 million tons. The global onion market is worth roughly 50 to 60 billion dollars per year.
Onions are grown in more than 170 countries. There are over 1,000 cultivated varieties, ranging from tiny pearl onions to giant exhibition onions weighing over 8 kilograms. So here is the answer. The onion never asked to be domesticated.
It resisted the process at every step, with every sulfur molecule it could produce. It was small. It was pungent. It deployed a chemical weapon that caused tears the moment any of its cells were damaged.
It burned the mouth. It was strong-smelling. It rotted so quickly that it left no archaeological trace. No one knows its exact wild ancestor.
The history of its domestication is uncertain, sometime before 3000 BC, possibly much earlier, perhaps 5,000 years or more. The location is approximate, Central Asia, Vavilov’s center of origin. And from that uncertain beginning, the onion became one of the most important plants in human history. Not because it is the most nutritious.
Not because it is the most caloric. But because it was the most useful. It stored for months without refrigeration. It grew in almost any soil, almost any climate, at almost any latitude.
It added flavor to every other food it touched. It could be eaten raw, cooked, pickled, dried, or fermented. It was used as medicine, as a preservative, as a religious symbol, and as currency. The Babylonians built their recipes around it.
The Egyptians buried their dead with it. The Romans carried it across their empire. The Indians built an entire culinary tradition on it. Ulysses Grant refused to march without it.
And every night, in every country on Earth, someone stands before a cutting board with tears running down their face, because a plant that developed a chemical weapon specifically to deter herbivores met the one herbivore stubborn enough not to take the hint. Humans did not defeat the onion’s defenses. They decided those defenses were the whole point. The burn, the tears, the sharp sting, the slow caramelization, the deep flavor of an onion slowly cooked until golden in the pan.
The defense system became the flavor. The weapon became the ingredient. And the plant that did everything it could to make itself inedible became the one thing no cuisine in the world can do without.