The History of the Onion
Because onions are small and their tissues leave little or no trace, there is no conclusive opinion about the exact location and time of their birth. Many archaeologists, botanists and food historians believe onions originated in central Asia. Other research suggests that onions were first grown in Iran and West Pakistan.
It is presumed that our predecessors discovered and started eating wild onions very early—long before farming or even writing was invented. Very likely, this humble vegetable was a staple in the prehistoric diet.
The origin of the name “onion” comes from the classical period when it was given the Latin name unio--which means oneness or unity, or a kind of single onion. The French call it oignon. Martin Elcort in his book The Secret Life of Food writes, “The word (onion) was created by adding the onion-shaped letter o to the word union, yielding a new spelling ounion. The letter u was later dropped to create the modern spelling. A union is something that is indivisible and which, if taken apart, is destroyed in the process, like an onion.”
Most researchers agree that the onion has been cultivated for 5000 years or more. Since onions grew wild in various regions, they were probably consumed for thousands of years and domesticated simultaneously all over the world. Onions may be one of the earliest cultivated crops because they were less perishable than other foods of the time, were transportable, were easy to grow and could be grown in a variety of soils and climates. In addition, the onion was useful for sustaining human life. Onions prevented thirst and could be dried and preserved for later consumption when food might be scarce.
While the place and time of the onion’s origin are still a mystery, there are many documents, from very early times, which describe its importance as a food and its use in art, medicine and mummification.
Onions grew in Chinese gardens as early as 5000 years ago and they are referenced in some of the oldest Vedic writings from India. In Egypt, onions can be traced back to 3500 B.C. There is evidence that the Sumerians were growing onions as early as 2500 B.C. One Sumerian text dated to about 2500 B.C. tells of someone plowing over the city governor's onion patch.
In Egypt, onions were actually an object of worship. The onion symbolized eternity to the Egyptians who buried onions along with their Pharaohs. The Egyptians saw eternal life in the anatomy of the onion because of its circle–within–a–circle structure. Paintings of onions appear on the inner walls of the pyramids and in the tombs of both the Old Kingdom and the New Kingdom. The onion is mentioned as a funeral offering and onions are depicted on the banquet tables of the great feasts—both large, peeled onions and slender, immature ones. They were shown upon the altars of the gods.
Frequently, a priest is pictured holding onions in his hand or covering an altar with a bundle of their leaves or roots. In mummies, onions have frequently been found in the pelvic regions of the body, in the thorax, flattened against the ears and in front of the collapsed eyes. Flowering onions have been found on the chest, and onions have been found attached to the soles of the feet and along the legs. King Ramses IV, who died in 1160 B.C., was entombed with onions in his eye sockets. Some Egyptologists theorize that onions may have been used because it was believed that their strong scent and/or magical powers would prompt the dead to breathe again. Other Egyptologists believe it was because onions were known for their strong antiseptic qualities, which construed as magical, would be handy in the afterlife.
Onions are mentioned to have been eaten by the Israelites in the Bible. In Numbers 11:5, the children of Israel lament the meager desert diet enforced by the Exodus: “We remember the fish, which we did eat in Egypt freely, the cucumbers and the melons and the leeks and the onions and the garlic.”
In India as early as the sixth century B.C., the famous medical treatise Charaka—Sanhita celebrates the onion as medicine—a diuretic, good for digestion, the heart, the eyes and the joints.
Likewise, Dioscorides, a Greek physician in first century A.D., noted several medicinal uses of onions. The Greeks used onions to fortify athletes for the Olympic Games. Before competition, athletes would consume pounds of onions, drink onion juice and rub onions on their bodies.
The Romans ate onions regularly and carried them on journeys to their provinces in England and Germany. Pliny the Elder, Roman’s keen-eyed observer, wrote of Pompeii’s onions and cabbages. Before he was overcome and killed by the volcano’s heat and fumes, Pliny the Elder catalogued the Roman beliefs about the efficacy of the onion to cure vision, induce sleep, heal mouth sores, dog bites, toothaches, dysentery and lumbago. Excavators of the doomed city would later find gardens where, just as Pliny had said, onions had grown. The bulbs had left behind telltale cavities in the ground.
The Roman gourmet Apicius, credited with writing one of the first cookbooks (which dates to the eighth and ninth centuries A.D.), included many references to onions.
By the Middle Ages, the three main vegetables of European cuisine were beans, cabbage and onions. In addition to serving as a food for both the poor and the wealthy, onions were prescribed to alleviate headaches, snakebites and hair loss. They were also used as rent payments and wedding gifts.
Later, the first Pilgrims brought onions with them on the Mayflower. However, they found that strains of wild onions already grew throughout North America. Native American Indians used wild onions in a variety of ways, eating them raw or cooked, as a seasoning or as a vegetable. Such onions were also used in syrups, as poultices, as an ingredient in dyes and even as toys. According to diaries of colonists, bulb onions were planted as soon as the Pilgrims could clear the land in 1648.
All About: Onion History — National Onion Association • http://www.onions-usa.org 
You can learn more about onions through the following publications:
Onions Onions Onions
Making the Most of Onions
Onions—Cooks Kitchen Reference
The Onion Cookbook