Arugula refers to several species, including Eruca sativa (Eruca means rough or harsh, presumably referring to the peppery flavor of Rocket; sativa refers to a cultivated rather than wild plant) and some spp. in the Diplotaxis ("double rows," referring to the double rows of seeds) genus. The Eruca, which I believe is the one almost universally called Arugula here in the States, is in the Brassicaceae, so closely related to cabbages and mustards.
It was a well-known herb in ancient times, gathered for eons as a potherb, and surely part of the horta (χορτα), the mixed greens gathered by Greeks and Etruscans that formed one of the bases of their diets.
We find it, for instance, mentioned in the Book of Kings:
4:39 One went out into the field to gather herbs [oroth, widely believed to be E. sativa], and found a wild vine, and gathered of it wild gourds his lap full, and came and shred them into the pot of stew; for they didn’t recognize them. So they poured out for the men to eat. It happened, as they were eating of the stew, that they cried out, and said, “Man of God, there is death in the pot!” They could not eat of it.Mmmm. Death in the pot. (My guess: Zucchini!) Guess they should have brought back the Arugula.
In most cultures where it is indigenous (the Mediterranean basin and some parts of western Asia, particularly Turkey), it has been considered an aphrodisiac, but it was more than a classical Spanish Fly: I've found it cited as a cure for freckles, baldness, and intestinal worms. Its healing virtues are lauded as a cure for children's coughs, as a way to increase milk flow, and as a way to "take away the ill-scent of the arm pits." Given that one book I came across referred to the smell of Arugula as "like rancid bacon," one wonders if this might not be another case of the cure being worse than the disease. (Personally, I like the bitter and peppery smell of Arugula, and I'll often mash a leaf between finger and thumb while watering or weeding.)
I read that Pliny advises one to eat Arugula seeds before being whipped: Anyone who does so shall be "so hardened, that he shall easily endure the pain." The Talmud teaches us that it can be used to treat eye infections.
Most cookbooks I've glanced in claim that Arugula first gained popularity here in the States in the 1990s, taking its place alongside roasted red peppers during the mania for all things Mediterranean. However, I read in a 1960 New York Times article ("A Green by Any Name; Pungent Ingredient Is Cause of Confusion for City Shopper Arugula -- or Rocket -- Is the Secret of Experts' Salads") that Craig Claiborne, the Times food critic, found the herb everywhere in New York City: "Most Italian chefs know [that Arugula] is the secret ingredient of their salads-about-town." He lists a half-dozen stores in the area that sell bundles of the stuff for fifteen to nineteen cents, about the price of spinach. (The Publix down the street sells a couple ounces of "baby Arugula" for close to $4!)
Galen, the Greek physician whose theories dominated 'medicine' for over a thousand years, proclaimed Rocket to be "hot and dry in the third degree" and therefore unfit to be eaten alone. He instructed that the herb be eaten with "cold herbs" like lettuce and purslane. Galen's advice aside, I eat the stuff straight with a cheesy vinaigrette or stacked high on a roast-beef sandwich. It's an undemanding herb, and the 'Apollo' cultivar is ready to pick within a few weeks. The seeds germinate at nearly 100% and transplant easily from a seedling tray into my garden. I've tried the wild stuff, but it grows much more slowly and the leaves are far smaller and have an unpleasant rubbery texture.
(The photographs are, from top down, 'Apollo', "wild" arugula (E. sativa) and 'Selvetica')