In the end, all this fuss comes down to a plant.

Where did it come from? How does it grow? Why was it forbidden fruit for such a long time? What does it really do to us? And why are we so obsessed with it? Herewith, a short biography of the cannabis plant.

It’s an annual − a hardy weed that grows almost anywhere, from seed or (these days) from clippings and clones, indoors under lights or outdoors under nothing more than sunlight. The plants can be anywhere from less than a metre to six metres high. Male plants are frailer, and often culled before they pollinate the females. This makes the sexually frustrated females overproduce flowers, upon which grow the trichomes that produce the plant’s 113 cannabinoids − especially the two we care about most, psychoactive tetrahydrocannabinol (THC, the stuff that gets you high) and non-psychoactive cannabidiol (CBD, the nonintoxicating compound that calms people down). In general, the hotter and sunnier the growing climate, the more psychoactive the plant. Outdoors, some cannabis needs a three to five month growing season; indoors, fed hydroponically under artificial lighting and induced darkness (which in turn prompts the plants to bloom), a clever grower can produce three pounds of high-powered flower from 100 knee-high cloned plants (which might cover a large dining room table) every eight weeks.

But you don’t have to be a clever grower. The Mary.ag, a slick grow-box that sits in your living room and looks like a stereo speaker, controls watering and fertilization and odour electronically, via cellphone. The Mary produces 45 grams of smokeable flower every seven weeks. It costs US$499.

In general parlance (and exacting taxonomists have continuing objections to this), there are two widely accepted psychoactive variants of cannabis: harder-to-grow sativa, which is taller and skinnier and looks slightly hapless; and indica (shorter, bushier, tough guy). Neither plant wins beauty contests. The two subspecies are marketed in dispensaries, respectively, as energizing (sativa) and narcotic (indica). But − breaking news! a small scandal in the small world of cannabis research! − according to Vancouver’s Jonathan Page, a world expert on the cannabis genome and CEO of Anandia Laboratories Inc. (acquired two weeks ago by Aurora Cannabis Inc. for $115-million): “We’re having a lot of trouble showing that they’re genetically different. And also that there’s different chemistry between the two types, dopey-sleepy versus uplift.” For all the media attention cannabis gets, research on the plant is still in its infancy − as you might expect of a shrub that has been illegal in one form or another since the 1920s.

Cannabis evolved 65 million years ago. It was one of the earliest plants to be domesticated by humans, who have cultivated it for at least 10,000 years, starting with Neolithic man. (It was the Stone Age, dude). In 2008, a team of researchers in northwest China uncovered the grave of a Caucasian shaman buried 2,700 years ago with 700 grams of quad (high-grade AAAA cannabis indica) − “the good shuzzit,” as Louis Armstrong and his long-time dealer, Milton Mezzrow, called such stuff, and also the earliest physical evidence of human use of the psychoactive form of the drug. The earliest medical evidence of cannabis dates to the same era, also in China, where cannabis tea was recommended for a hundred ailments, including constipation, “female weakness” and absent-mindedness. One early Chinese doctor said cannabis in moderation lets users speak to spirits; in excess, he added, they become demons. Apparently, the advice to have “just one hit” has been ignored for centuries. The first cannabis plants grew wild − so say Robert Clarke and Mark Merlin, in their door-stopping Cannabis: Evolution and Ethnobotany − in the valleys of the Hindu Kush mountains between Afghanistan and Pakistan. From there, cannabis was traded and cultivated across Europe and Asia. Herodotus writes of Scythians (they were early horsemen, and established the Silk Road) “howling with pleasure in their hemp vapor baths.” As Martin Lee observes in Smoke Signals: A Social History of Marijuana (another excellent book upon which this article relies heavily), “Something about the herb resonated with humankind.”

“What is our society missing that we are so keen to bring cannabis back into it? What is it in the DNA of our society that puts cannabis on the front page of our newspapers every day?”

— Jonathan Page, world expert on the cannabis genome and CEO of Anandia Laboratories Inc.

The weed that went west and north, into Europe, tended to be hemp − that is, the leggy cannabis subspecies low in THC but excellent for rope, sails, clothing, paper, “hempcrete” and at least 3,000 other known uses that hemp fanatics will happily list. (Henry Ford built a hemp car that ran on hemp oil in the early 1940s). The plants that moved south into tropical India and Africa tended to be the psychotropic cultivars employed as medicine and for spiritual yayas. Daga has been used in Africa for at least 2,000 years: Zulus smoked it to relax and before battle, while pygmy tribes inhaled it through a mound in the earth and called it “earth smoking.” Today, Canadians vaporize cannabis oil through a water pipe and refer to it as dabbing. We’ve all been doing this a long, long time.

Most marijuana smoked in North America before the 1970s was grown in Mexico. When Mexico (under pressure from Ronald Reagan’s state department) sprayed its marijuana crops with the toxic plant killer paraquat, North Americans began to grow their own − indoors, to avoid detection. The war on drugs created a botanical revolution of historic proportions. Cannabis sativa, which produced a lighter, brighter, “talkier” high, was harder to grow in colder climates; the more narcotic and stonier indica subspecies could be grown everywhere, but it tended to induce couch lock. Growers (many of B.C.’s earliest were U.S. draft dodgers) soon combined the best of both worlds and produced seedless (hence sensimilla) hybrids of the two.

Indoor growing really took off in the 1980s with the invention of metal-halide and sodium grow lights. In 1982, U.S. law-enforcement agencies learned with alarm that the record tonnage of marijuana they seized was 38 per cent larger than the government’s estimate of the entire U.S. national crop that year. Cannabis plants that averaged 5 per cent to 8 per cent THC before the war on drugs were now capable of producing flowers with THC levels of 30 per cent and more.

Canada may be the first industrialized country to legalize cannabis for adult recreational use, but this isn’t the first time cannabis has been legal. From Pakistan, weed made its way around the world: to Europe and England both overland and from India, and to North and South America and everywhere in between via colonizing settlers and the slave trade from Africa. Richard Burton, the explorer and writer, used it for depression. Friedrich Nietzsche used it to wind down, Yeats and Wilde to wind up. Alexandre Dumas and Eugène Delacroix, the painter, smoked it with their famous pals at Paris’s Club des Hashischins.

The first crop of hemp cannabis in Canada was planted in Port Royal (now Nova Scotia) in 1606 by an apothecary accompanying Samuel de Champlain. France pressured the colonies to grow hemp for its navy, exempting the crop from the tithe paid to the Catholic Church, which in turn made the Catholic Church a sworn enemy of cannabis.

William Brooke O’Shaughnessy helped introduce cannabis to modern medicine after encountering the drug in Calcutta as an agent for the British East India Company. Upon his return to London in 1842 with a large stash of cannabis indica, he commissioned the manufacture of Squire’s Extract. Sir William’s chronic tonic actually relieved pain from rheumatism, quelled infant convulsions and calmed the spasms brought on by rabies and tetanus. Queen Victoria’s physician prescribed it for her menstrual cramps, and to others for what we would call Alzheimer’s.

The United States, so rabidly anti-drug for so long, was as keen on cannabis in the past as states such as California, Oregon and Washington are today. Mary Lincoln used it as a sedative after Abe’s assassination. The first two drafts of the Declaration of Independence were written on hemp paper. According to Dana Larsen’s Cannabis in Canada, F.W. Goodwin gave a lecture in 1897 that touted cannabis as a remedy for a raft of ailments, and also recommended it recreationally: It stimulated the appetite; induced sleep; gave users a sense of well-being “as if he had heard good tidings of great joy.” It also enhanced sexual pleasure, inducing a “powerful erection when the necessary mental stimulus is at hand,” which is an interesting way of putting it. However, it reduced the sensitivity of the member, which in turn lessened “premature discharge.” Dr. Goodwin was president of the Nova Scotia Medical Association.

“No one had a problem with cannabis then,” the legendary California cannabis activist Steve De Angelo told me recently. “Because it was being used by white people.”

How did such a popular plant become an object of hysterical hatred? The answer is no surprise given the current standoff at the U.S.-Mexico border: Cannabis became associated with immigrants and foreign labour during what Mr. Lee calls “an early twentieth century upsurge of nativism, scapegoating and political repression.”

Britain tried banning cannabis in the 1800s, over complaints about what it did to work habits on plantations: Slaves grew pot between rows of sugarcane and smoked it to lighten the drudgery of long hours of repetitive labour. By the early 20th century, after the prolonged Mexican revolution drove thousands of fleeing Mexicans into California, prohibitionists invented the threat of marijuana-crazed Mexicans and African-Americans to rail against immigration, tainting cannabis in the process. The anti-pot mania also infected Canada, where the spectre of black men puffing reefer and unemployed Chinese railway workers smoking opium were commingled and demonized by hugely popular writers such as Emily Murphy, who worked for Maclean’s Magazine under the byline Janey Canuck. She was also in favour of sterilizing “inferior” women.

Manias take hold of entire nations. Cannabis has inspired some notable developments in human culture − from Louis Armstrong’s improvisational jazz and Ella Fitzgerald’s scat singing and the Beatles’s Sgt. Pepper album to Jack Kerouac’s novels and Tommy Chong (a Canadian) and a good part of Woody Harrelson’s acting career. But Canada banned cannabis under the Opium and Narcotic Act as early as 1923. The prohibitionist frenzy didn’t abate until 70 years later, when the medical cannabis movement, in the form of compassion clubs, turned up in B.C., partly in response to the AIDS crisis.

By then, in the United States, Mr. Lee maintains, fifty separate government agencies were dedicated to inhibiting research into the therapeutic use of pot. Even in 2003, when Mr. Page took a job in Saskatoon at the National Research Council to study cannabis, he wasn’t allowed to buy or grow any: The NRC was an arm of the government, and it focused on the illegality of cannabis. Recent polls say a quarter of Canadians are still against legalization.

The problem is that prohibition has never worked. In 1937, 50,000 Americans smoked pot. By 1947, the number had doubled. In 2005, the year before California licensed its first six medical-marijuana clinics, more than 750,000 Americans were arrested on marijuana-related charges, most for simple possession, and most of them not white. In all, roughly two million people have been arrested for growing and selling pot in Canada; as recently as 2013, more than 59,000 people were arrested for possession. And yet somewhere between 183 million and 238 million people worldwide have tried or are users of cannabis. In the meantime, the governments of the United States and Canada have spent an estimated US$60-billion a year on the war on drugs. “Thirty years from now,” the writer and anthropologist Wade Davis has said, “the entire war on drugs will be seen as one of the greatest acts of folly in the history of public policy.” Meanwhile a third of Canadians plan to use cannabis when it goes legit on Oct. 17. People are already planning their parties. Big ones.

Cannabis is the only plant known to manufacture THC. It does this by siccing a series of enzymes upon a fatty-acid molecule and transforming it into an acid form of THC that, when it is heated or smoked, becomes psychoactive. (You can eat the leaves raw and not feel a thing.) All this action takes place in the trichomes on the surface of the resin-heavy flowers.

Things get even more interesting once THC is ingested into the human body by smoking or vaping or eating. It turns out − this is a fairly recent discovery − that the human body has its own (very ancient) set of endocannabinoid receptors in our central and peripheral nervous systems. These receptors bind to (among other agents) a neurotransmitter called anandamide, an endogenous cannabinoid − that is, one produced by our bodies. (Anandamide derives from the Sanskrit word “bliss.”) THC binds to the same receptors anandamide does, and does the same general thing anandamide does, except that it does it in a much blunter and more aggressive and more discombobulating way.

And what is that? That’s a question researchers are still answering in full. Like its in-house double, the retrograde anandamide, THC is a signal inhibitor − what Mr. Page refers to as “a kind of presynaptic dimmer switch that reduces the excitation of the neurons.” To put it (too) simply, cannabinoids such as THC (in conjunction with CBD and terpenes and many other cannabinoids, all of which work on one another to produce an “entourage effect”), slow down the messages barrelling back and forth through our synapses. This accounts for many of cannabis’s indicated medical effects − because cannabinoids reduce inflammation and pain, lower blood pressure, can stop seizures and relax muscles and quell epilepsy (and maybe migraines and fibromyalgia), seem to reduce spasticity and stiffness in people with multiple sclerosis, help with cramping, improve sleep, reduce nausea and calm PTSD. They may even be useful in weaning people off harder drugs. Opioids take pain away from patients, along with most alertness; cannabis, as one user once said to me, “takes you away from the pain.”

Or, to put it psychoactively, in addition to lowering the body’s pent-up physical pressures, cannabis spaces out the messages the mind and body send to each other incessantly, which in turn gives us time to notice what’s going on − that sense one has, using pot, that everything is happening for the first and most remarkable time. Michael Pollan, in his astonishing book The Botany of Desire, called it “the italicization of experience.”

Some researchers speculate that anandamide and other cannabinoids may be implicated in helping us forget − the short-term memory problem pot smokers experience − but in ways that are necessary and clarifying. Mr. Pollan thinks the main attraction of THC may be, in fact, the disarray and short-term memory loss it creates in the human brain. “The cannabinoid network appears to be part of that mechanism, vigilantly sifting the vast chaff of sense impressions from the level of perception we need to reach if we’re to get through the day and get done what needs to be done. All depends on forgetting.”

Cannabis does the same thing, just harder and faster. Cannabis makes it impossible to remember all the details that threaten to drown us, and lets us concentrate on them one after the other, laterally and forgetfully. It impairs us, but in doing so allows us to experience the world not as masters of the entire universe but as liberated goofball bystanders, freed from the world’s and our own blinding compulsions and expectations. Physiologically, cannabis disarms the bully Time, quiets its insistent tattoo of tick tick tick − leaving us to respond to the mere moment, possibly while laughing. And not just to respond to it, but to feel it, emotionally. It’s as if THC were a converter that transforms the matter we see and hold and hear and smell and taste into the wow of a more − for lack of a more comprehensive word − spiritual experience. At the very least, THC lets us be in the here and now, and experience the moment not as something to be rushed past but as something worth paying attention to.

The last time I spoke to Mr. Page, he ended our conversation by asking me a question. “What’s so special about cannabis?” he said. “What is our society missing that we are so keen to bring cannabis back into it? What is it in the DNA of our society that puts cannabis on the front page of our newspapers every day?” I thought about that for a while. Then he said “Someone characterized cannabis to me recently as the cure for the human condition.”

Maybe we can’t take our eyes off the cannabis plant because it serves a purpose. Maybe the evolutionary purpose of humanity’s ancient obsession with cannabis is that it can free us (momentarily) from the job of evolution − or at least from the relentless grind of it, of trying to survive with the fittest − which in turn allows us to simply be who we are, briefly, without regret, while high. Maybe it’s a spur to keep going. If cannabis is the cure for the human condition, which is that we are born to die, that we live only in order to sadly leave, cannabis may be all about forgetting. But it might also be a way to repeatedly forgive ourselves for our unwitting part in the calamity of being human.