Plants have always excelled at the science of biochemistry, drawing sustenance by converting the sun's energy into organic compounds, discovering ways of poisoning or sickening their predators, evolving sights, tastes and smells that enlist animals in their reproduction. But a few plants have hit upon an especially ingenious approach to ensuring their survival, producing chemicals that have the power to alter how humans experience the world.
Cannabis—more commonly known as marijuana—seems to have long ago adopted a strategy of tying its fortunes to humans, appealing in particular to our innate desire to alter consciousness, a desire that spans nearly every culture and historical period. In exchange, humans have gone to extraordinary lengths, often at their own peril, to help the plant grow and reproduce.
Though marijuana has been in use in one form or another for as long as history has been recorded, the plant has undergone its greatest transformation only in the last few decades. Ironically, that change occurred just at the moment when the future of the plant seemed most in doubt.
In the 1960s, the U.S. government decided to crack down on the growing popularity of marijuana by declaring a war on drugs and launching a fierce assault against it. Border agents stepped up interdiction efforts. Crop dusters sprayed poisonous pesticides wherever the plants were spotted.
But rather than surrender, marijuana did what few people expected: It moved indoors, adopting a new evolutionary strategy that not only seems to have ensured its survival, but also has left it stronger than ever before.
The reason is fairly simple. As marijuana cultivation moved indoors, highly skilled gardeners had to learn how to tend their crops in delicately maintained artificial environments, cross-breeding it with other distant varieties and constantly selecting the strongest strains. Protecting these indoor plants from pests and disease requires constant vigilance, often by expensive, computerized systems that monitor every aspect of the environment and ensure the most optimal growing conditions. But the desire for intoxication is so strong that people will take great risks to satisfy it. So thanks to the efforts of these high-tech gardeners, marijuana has become a much faster-growing, far more potent plant than at any other time in its history—and one of the biggest cash crops in America.
But there is another part of this story that is ultimately even more important: Our long relationship with marijuana has caused scientists to ask what it is about the plant that enables it to affect people the way it does—and in their efforts to answer that question, they stumbled upon a whole new network of brain receptors we otherwise might never have discovered. The main psychoactive molecule in marijuana, tetrahydrocannabinol, known as THC, binds to these receptors. But so, it turns out, does a molecule our own brain produces—a discovery that is offering new insights into the workings of our memory, emotion and consciousness.
The human desire for intoxication may have transformed a pygmy weed into one of the most valuable crops in the world. But in a kind of co-evolutionary quid pro quo that is as fascinating as it is surprising, the plant has in turn enabled us to unlock some of the deepest secrets of our own brains.