Toxic Fumes
About 10 years ago, I saw M. Night Shyamalan's film The Happening. This film starts with large numbers of people killing themselves for some inexplicable reason. They seem possessed, in a trance-like state, before walking off the ledge of a tall building or jamming a pencil into their own necks. Like most of M. Night's movies, this is left as a mystery for the first part of the film.
Abandon all hope of enjoying the movie's twists and turns, ye who read on. (e.g. I'm going to spoil this thing for you, if you haven't seen it.)
What we later come to find out is that plants are causing all of this odd behavior. Sensing that human beings are a threat to the global ecosystem, and therefore the survival of plants, they manufacture and release an airborn neurotoxin designed to rid the world of the plague of humanity.
It wasn't the worst movie that M. Night has made, but the overall story didn't exactly captivate me. I watched it as one of many DVDs that were flowing in and out of my apartment via the mail in Blockbuster envelopes. (Remember the world when we weren't all just streaming Netflix?)
A few years later, I met a kid in college who was really interested in plants. He told me all about how plants communicate with one another via chemicals. It's not quite the same as the verbal communication that human beings use, but plants are capable of sensing a threat in their environment and generate chemicals as a defense mechanism. In response to a bug chewing on their leaves, some plants will secrete a toxin in the leaves that will make the bug sick. Some will even release a chemical into the air that lets other plants know that an insect is threatening them, so that they can preemptively ward off the insects by secreting chemicals before the insects reach them and take a single bite.
In hindsight, this isn't as crazy as it sounds. I had a landscaping job in college, and on one particular day, a couple of co-workers of mine took weed whackers to a whole mess of plants that were growing around a creek on the property of one of our clients. They emerged from the trees covered in wet trimmings. They learned later that evening that a lot of the plants they had hacked down were poison ivy.
Tobacco evolved nicotine not for our Pleasure to Burn™ but as a neurotoxin to deter herbivores from consuming their leaves. Caffeine evolved for the same reason. I'd be willing to wager that cannabis, with its THC, is probably meant to disorient an animal that eats its flowers so it won't be a repeat diner. Plants use all kinds of chemicals to defend themselves.
What I haven't heard of before is any plant releasing a chemical to directly manipulate human behavior. This has always felt like a stretch.
Recently, I learned about Toxoplasma gondii, a parasite that can infect all mammals. In order to reproduce, it has to get into the system of a feline. Here where things get interesting: if it infects a rodent, there seems to be evidence that the rodent will lose its natural aversion to the presence of cats into an attraction, which will make it more likely to be preyed upon by a cat, thereby making it more likely that the parasite will end up in a host for reproduction.
There seems to be some evidence suggesting that if the parasite infects a human, that human will be more likely to seek out the company of cats. I'm simplifying the details of this, but the key point is that the notion of the "crazy cat lady" might actually have a scientific explanation.
I'm willing to wager that there are dozens, perhaps hundreds, of other parasites that are present in some human beings which are influencing their behavior in some way, but we don't know what they are or just how they are affecting the behavior of the hosts.
The plot of The Happening really doesn't seem so crazy...at least in concept. The one aspect of the storyline that I take issue with is the idea of several plant species around the world suddenly engaging in a coordinated attack on humans via a toxin. This idea is Biblical; think of the Noah and the Flood. God is a central authority figure who could (assuming he exists) decide to wipe out humans if he was dissatisfied with them. This isn't how nature works. The mechanisms of evolution are decentralized, not centralized. A species is separated by some geographical boundary, and on each side of the divide, each population slowly evolves to be better adapted to the conditions it finds itself in. After enough time, the two populations might end up so different that they are reproductively isolated (that is, if they came into contact with each other, they couldn't produce offspring by mating), and so end up becoming different species. The environmental mechanisms operating to evolve each separate population are local. There is no grand plan coordinating these kinds of changes.
So it's possible that a chance mutation in some particular plant might cause it to release a chemical that causes human beings to kill themselves. It's interesting to imagine how this might play out if this really occurred some place in the world. The chemical would have to confer some kind of evolutionary advantage in order to start spreading. It's unlikely this would happen; if there were some particular area where human beings suddenly killed themselves in a zombie-like state, scientists in Hazmat suits would probably come in and figure out what the offender was, and it would be destroyed.
If anything, plants would be more likely to evolve a chemical that had the opposite effect on humans. Human beings are a remarkably successful species, having settled in just about every area of the planet, however unhabitable it may have seemed a few hundred years ago. I mentioned nicotine, caffeine, and THC earlier. The plants that produce these chemicals are grown in abundance all around the world by human beings because they offer us pleasure, not pain or death, and so they have co-evolved with us, piggybacking onto our adaptive capabilities.
A parasite is more mobile, and could wreck this kind of havoc on a species, even humans, who might be capable of detecting it. This actually seems more sly and pernicious than a lot of parasites that infect humans. The Guinea worm is pretty terrible (don't Google that unless you have a strong stomach), but it's obvious to us that a parasite is at work, and we work to get rid of it. If masses of people suddenly got depressed and started killing themselves, this could go quite far before anybody even realized that a parasite was somehow involved. (Tangentially, we currently seem to believe that social media and smartphones are causing such a thing among young people, but we aren't doing much to reduce their usage.)
Of course, there is the small problem that any parasite that destroys its host is also unlikely to thrive. Ebola is not terribly widespread, in large part due to the fact that people that contract it die before they have a chance to spread it to too many other people. What our depression parasite could do, instead, is cause its hosts to seek out the company of happier people, who are less likely to be infected.
I could keep going with this, but I have to cut myself off at some point. I find musing about the possibilities here fascinating. This would be a difficult thing to study; for statistical reasons, a scientist couldn't just do a blood panel of several individuals to find pathogens in their bodies, study their behavior for a few years, and look for correlations. There is probably a very compelling bestselling thriller novel that could be written about some such parasite that affects human behavior in some interesting way, that wouldn't make an evolutionary biologist roll their eyes.
It's probably a story we're all living right now, in more ways than one, whether or not we'll ever know it for certain.
Abandon all hope of enjoying the movie's twists and turns, ye who read on. (e.g. I'm going to spoil this thing for you, if you haven't seen it.)
What we later come to find out is that plants are causing all of this odd behavior. Sensing that human beings are a threat to the global ecosystem, and therefore the survival of plants, they manufacture and release an airborn neurotoxin designed to rid the world of the plague of humanity.
It wasn't the worst movie that M. Night has made, but the overall story didn't exactly captivate me. I watched it as one of many DVDs that were flowing in and out of my apartment via the mail in Blockbuster envelopes. (Remember the world when we weren't all just streaming Netflix?)
A few years later, I met a kid in college who was really interested in plants. He told me all about how plants communicate with one another via chemicals. It's not quite the same as the verbal communication that human beings use, but plants are capable of sensing a threat in their environment and generate chemicals as a defense mechanism. In response to a bug chewing on their leaves, some plants will secrete a toxin in the leaves that will make the bug sick. Some will even release a chemical into the air that lets other plants know that an insect is threatening them, so that they can preemptively ward off the insects by secreting chemicals before the insects reach them and take a single bite.
In hindsight, this isn't as crazy as it sounds. I had a landscaping job in college, and on one particular day, a couple of co-workers of mine took weed whackers to a whole mess of plants that were growing around a creek on the property of one of our clients. They emerged from the trees covered in wet trimmings. They learned later that evening that a lot of the plants they had hacked down were poison ivy.
Tobacco evolved nicotine not for our Pleasure to Burn™ but as a neurotoxin to deter herbivores from consuming their leaves. Caffeine evolved for the same reason. I'd be willing to wager that cannabis, with its THC, is probably meant to disorient an animal that eats its flowers so it won't be a repeat diner. Plants use all kinds of chemicals to defend themselves.
What I haven't heard of before is any plant releasing a chemical to directly manipulate human behavior. This has always felt like a stretch.
Recently, I learned about Toxoplasma gondii, a parasite that can infect all mammals. In order to reproduce, it has to get into the system of a feline. Here where things get interesting: if it infects a rodent, there seems to be evidence that the rodent will lose its natural aversion to the presence of cats into an attraction, which will make it more likely to be preyed upon by a cat, thereby making it more likely that the parasite will end up in a host for reproduction.
There seems to be some evidence suggesting that if the parasite infects a human, that human will be more likely to seek out the company of cats. I'm simplifying the details of this, but the key point is that the notion of the "crazy cat lady" might actually have a scientific explanation.
I'm willing to wager that there are dozens, perhaps hundreds, of other parasites that are present in some human beings which are influencing their behavior in some way, but we don't know what they are or just how they are affecting the behavior of the hosts.
The plot of The Happening really doesn't seem so crazy...at least in concept. The one aspect of the storyline that I take issue with is the idea of several plant species around the world suddenly engaging in a coordinated attack on humans via a toxin. This idea is Biblical; think of the Noah and the Flood. God is a central authority figure who could (assuming he exists) decide to wipe out humans if he was dissatisfied with them. This isn't how nature works. The mechanisms of evolution are decentralized, not centralized. A species is separated by some geographical boundary, and on each side of the divide, each population slowly evolves to be better adapted to the conditions it finds itself in. After enough time, the two populations might end up so different that they are reproductively isolated (that is, if they came into contact with each other, they couldn't produce offspring by mating), and so end up becoming different species. The environmental mechanisms operating to evolve each separate population are local. There is no grand plan coordinating these kinds of changes.
So it's possible that a chance mutation in some particular plant might cause it to release a chemical that causes human beings to kill themselves. It's interesting to imagine how this might play out if this really occurred some place in the world. The chemical would have to confer some kind of evolutionary advantage in order to start spreading. It's unlikely this would happen; if there were some particular area where human beings suddenly killed themselves in a zombie-like state, scientists in Hazmat suits would probably come in and figure out what the offender was, and it would be destroyed.
If anything, plants would be more likely to evolve a chemical that had the opposite effect on humans. Human beings are a remarkably successful species, having settled in just about every area of the planet, however unhabitable it may have seemed a few hundred years ago. I mentioned nicotine, caffeine, and THC earlier. The plants that produce these chemicals are grown in abundance all around the world by human beings because they offer us pleasure, not pain or death, and so they have co-evolved with us, piggybacking onto our adaptive capabilities.
A parasite is more mobile, and could wreck this kind of havoc on a species, even humans, who might be capable of detecting it. This actually seems more sly and pernicious than a lot of parasites that infect humans. The Guinea worm is pretty terrible (don't Google that unless you have a strong stomach), but it's obvious to us that a parasite is at work, and we work to get rid of it. If masses of people suddenly got depressed and started killing themselves, this could go quite far before anybody even realized that a parasite was somehow involved. (Tangentially, we currently seem to believe that social media and smartphones are causing such a thing among young people, but we aren't doing much to reduce their usage.)
Of course, there is the small problem that any parasite that destroys its host is also unlikely to thrive. Ebola is not terribly widespread, in large part due to the fact that people that contract it die before they have a chance to spread it to too many other people. What our depression parasite could do, instead, is cause its hosts to seek out the company of happier people, who are less likely to be infected.
I could keep going with this, but I have to cut myself off at some point. I find musing about the possibilities here fascinating. This would be a difficult thing to study; for statistical reasons, a scientist couldn't just do a blood panel of several individuals to find pathogens in their bodies, study their behavior for a few years, and look for correlations. There is probably a very compelling bestselling thriller novel that could be written about some such parasite that affects human behavior in some interesting way, that wouldn't make an evolutionary biologist roll their eyes.
It's probably a story we're all living right now, in more ways than one, whether or not we'll ever know it for certain.