Tobacco, marijuana, coca, nightshade*, and a variety of others all evolved to produce substances that are narcotic/poisonous (or have a neurologic effect in humans). My question is why.
Why did these plants evolve to produce what (I assume) are fairly complex chemicals? Is there some benefit for the plant? or is it just the result of a random mutation that doesn’t hurt the plants’ survival in their environmental niche overly much?
The second option includes the assumption that humans (and some animals) experiment with every plant they come across for it’s usefulness. Thus they didn’t evolve for that aspect, rather we sought them out for an aspect that’s an incidental part of their biological processes.
*Yes, I know nightshade refers to an entire family of plants including several common vegetables/fruits that aren’t poisonous (bell peppers, eggplant, etc.)
Many alkaloids serve to deter insects and other herbivores from eating plants that produce them. In fact, nicotine (an alkaloid from the nightshade family) was used as a commercial pesticide because of this effect.
As to the evolution of these chemicals, it’s likely that the complex interaction between plants and herbivores (who themselves were adapting to such chemicals due to selective pressures) suggests that your second scenario ultimately played a role in the development of narcotics.
One fairly recent example is cannabis. For obvious reasons, selective breeding by humans has led to the development of plants with much higher levels of THC than would have existed in nature. While this has not led to a different chemical structure, you can see how such a process could lead to the evolutionary development of psychoactive compounds.
Plants produce neurotoxic substances in order to poison animals or fungi that try to eat them. It’s really that simple. Humans aren’t especially the target, and in general plants don’t give a toss about humans.
Plants don’t want to be eaten. Even if being grazed/browsed doesn’t kill the plant directly, it opens it up to infection and makes it less competitive. So any plant that can evolve chemicals that disable the nerve system of animals that eat it is at a huge evolutionary advantage.
The fact that humans can sometimes manage to get some sort of high out of those chemicals is entirely coincidental. We are simply trying to get a small enough dose to disable some nerves, without getting a high enough dose to disable the nerves that lee us breathing. We can do that precisely because we don’t feed on those plants, so we can regulate the dose. Caffeine or nicotine give us a buzz because we consume tiny quantities of the leaf. If we tries to consume enough tea or tobacco leaves to get any measurable nutritional value we would be dead in short order.
We take this to the most extreme with the use of “Botox”. The substance itself is produced by a soil bacterium, and certainly didn’t evolve for human use. In natural conditions, the toxin is very effective at killing protozoa and fungi that both compete with and prey upon the bacterium. The fact that it also disables human nerve cells is due to the shared evolutionary pathway involved between all us aerobic eukaryotes. What it *evolved *to do was stop fungi and protozoa from pumping digestive enzymes in to the environment, which has obvious advantages to a bacterium living with fungi and protozoa. The fact that human nerve cells use the exact same mechanism to pump signalling chemicals into the gap between nerve cells is the result of shard evolutionary history. Nonetheless, it means that our nerves are strongly affected by the toxin.
Now, we’ve exploited this effect to use *Botulinum *toxin to treat diseases and for cosmetic reasons, but we do that by very carefully regulating the dose. The chemical didn’t evolve randomly, it evolved to kill predators. But humans aren’t the target, we’re just bycatth because we are related ot the predators it evolved to kill. We are similarly effected by compounds like nicotine, THC and so forth because we are related to the caterpillars and cows that those substances evolved to kill.
While I agree with you overall, you might want to ask wheat, rice, grass etc how they have done out of being eaten by people (or eaten by animals eaten by people).
Isn’t this question (and most others framed the same way) akin to “why did ash trees evolve to make great baseball bats?”? (Not sure how to punctuate that ending…)
I don’t see any logical, meaningful way to connect the compounds a particular plant produces with the way they affect another life form not part of its life cycle. Of the billions of organic compounds on earth, some taste good, some make us feel good and an awful lot of them kill us… but to impute that there’s any “reason” or design to that distribution is, IMHO, nonsensical.
In the wild, sure. Domesticated plants have “figured out” that a better survival strategy was to be adapted in such a way that the majority of seeds will be eaten, but some proportion will be deliberately replanted in favorable conditions.
Also, plants that produce fruit want that fruit to be eaten- hence the bright colors, strong pleasant aromas and usually sweet taste.
But yeah, the majority of plant compounds are either things like antifreeze or are defensive against insects, animals and fungus.
Some other reasons plants have chemical compounds that humans have found useful for their own purposes: Infection control, humidity control, hormonal effects, wound healing, communication across distances (some plants will release chemicals into the air which other plants will pick up, and those chemicals fit into receptors which change something the receiving plant is doing - make more antifreeze, make more bug killing stuff, etc.) and attracting or repelling insects, birds and animals
By and large, the poisonous and recreational stuffs are alkaloids, which make plants bitter, reducing the chances they’ll be eaten prematurely and act as pesticides against insects. It doesn’t always work, obviously - some animals aren’t bothered by the alkaloids at all, and some (humans especially) seek them out, but evolution is a constant arms race.
As has been said, the presence of such compounds is adaptive to the plants that make them, because they serve to deter insects and other herbivores that try to eat them.
The effect of the compounds on other animals, such as humans and other mammals, is fortuitous but not exactly “coincidental.” That is, many of these compounds affect the nervous system, so it is not a matter of chance that they might be psychoactive in other organisms.
Recently I read an article about Alkaloid Content of honey.
It is believed that plants produce Alkaloids to make some “vulnerable” insects and mammals Addicted. So theses animals seek out those plants to get the alkaloid.
This has an evolutionary advantage for the plant because the plants are badly in need of vectors (bees or mammals) to transfer their Spores. Thus Alkaloid producing plant lure their vectors to prefer them to other non alkaloid producing Plants to feed on.
A related question: Why are hot peppers hot. The selective advantage to fruit-bearing plants is dispersal by animals that eat them and pass the seed undigested. But what is the advantage of a chemical such as capsaicin, that makes the fruit unpalatable? Similarly, what is the advantage to deadly nightshade of having their fruit be poisonous. Fruit wants to be eaten, to be teleological.
The plant found a defense that happens to be poisonous to its enemies and innocuous to its friends. Exactly what would be predictable. That it also has effects in species irrelevant to its life cycle is irrelevant.
I realize this is a semi-zombie and I’ve already said much of what I think in prior posts, but I am always a little troubled by simplistic questions of “Why did X evolve [to do Y]?”
In some, maybe many cases, you can draw a pretty straight line between need/purpose and evolved feature, and it makes sense in light of natural selection. But I think all that logic goes astray when you try to connect the complex chemicals lower life forms have developed with, say, psychoactive response in humans. Opium did not “evolve” to give humans fantastically warm, satisfying sleep - and there are ten thousand times more complex chemicals that do nothing, or are toxic, or even fatal to humans. You might as well question why milkweed sap is benign to humans. (At least, I think it is - substitute your own plant goop.)
Isn’t it fair to say that plant chemicals that are not intended to make the plant attractive to pollinators and seed-carriers are intended to be toxic (for those plants that don’t “want to be eaten”) - and that some forms of toxicity just happen to make humans feel real, real good? No particular evolutionary path intended or taken?
I’d also read that capsaicin in chilies evolved the way it did because mammals tend to chew up the chilies and destroy the seeds, while birds tend to swallow and poop the seeds out.
So maybe the initial capsaicin evolved as an anti-fungal, and it turned out also to be beneficial in terms of allowing the less destructive consumers of the chilies to eat it, while repelling the ones that destroyed seeds.
