One Sunday morning sun came up and pop out of the egg came a very hungry caterpillar. On Monday it ate through a whole apple, but it was still hungry. On Tuesday it ate some broccoli and died. [MUSIC] Plants are the richest source of nutrients on earth. Bacteria, fungi, insects and animals of all shapes and sizes depend on them for energy. Even animals that eat meat ultimately rely on plants, and we’re no exception! [SALAD SCREAMING] But it’s not always in a plant’s best interest to get eaten. They lack an immune system like we have to fight off foreign invaders, and while some of them put on intimidating outfits, many plants have turned to chemical warfare in order to fight off nibbling and gnawing enemies.
Like Paracelsus said, “the dose makes the poison”, and unfortunately for plants, we don’t really mind many of their chemical countermeasures. We find them delicious, actually, or sometimes stimulating, and in trying to avoid becoming food they’ve accidentally created some of our favorite flavors. Or like you say in Canada and the UK, flavours. Fla-voooours. Take mustard. Now humans love that spicy zing, but insects hate it. Plants in the genus Brassica, which includes a whole host of our favorite edible plants, manufacture chemicals called glucosinolates that lead to bitter flavors. To most insects these spicy compounds are toxic, even deadly, so mustard and its relatives are usually off the menu.
But that’s not true for one group of insects. Cabbage butterflies have stumbled upon a way to detoxify these spicy poisons, their own chemical countermeasure countermeasures it let their caterpillars feed on a food source that’s unavailable to other insects. It’s coevolution, culinary edition. In a coevolution relationship, a change in one organism drives the natural selection of a new trait in another organism. We see this all over nature. The idea of coevolution was introduced about 50 years ago by Paul Ehrlich and Peter Raven. Although even Darwin had predicted it, they were among the first biologists to describe this sort of cooperative evolutionary arms race in detail, and they used cabbage butterflies and Brassica plants as one of their prime examples.
Just this year, scientists were able to peer inside the genomes of these two species and In evolution, new traits don’t just fall out of the sky. Long ago in the mustard plant family, thanks to an error in DNA replication, a gene was duplicated. Since the plant now had an extra copy that it didn’t need, this new gene was free to explore some mutations, try something new, you know, really find itself. In some individuals, that new function was synthesizing a toxin, and the plants with that new trait survived and reproduced better than plants that didn’t have it. Caterpillars who, through their own random mutations, were able to better tolerate or inactivate the toxin also survived better and reproduced more. This cycle continued, with new mutations making more or different toxins, and the hungry caterpillars who couldn’t adapt went sayonara. We see this all over edible plant world. Fruits like apricots and peaches contain actual cyanide in their seeds to defend against herbivores.
Castor beans contain ricin for crying out loud. If you’re an insect, alkaloids like nicotine and caffeine are potent neurotoxins, we even created a whole class of insecticides based upon them, but they’re pleasurable stimulants to us. Citrus plants have devised a second use for caffeine, not using it for defense, but lacing their nectar with the chemical so bees become flying flower junkies. Monarch butterflies have figured out how to use a plant toxin for their own good, the milkweed they eat as caterpillars is poisonous to other animals, but they store away chemicals from the plant’s latex sap inside their bodies, basically becoming flying little poison nuggets. The tannins that give wine and tea their sharp flavor grab on to and disrupt the digestive enzymes of bugs. They also bind to proteins in our saliva, which is why you get that sort of astringent mouthfeel when you drink a glass of cabernet. The hops we add to beer offer antimicrobial compounds along with their bitter flavor, fighting off bad bacteria while letting the brewers yeast flourish.
Most of the defensive chemicals used by plants have a bitter taste, because bitter is nature’s way of saying: maybe don’t eat this and also because plants don’t have hands so they can’t draw that skull and crossbones thing. Not all botanical weaponry is bitter, though. Chili peppers produce a compound called capsaicin, which gives them their fiery burn. But capsaicin only works on certain targets. Feathered dinosaurs, I mean birds, lack the molecular receptors necessary to feel the heat, so they can pop chili peppers like candy, but if mammals like us eat them, with our spice-sensitive taste receptors, then Lucky for us, our bodies have figured out nutritious uses for many of these chemicals, from vitamins to antioxidants. Even though most of our favorite flavorable plants are technically little poison factories, don’t worry, the benefits of eating fruits and vegetables far outweigh any risks. So don’t use this an excuse to not finish your broccoli.