The evolutionary spice trade

Hanson T. 2015. The triumph of seeds: how grains, nuts, kernels pulses & pips conquered the plant kingdom and shaped human history. New York (NY): Basic Books. p. 128-142.

“Scholars often compare the historical craving for spices to the modern appetite for petroleum” (p. 131). The irony is that these scholars were more right than they know. In this context, the quote is referring to the intense desire humans have for spices and how it started wars, lead to exploration missions and made countries rich (p. 129). But as Hanson shows us, spices have been apart of a war for far longer than human history documents. This legendary war pits the Kingdoms Fungi and Plantae against each other. The two species fighting, a fungal seed pathogen and our beloved chili-pepper.

Let me explain.

As we know, seeds hold the potential offspring of plants. Being so, plants protect them with the same vigor as a grizzly protecting her cubs. However, since plants don’t have the luxury of moving, they resort to chemical warfare (p. 139). In chili-pepper terms, their proverbial mustard gas is capsaicin.

The presence of capsaicin has been well studied in Bolivia. Noelle Machnicki, a mycologist who had done much research on interactions between chili-peppers and this fungus walks us through what’s going on. “All modern species [of chilies], no matter how spicy, descended from a mild ancestor” (p. 136). So why did peppers like habanero get so hot? Noelle, tells us that a fungal pathogen infects the seeds, effectively killing their potential offspring. It was the capsaicin that made the peppers resistant (p. 137). So it makes sense for all the peppers to become spicy then, right?

Not exactly.

Capsaicin affects the chilies ability to retain water and make the seeds more vulnerable to ants (p. 137-138). Not to mention it is made from a nitrogen-based structure, an important nutrient for plants. The advantage of capsaicin is that it deters fungal growth on the chili-pepper’s seeds. As in any coevolutionary relationship, the fungi developed defenses. They created their own chemicals, making them resistant to the capsaicin (p. 137). In these circumstances, the peppers with enough capsaicin to resist fungal growth will be selected for thus causing directional selection towards increased capsaicin. These are not always the circumstances though. As most of us intuitively know, fungi grow better in moist habitats, and worse in dry habitats. This is why Bolivia becomes such a good place to study chili-peppers. The topography and climatic variation of Bolivia has both moist and dry habitats, allowing researchers to study the variation of peppers. In moist habitats (or areas better for fungal growth), the peppers must go on the defence, producing high amounts of capsaicin to deter the fungus (p. 138). In arid climates (or areas worse for fungal growth), peppers with high capsaicin are selected against because infection becomes less likely and due to the pepper’s reduced seed production and increased vulnerability to ants. So if you want to find spicy peppers in Bolivia, you’re best to go to wet regions.

However, capsaicin isn’t just evolving with fungi, they’re evolving with the organisms that eat them too. As Hanson mentions, future research is looking at how the birds that disperse chilies don’t seem to be impacted by the capsaicin and the seeds pass through unharmed. Even more surprising, the capsaicin slows down the digestion of the birds, increasing their holding time, allowing their seeds to be dispersed further! (p. 139)

Of course, even we take advantage of the chilies capsaicin. When capsaicin reaches the mammalian tongue, it tricks the body into thinking it has detected heat causing a burning sensation and a rush of endorphins (p 140). However, what’s more interesting is that Noelle tells us small quantities of capsaicin can act as a preservative (p. 140). “[So] people [may have] started eating capsaicin for the very same reason it evolved: to ward of the fester of fungus and rot” (p. 141). This makes me wonder if this is the reason people descended from the South seem to have a greater like for spice than us.

I liked this section of the Triumph of Seeds much more than the first passage we read. Hanson gave great background information on the chili and was able to illustrate his conversation with Noelle very well, even letting us in on some of their banter. An example of this is on p. 136, “I pressed the question, she lauged, and confessed to keeping a bottle of hot sauce in her desk drawer at work. ‘Josh does too!’ she added”. He also sets the scene for their interaction well too. He tells us about her busy life, and talks about how her demeanor shows it. “‘I’m sort of living a double life right now,” she admitted wearily, sipping from a large coffee” (p. 134). Then he talks about how she sparked right up when they started talking about chilies. I can connect to this very easily, and feel I have a larger understanding of who Noelle was and it felt like I was an observer of the conversation, rather than a reader.

The big idea behind this chapter of the book is to show us an example of why some plants invest so much energy into their secondary metabolites. In this case it is to ward off microbes but in other cases (such as poisons) it could be to ward off mammals, whose digestive systems may ruin their seeds. Another point Hanson was trying to illustrate was how evolutionary relationships are a complicated web, not a simple interaction between two species.

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