字幕列表 影片播放 列印英文字幕 When the first people arrived in the Americas, they encountered many strange new species, from armadillo-like animals the size of a car called glyptodonts, to the humble but nutritious potato. But they seem to have been struck by one group in particular - a genus of plant with the bizarre ability to make mammals feel like their mouths are on fire: Capsicum, better known as the chili peppers. And starting about 6,500 years ago, archaeological and genetic evidence show that groups of people throughout the Americas independently domesticated different chili peppers over and over again This makes it one of the oldest known domesticated plants in the Americas, and possibly the oldest domesticated spice ever While humanity’s love affair with chilis has its roots in ancient Mexico, Central and South America, they have now reached every part of the globe. from their native home in the Americas to Europe, Africa, and Asia. But how and why did chilis evolve this weird, fiery trick in the first place? And why did we learn to love that spicy burn? Well, like many of the best love stories, it happened against all the odds. From an evolutionary perspective, our relationship with chilis was never supposed to be. The story of chili plants and their unique red-hot fruit began in the middle of the Miocene 0:01:21.457,1193:02:47.295 Today, chilis are the most widely cultivated spice crop in the world - grown everywhere Epoch, somewhere between 10 and 20 million years ago, when the genus split off from its closest relatives and developed its characteristic spiciness. Researchers think Capsicum originated in western or north-western South America, around Peru, Ecuador, and Colombia. Over millions of years, it spread and diversified throughout South America and eventually expanded north into Central America and Mexico By the time people arrived, the genus contained dozens of wild chili species that were widespread throughout the region. Now, the fiery kick that many chili species are known for comes from a specific compound unique to the genus called capsaicin, which is mostly produced in the tissue that surrounds its seeds. And exactly how these plants make capsaicin is still kind of unclear, but it looks like a few of their genes went through a series of rearrangements and duplication events over time. These duplications allowed the extra copies to evolve new functions, like making capsaicin. But, producing this compound comes at a cost to the plant: it’s a pretty large molecule that requires valuable resources, like nitrogen, to create. And some studies suggest that spicy chilis that make capsaicin seem to be much less efficient at using water than non-spicy ones. This means that when there’s less water available, like during a drought, spicy chilis do much worse - like producing half as many seeds as their non-spicy relatives. So, why did some chilis become spicy in the first place? Well, there’s evidence that the advantages of capsaicin may outweigh the costs. The compound seems to protect the plant from certain insect pests and plant pathogens, including a devastating plant-killing fungus. Studies have shown that spicy chilis are much less affected by the fungus than non-spicy ones, and the natural geographic distribution of spicy chilis matches up pretty well with the distribution of the fungus. And, the fungus thrives in wet environments - exactly the places where the reduced water efficiency trade-off that comes with being spicy is less of a big deal for the plants. This antimicrobial trait of spicy chilis may have been one of the key reasons that people were so quick to domesticate them over and over - they would’ve been a valuable way of keeping food fresher, longer. And we have some archaeological evidence of this. In 2007, researchers identified microfossils of starches from domesticated chilis from seven sites throughout the Americas, always alongside microfossils of maize. The oldest evidence of these two foods being associated goes back 6,100 years to a site in southwestern Ecuador. And while our relationship with chilis was clearly useful...and delicious, from an evolutionary perspective, it was never meant to happen. Because! Another one of the probable functions of capsaicin was to keep organisms like us away. Like many fruiting plants, the seeds of chili peppers are spread, or dispersed, by animals. An ideal seed disperser, from the plant’s point of view, doesn’t have teeth that might crush the seeds, has a digestive tract that doesn't destroy them, and has the ability to disperse the seeds over a wide area. For chili peppers, that meant birds were the best candidates. So early chili pepper plants faced a challenge: how could they keep other animals, like mammals with seed-crushing teeth and small dispersal ranges, away, while still being attractive to birds? Enter capsaicin. You see, this compound binds to a receptor in mammals called TRPV1. This is an ancient receptor that appeared early in the evolution of vertebrates - over 400 million years ago - and is widely shared among vertebrates living today. Its function is to sense dangerously high levels of heat and warn the organism by stimulating a painful burning sensation Through an oddity of biochemistry, capsaicin is also able to activate this receptor, which tricks mammals, including us, into feeling like their mouths, stomachs, or skin are on fire. It doesn’t actually cause any physical damage - it’s just a sensory illusion created by hacking an ancient pain pathway shared across many different species But the same heat-sensing receptor in birds has small structural differences that make it insensitive to capsaicin. This means that while mammals quickly learned to stay away from this group of irritating fruits, birds remained completely unaffected, and continued to eat the fruit and disperse the seeds far and wide. This is known as the Directed Deterrence Hypothesis, and the trick worked well for millions of years... Until, of course, we came along, took a bite of a chili, and thought to ourselves, “It hurts so good!” The rest is history. And a key reason that we learned to love the pain may be because, in a sense, it kind of gets us high. When TRPV1 is triggered by capsaicin, your nervous system is completely fooled into perceiving dangerous levels of heat and sends a message like: “Hey you, so….uh.….some bad news - you’re on fire right now. No worries...sending some chemical relief your way to help you through these trying times.” Levels of two neurotransmitters suddenly rise - endorphins, which help to reduce pain and stress, and dopamine, which gives a sudden rush of pleasure following the initial pain. Their rapid release makes eating hot peppers a sensory rollercoaster - one that potentially becomes enjoyable, you know, once you’ve been through it a few times and realized that the danger isn’t real. This is an example of a ‘constrained risk’ - where our body thinks we’re in danger, but our mind knows there’s no actual threat. The thrill of the experience and the rush of chemicals that comes with it is pleasurable, and even kind of addicting - think horror movies and bungee jumping. And this may be why our appreciation of chili peppers and their heat generally increases with time and exposure. The more chilis, and the hotter ones that you eat, the more you come to like them and tolerate ever-hotter varieties. The burning sensation doesn’t disappear, but you increasingly associate the pain with the thrill of the experience Only one other mammal is known to snack on chili peppers - a treeshrew. These little guys aren’t thrill-seekers like us. Instead, their tolerance of the heat comes from a mutation in their TRPV1 receptor that gives them reduced sensitivity to capsaicin - much like birds. Their range overlaps with a plant in Southeast Asia that has independently evolved a form of ‘spiciness’, so it’s thought that this mutation helped the shrew expand its diet by side-stepping the ‘burning defence’. The same adaptation works on Capsicum too, so the treeshrew will happily feast on chilis all day long. But, aside from that one exception, we’re the only mammal that isn’t scared off by that burning sensation. And this wouldn’t be a very good love story if Capsicum didn’t also get something out of the relationship too. From a certain point of view, Capsicum is the real winner here - while their spicy defences may have originally evolved to keep us away rather than to attract us, our infatuation with them has definitely worked out in their favour. Without us being very weird heat-loving, thrill-seeking mammals, it’s pretty unlikely that the genus would be as widespread as it is today. So, in a sense, becoming spicy was key to its eventual global domination. After millions of years of seed dispersal by birds, chilis found their new disperser of choice in the Holocene Epoch: a species of Great Ape that would take them all over the world. Move over, birds, we are the seed dispersers now. Now, we assisted chilis with their global domination, but what about us? Find out more on our episode, “When We Took Over the World”. And thanks to this month’s Eontologists for being the spice of our life: Sean Dennis, Jake Hart, Annie & Eric Higgins, John Davison Ng, and Patrick Seifert! You can become an Eonite at patreon.com/eons to get fun perks like submitting a joke for us to read, like this one from Sarah M Why was the palaeontologist laughing? Because they found this humerus. And as always thank you for joining me in the Konstantin Haase studio. Subscribe at youtube.com/eons for more evolutionary escapades.