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  • 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 wouldve 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 - youre 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 youve 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 were 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

  • ofspiciness’, so it’s thought that this mutation helped the shrew expand its

  • diet by side-stepping theburning 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, were 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.

When the first people arrived in the Americas, they encountered many strange new species,

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Chilis如何变得辣(How Chilis Got Spicy (and Why We Love the Burn))

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    joey joey 發佈於 2021 年 05 月 03 日
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