Thank you to Wondrium for Supporting PBS. According to legend, over a thousand years
ago, in the mountains of Ethiopia, there lived a goatherder named Kaldi. One day, he noticed something kind of strange. His goats had been browsing on the berries
of a small tree, and this seemed to make them unusually…peppy - jumping around and unwilling
to go to sleep at night. After trying the berries himself, Kaldi reported
them to others who roasted, dissolved, and drank them. Thus, the cup of coffee was born. Now, whether or not these events actually
happened, we don’t know for sure - but there is a part of this story that’s rooted in
fact. Throughout history, plants that produce caffeine
have been eagerly adopted into the traditions of many different cultures around the world:
from tea in Asia, to coffee in Africa, to guaraná in South America, just to name a
few. And the reason why is pretty simple. It's a seemingly magic molecule that, through
a quirk of biochemistry, happens to make our brains go zoom zoom. Today, billions of people around the world
start their day with it - including me, and statistically, probably you, too! But how and why did the ability to produce
this molecule independently evolve in multiple, distantly-related lineages of flowering plants,
again and again? Its benefit to us is pretty profound - but
what is its benefit to them? Well, as it turns out, part of the answer
is that while it gives us the buzz we need to get going, for insect pests, it's a literal
buzzkill. At the molecular level, caffeine is what's
known as an alkaloid - a naturally-occurring organic compound that’s often bitter-tasting. But to actually make it, plants need a specialized
molecular toolkit to do the work. They start with what’s called a precursor
molecule, which is then modified by a series of chemical reactions carried out by specific
enzymes. Step by step, these enzymes build the caffeine
molecule that we all know and love. And we can tell that caffeine has evolved
convergently in different plant groups in part because the routes that they take and
the enzymes they use to produce the exact same molecule are often different! In addition to tea and coffee, caffeine is
also produced by the guaraná plant, cacao tree, some species of holly from the Americas,
the cola tree from Africa, and even some citrus plants that produce it in their leaves and
flowers. Now, while these are all flowering plants,
they’re not particularly closely related. Coffee and citrus plants, for example, last
shared a common ancestor around 100 million years ago! And in recent years, genome sequencing and
molecular analyses have helped to shed light on how these plants converged on the production
of caffeine via different evolutionary journeys. In many cases, ancient genes that the plants
already had were duplicated and modified - ones that were originally involved in other metabolic
pathways. The enzymes that those genes coded for were
then re-purposed for making caffeine. This process is known as exaptation - where
features an organism already has are co-opted for new roles. It’s a pretty common route by which evolution
generates novelty. Instead of starting from scratch, natural
selection works on existing traits and adapts them for new purposes. Ok, but why did this happen? What were the selective pressures that drove
distantly related plants to converge on the production of the same molecule by tweaking
different genes? Well, for one, caffeine seems to act as a
natural pesticide. The concentration of caffeine found in plants
like tea and coffee is often high enough to paralyze or even kill insects that try to
take a bite, especially when they’re still in their larval stages. And in coffee plants, at least, caffeine concentrations
seem to be highest in new buds and leaves that are still developing - the parts of the
plant that are most vulnerable to being eaten. Beyond insects, experiments have shown that
caffeine also works as a defense against slugs, snails, and some fungi and bacteria, too! But, in at least one case, this defense has
triggered a bit of an evolutionary arms race. A species of beetle called the coffee berry
borer has formed a symbiotic alliance with some bacteria that live in its gut. In exchange for room and board, the bacteria
detoxify the caffeine molecule, which they then use as their main food source. This allows the beetle to feast on coffee
plants without any negative effects. Along with being a pesticide - though not
a foolproof one - caffeine also seems to offer other advantages, too. Some caffeine-producing plants spike their
nectar with small concentrations of the molecule, which seems to actually reward pollinators
that visit the plants, and encourages them to return! In experiments on bees, researchers have found
that once they’ve had a sip of caffeine-laced nectar, it boosts their memory, makes them
preferentially revisit the plants, and even helps them pollinate faster and more efficiently. So, for plants, caffeine is a multi-purpose
molecule with a lot of adaptive benefits. But, for us, it has one main benefit: it gets
us through Monday morning. See, caffeine has a similar shape to a neurochemical
called adenosine. And in our nervous system, one of the roles
of adenosine is to make us drowsy. It builds up over the course of the day and
binds to specific receptors on our neurons. The more receptors it binds to, the sleepier
we feel. But because caffeine is shaped like adenosine,
it can bind those receptors, too, blocking the adenosine. This keeps us from feeling as tired as we
otherwise would. Caffeine doesn't actually give us extra energy,
it just stops adenosine from making us sleepy, at least temporarily. And once we’ve tried it, like the bees,
we keep coming back for more. Just like different kinds of plants convergently
evolved to produce caffeine, many cultures on different continents independently converged
on using it. Now, a lot of these cultures have their own
myths and legends surrounding the first use of caffeine by their ancestors. In some, it began serendipitously - like Kaldi
with his hyperactive goats. In others, it was a literal gift from the
Gods. And, in addition to these oral traditions,
in some cases we also have physical evidence of its early use around the world in ancient
times. For example, Cacao domestication in South
America has been traced back around 5,300 years to the mid-Holocene, based on analysis
of starch grains, chemical residues, and ancient DNA. And in 2016, scientists announced the discovery
of the earliest known evidence of tea consumption, dating to 2,100 years ago. They found the partial remains of leaves buried
in the tomb of a Chinese emperor, which biomolecular analysis confirmed as being tea leaves. In North America, chemical residues found
on pottery fragments in what is now Illinois show that a caffeinated drink made from species
of holly was being consumed as far back as 1000 years ago. We know from later historical records that
this ritual ‘black drink’ had ceremonial importance, and similar drinks are still consumed
by some Indigenous communities today. So while these plants evolved caffeine primarily
to manipulate their insect friends and foes, it was the molecule’s incidental effect
on one species of grumpy, tired, hairless ape that led to their global success. Today, many of these plants are grown in huge
quantities well outside of their native ranges, and they’ve earned their place among our
favorite crops. Without them, life would be, well, generally
just a lot harder. At its heart, the story of caffeine is a story
of convergence - both the evolutionary convergence of plants that produce it and the cultural
convergence of the people that fell in love with it, again and again, and again. Thank you to Wondrium for Supporting PBS. The folks behind The Great Courses are making
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biology of the human body and how we ended up with all these… quirks . Check the links in our description
below to see what’s happening over on Terra, and if you haven’t subscribed to them…we’re
sorry you’re missing out. We’ve gotta thank this month’s brew-tiful
Eontologists! Annie & Eric Higgins, Chase Archambault, Colton,
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Adam Lowe studio. Subscribe at youtube.com/eons for more journeys
in deep time! For insect pests, it's a literal buzzkill The jokes, oh we've got jokes [laughs] I love it, I love it, ok
