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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.
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For insect pests, it's a literal buzzkill
The jokes, oh we've got jokes
[laughs] I love it, I love it, ok