Around 2,500 years ago, at a cemetery high  in the mountains of what’s now western China,   a group of people gathered for a funeral ritual.

They placed hot stones inside ten wooden  containers scattered around the room,   then laid leaves on top of the stones.

As the leaves started to burn, they filled  the enclosed space with a haze of smoke.

And, as the funeral-goers inhaled,   they found their minds being  shifted to a new, peculiar state.

Thousands of years later, when archeologists  analyzed the charred plant remains,   they realized they had stumbled upon the  earliest known example of people smoking   a plant that over a hundred million still  use in much the same way today: Cannabis.

But how did such a strange plant  come to be in the first place?

When and where did we first domesticate it?

And why oh why does it get us high?

Well, the plant that manages to mysteriously hack   a biological system shared by almost all  animals has undergone some hacks itself.

The story of cannabis evolution  began around 420 million years ago.

Just kidding.

Can you imagine?!

In reality, the deep time origins of cannabis had  been, well, hazy, for a long time, thanks to the   fact that fossil prints of their distinctive  leaves are extremely rare…only 2 are known.

But in recent years, analyses of  specific DNA markers and studies   of fossil pollen have suggested that  the genus got its start around 28   million years ago in the mid-Oligocene  epoch, high on the Tibetan Plateau.

This is when it diverged from  its sister genus Humulus,   which contains species commonly known  as hops - a key ingredient to beer.

That family reunion must be lit.

And the formation of the Tibetan  Plateau may actually have been a   key driver behind the origin of the plant itself.

See, cannabis thrives in what are known as steppe  environments - open, arid, and tree-less habitats.

And when India collided with Asia around  35 million years ago, forming the Tibetan   Plateau, it caused those kinds of steppe  environments to develop throughout the region.

This set the stage for Cannabis  to emerge and flourish.

So, in a sense, plate tectonics gave us weed.

Following its lofty origins  on the Tibetan Plateau,   the plant slowly spread both east and  west, with its first fossil traces   appearing in Europe around 6 million years ago  and eastern China around 1.2 million years ago.

By the time we humans arrived  over a million years later,   Cannabis was already an ancient  feature of many parts of Eurasia.

But it wasn't exactly like the  cannabis we’re familiar with today,   which has been shaped by thousands of years  of human cultivation and artificial selection.

Exactly how and when our relationship  with this strange plant first began,   and how much we changed it over time, has  been a mystery, for a few key reasons… For one, we haven't been able to find any  living populations of the wild ancestor,   if it even still exists at all.

Sure, there are plenty of places where  cannabis grows in the wild today,   but all of those plants seem to  be escaped lineages that were   once domesticated - making them  feral, rather than truly wild.

In fact, the plant’s tendency to go feral and grow  in the wild is where it got the nickname ‘weed.’ Plus, considering its use has  been taboo in many cultures,   a lot of cannabis breeding has  historically been done in secret.

And samples are often hard to legally collect,   making its domestication history  difficult to reconstruct.

Additionally, we humans have  found multiple uses for cannabis.

People not only grow it for  its psychoactive properties,   but also use some lineages for  their hemp fibers and oils.

In fact, for a long time,   taxonomists couldn’t agree on how many  species of cannabis there even are today.

The current scientific view, though, is  that there’s just one: Cannabis sativa,   which contains a variety of  different cultivated lineages.

For all those reasons and more,  tracing the details of the story   of people and pot using conventional  methods proved extremely tricky.

But in 2021, researchers published  a new study using methods that only   became available recently: genome sequencing.

They reconstructed the domestication history of  Cannabis by comparing the genomes of 110 plants   from around the world - including feral lineages  as well as those grown for both hemp and drugs.

And by looking at the genetic differences  between the plants and using molecular   dating to work backwards, they were  able to pinpoint a single origin of   Cannabis domestication in what's  now China around 12,000 years ago.

This makes cannabis one of the single  oldest known plants that we domesticated,   right around the dawn of the  agricultural revolution itself.

For context, this means cannabis was domesticated   earlier than staples like  wheat, corn and potatoes.

Priorities, right?

The genomic analysis also suggested that  early domesticated cannabis was probably   first used as a multi-purpose crop,  with the hemp and drug-type lineages   only separating from one another  genetically around 4000 years ago.

This is when people began deliberately breeding  different lineages for two different purposes,   leaving a strong signal of divergent  selection in the genomes of each type.

The researchers were even able to pinpoint  signals of selection on specific genes   responsible for the characteristic  differences between the two types.

Like, for example, the genes responsible for the   taller and unbranched stems with higher  fiber content of hemp-type cannabis.

And conversely, the genes behind the  shorter, branching stems of drug-type   cannabis with woodier cores, more  flowers, and greater resin production.

After these two cannabis types diverged  around 4000 years ago, we start to find   clear archeological evidence of people using  both types throughout Eurasia and beyond.

Multiple fiber artifacts from hemp-type  cannabis appear in East Asia around this   time and spread westward into  Europe and the Middle East.

The first clear evidence of people using  drug-type cannabis to get high comes from   that mountain cemetery in western China 2,500  years ago, which researchers reported in 2019.

And written evidence from India indicates that   people were using drug-type cannabis  as far back as 2000 years ago - with   it reaching Africa by the 13th century  and Latin America by the 16th century.

Clearly, it’s a plant that we humans  have had a very long history with.

But despite being able to figure  out how and when we domesticated it,   why it gets us high in the first  place is still pretty mysterious.

Is it just an accidental quirk of biochemistry,   or some kind of adaptive evolutionary  strategy on the part of the plant itself?

And if so, what could that  strategy originally have been?

Like, surely the plant doesn't want to be burned   and inhaled or baked into brownies  by a species of bipedal ape, right??

Well, to dig into that question,   we first need to look at exactly how  cannabis gets us high in the first place.

See, the primary molecule cannabis  produces that’s responsible for its   psychoactive effects is called  tetrahydrocannabinol, or THC.

And it’s just one of many chemicals  that cannabis produces, which has led   it to being described as ‘the plant  of a thousand and one molecules’.

Studies have shown that some  of them have antimicrobial and   antifungal properties that help  defend the plant against disease.

Others seem to act as a natural  sunscreen, protecting the plant   from exposure to dangerous levels of  ultraviolet radiation at high altitudes.

Which makes a lot of sense, seeing  as its original Oligocene habitat   was on the Tibetan Plateau, where  UV exposure is especially high.

Plus, there’s also evidence that these  molecules help to deter some insect   pests and grazing herbivores, thanks to their  off-putting taste and disorienting effects.

But we still don’t completely  understand the full ecological   context that drove cannabis to  become such a prolific chemist.

What we can say though, is that  some of those molecules, like THC,   happen to be the right shape to  bind to specific receptors in our   bodies - ones involved in what’s  called the endocannabinoid system.

This is an ancient cell-signaling network  that’s found in almost all animals,   from us primates to sea urchins, possibly  dating back as far as 600 million years.

And the system uses a class of molecules  produced by the body called endocannabinoids.

Many of them are similar to  the ones cannabis produces,   and they regulate a wide range of  important biological functions.

These include appetite, sleep,  mood, learning and memory,   metabolism, and various aspects  of overall growth and development.

Which means that animals had been producing  and using cannabinoid molecules for hundreds of   millions of years before plants like cannabis  independently evolved their own versions.

And these versions, like THC, mimic our  own internally produced cannabinoids.

They bind to the same receptors and influence  many of the functions we just mentioned,   resulting in the characteristic  ‘high’ that cannabis gives its users.

So while the plant originally produced these  molecules for a range of defensive roles,   we humans found their effects…intriguing.

And for thousands of years, through a process  of careful and sometimes clandestine selection,   we’ve been further refining those effects,   making what was supposed to be a  deterrent ever-more attractive.

So, because of its ability to hack an  ancient cellular signaling system of animals,   cannabis became a favorite of many early  cultures and a taboo in many modern ones.

Whatever your view on its use today,   cannabis is a plant that humans have been  growing for longer than almost any other.

And one that we’ve been actively changing