Scientists find evidence humans were getting high 25,000 years ago: Here's what they used

Humans may have been using mind-altering drugs as far back as 25,000 years ago, long before farming, cities or written language even existed.

Scientists have found what they believe is the oldest known evidence of habitual drug use in the teeth of prehistoric hunter-gatherers from the Indonesian island of Sulawesi.

And their drug of choice is still being used by hundreds of millions of people today.

Researchers believe the ancient foragers regularly sucked whole betel nuts, the seed of the areca palm which contains a psychoactive chemical called arecoline.

The discovery started with something strange in their teeth.

Two individuals, who lived thousands of years apart, had unusually deep, rounded grooves worn into their teeth.

The older individual lived between 16,000 and 25,000 years ago, while the other dated back around 6,300 to 7,600 years.

Scientists eventually worked out that repeatedly holding and sucking a hard, round object could have produced the marks. Betel nut was a near-perfect match.

Then they found something much harder to explain away.

Chemical analysis of the ancient teeth detected arecoline: the main psychoactive ingredient in betel nut.

Researchers even soaked whole betel nuts in artificial saliva and found that simply sucking them was enough to release the chemical and affect receptors found in the human brain.

The effect probably wasn't particularly intense. Betel nuts can produce mild euphoria and increased alertness, while also numbing the mouth.

That last effect may explain why the ancient habit started.

Both individuals had serious dental problems, and researchers believe they may have sucked the nuts to ease tooth pain.

There was just one problem.

Constantly sucking the hard nuts damaged their teeth even further, possibly creating a cycle in which they used more betel nuts to relieve pain partly caused by the habit itself.

The discovery, published in Science Advances, pushes direct evidence of habitual psychoactive plant use thousands of years further into the past.

Betel nut remains one of the world's most widely consumed psychoactive substances, behind alcohol, caffeine and nicotine.