If the US team is right, they may have found a new route towards treating and preventing neurodegenerative diseases such as Alzheimer's .
Their work, in the Proceedings B journal, lends weight to a scientific theory experts have been chasing for decades.
The story began in the 1950s on a small Pacific Island called Guam.
Many of the indigenous people who lived there - the Chamorros - were dying from a perplexing paralytic disease.
US Army doctors described symptoms similar to dementia, Parkinson's and motor neuron disease.
Post-mortem examinations revealed abnormal collections of proteins in the brain that can also be seen in patients with Alzheimer's.
Researchers began to look at the island environment and what the Chamorros were eating to see if there was any link.
And a hypothesis began to emerge that an environmental toxin called BMAA could be to blame.
Beta-N-methylamino-L-alanine is made by bacteria found on and around the island.
Experts identified it in algae-rich pools of water and in the roots and seeds of the native cycad palm trees.
Fruit bats, known as flying foxes, a local delicacy, which feast on the cycad seeds, also harboured the toxin.
What was missing, until now, was experimental evidence that BMAA could trigger the telltale brain changes seen in the villagers.
So, Dr Paul Cox, from the Institute of Ethnomedicine, and colleagues from the University of Miami fed monkeys fruit laced with BMAA.
After 140 days, they all had abnormal proteins (tangles) in the brain.
The control monkeys, who received only regular fruit, had none of these brain changes.
Dr Cox repeated the experiment with more monkeys - 32 in total - and found the same.
"Every single one that had eaten the BMAA bananas developed the brain tangles," he said, "even the cohort given low-dose BMAA."
"If what we found in these animals holds up in man, then it means a few things.
"We need to protect people from this toxin.
"We have to get very serious about clean water supplies.
"And it may be possible to prevent some other neurodegenerative diseases."
Dr Cox has been focusing on motor neuron disease - a progressive condition that attacks the nerves.
Working with Dr Ken Rodgers and Dr Rachael Dunlop in Sydney, his team has found BMAA mimics an amino acid called L-serine and inserts itself into brain proteins, causing them to misfold and tangle.
And clinical trials are testing whether giving patients L-serine tablets might prevent this.
"We do not know if it will work, but we really hope so," Dr Cox said.
Dr Laura Phipps, from Alzheimer's Research UK, said: "This research in animals suggests that BMAA exposure could directly lead to hallmark features of neurodegenerative disease, providing new insight into the likely cause of this condition on Guam.
"While investigating rare forms of dementia can lead to insights into the more common causes of the condition, further research is needed to understand whether the findings have relevance to diseases like Alzheimer's or motor neurone disease in other parts of the world.
"The research suggests that L-serine could reduce the build-up of toxic proteins in the brain associated with high levels of BMAA exposure.
"There are early stage clinical trials for L-serine in motor neuron disease, and similar trials would be needed to explore whether L-serine could have any benefit for typical forms of Alzheimer's, not associated with the toxin.
"There is currently no evidence to suggest that taking L-serine supplements could help improve symptoms in Alzheimer's disease."
Most cases of Alzheimer's are caused by a mix of age, genetic and lifestyle factors.
The risk can be cut by:
Scientists say they now have good evidence in animals that exposure to a toxin from algae can trigger dementia-like changes in the brain.