Scientists in California claim to have transformed understanding of Parkinson's disease.
The researchers conducted experiments on animals which were published in the journal Cell. The results suggest the brain disorder might be caused by bacteria living in the gut.
The findings could eventually lead to new ways to treat the condition, such as drugs to kill gut bugs or probiotics, according to commentators.
According to experts, the results opened an "exciting new avenue of study".
In Parkinson's disease the brain is progressively damaged, which causes patients to experience a tremor and difficulty moving.
Researchers used mice genetically programmed to develop Parkinson's as they produced very high levels of the protein alpha-synuclein, which was associated with damage in the brains of Parkinson's patients.
However, those animals with bacteria in their stomachs developed symptoms while sterile mice remained healthy.
In further tests, when bacteria were transplanted from Parkinson's patients to mice, the researchers found more symptoms than with bacteria taken from healthy people.
One of the researchers at the California Institute of Technology, Dr Timothy Sampson said , "This was the 'eureka' moment, the mice were genetically identical, the only difference was the presence or absence of gut microbiota, BBC reported.
"Now we were quite confident that gut bacteria regulate, and are even required for, the symptoms of Parkinson's disease."
According to experts, the study could explain some of the strange coincidences seen in the disease, such as why most Parkinson's patients complained of constipation up to a decade before other symptoms became apparent.
Parkinson's disease is most commonly associated with tremors, stiffness, and difficulty moving, caused by neurons deep in the brain being killed off.
For years, scientists had limited the search for the cause of Parkinson's to the brain, though a growing body of evidence suggests that might be the wrong approach.