We know about the devastating effect that Alzheimer's and other types of dementia can have on people – but what's less clear is how they get started in the brain and what can be done to cure them or prevent them from happening in the first place.

Some clues could be found amongst indigenous people who live in the Amazon rainforest. In a new study, two of these tribes have turned out to have some of the lowest rates of dementia in the world.

One of the suggestions prompted by such a discovery is the idea that there might be something in our post-industrial life that brings on higher rates of these brain disorders. While it doesn't mean we should all take up hunting and gathering, it's a consideration for those studying dementia and its impact.

Two groups from the Bolivian Amazon were studied: the Tsimane and the Moseten people. Dementia rates among older people were found to be around just 1 percent, compared with 11 percent of those 65 and older in the United States.

"Something about the pre-industrial subsistence lifestyle appears to protect older Tsimane and Moseten from dementia," says psychologist Margaret Gatz, from the University of Southern California (USC).

The assessments of dementia were made with a combination of computed tomography (CT) scans, cognitive evaluations, and questionnaires tailored for the culture they were being targeted at.

While dementia rates were relatively low, a less severe form known as 'mild cognitive impairment' (MCI) was about as prevalent as it is in high-income countries like the US: around 8 percent of Tsimane people and 10 percent of Moseten people over the age of 60 were found to be showing signs of it.

The next avenue of enquiry for researchers is to look more precisely into what's causing the disparity. Previous research in other parts of the world has dementia prevalence ranging from 0.5 percent to 20 percent in indigenous older adults.

There's obviously a huge amount of difference between a post-industrial lifestyle and that of an indigenous Amazonian tribe – in terms of lifestyle, diet, exercise, sanitation, medication and more – but it's difficult to pick out individual risk factors from the pile.

What's more, some indigenous groups have adopted more contemporary lifestyles than others: the Moseten people are less isolated than the Tsimane group, for example, living closer to towns and schools, with better access to clean water.

"What we do know is the sedentary, urban, industrial life is quite novel when compared with how our ancestors lived for more than 99 percent of humanity's existence," says anthropologist Benjamin Trumble, from Arizona State University.

Even though there's still a lot we don't know, our understanding of dementia is improving. Previous studies have shown that both physical inactivity and increased air pollution can play a role, for example, and they might be involved here.

One clue that might be offered by the new research is the unusual calcifications or hardening of the brain arteries found in those participants with dementia or MCI. Scientists are still trying to understand how blood flow and dementia might be linked.

Then there's heart disease and a possible association with brain aging. The Tsimane people, for example, are known to have particularly healthy hearts in old age compared with the rest of the population.

"We're in a race for solutions to the growing prevalence of Alzheimer's disease and related dementias," says anthropologist Hillard Kaplan, from Chapman University in California.

"Looking at these diverse populations augments and accelerates our understanding of these diseases and generates new insights."

The research has been published in Alzheimer's & Dementia.