The practice of ritualistic mortuary cannibalism used to be common amongst the Fore peoples of Papua New Guinea. When a member of the tribe died, the women in the village used to dismember and prepare the body, which was then eaten. They would often feed bits of the brain to the children and elderly.

It was this custom of eating the brain of the deceased that is thought to have caused the epidemic in the 1950s of “human mad cow disease,” known as kuru, within the Fore peoples. Now, scientists have identified a genetic mutation that likely helped to protect the Fore against developing the illness, a type of prion disease caused by misfolded proteins. This specific mutation was also shown to protect against all other forms of prion disease, such as Creutzfeldt-Jakob disease (CJD).

“This is a striking example of Darwinian evolution in humans, the epidemic of prion disease selecting a single genetic change that provided complete protection against an invariably fatal dementia,” John Collinge from University College London, who co-led the work, told Reuters.

The word ‘kuru’ is derived from the Fore language meaning “shaking death,” and the disease belongs to a class of progressive neurodegenerative diseases that includes the so-called “mad cow disease” bovine spongiform encephalopathy (BSE) and variant Creutzfeldt-Jakob disease (vCJD). They are caused by a specific type of infectious protein called ‘prions.’ These are misshapen proteins that accumulate over time, forming clusters in the brain and irreparably disrupting it. Often this leads to lethal damage.

When exploration of Papua New Guinea intensified during the 1950s, an epidemic of kuru was found to be spreading through the population of the southern Fore peoples, infecting around 2% of the population. As soon as symptoms began, mainly identified through uncontrollable shaking, sufferers had around 6-12 months to live. The prion can, however, incubate within the body for up to 40 years.

Previous studies had identified that if a certain amino acid – codon 129 – in the prion-forming protein was swapped for another, then people were less susceptible to the disease. After studying the Fore peoples, Collinge found that they also have a another mutation, this one at codon 127. In this new study, published in Nature, he discovered that the replacement of the amino acid glycine with valine at codon 127 makes transgenic mice completely resistant to both kuru and CJD.

It’s thought to work by preventing the protein from folding, and thus causing the disease, and could have implications for how we treat not only CJD, but also other neurodegenerative diseases. When BSE infected beef in Britain in the 1990s, there were fears that tens of thousands of people would be infected. So far, only 177 people have died of the disease, though it’s thought that up to one in every 2,000 people might be incubating it.

“Thirty thousand people are silently carrying the disease and we don't know whether they will carry on carrying the disease without developing symptoms or go on to develop the disease,” Collinge told BBC News.