LONDON, April 10 (UPI) -- Prehistoric humans might have suffered repeatedly from deadly mad cow disease-like epidemics due to widespread cannibalism, an international team of researchers reported Thursday.
Such ancient brain-melting diets would help explain why many modern-day humans possess a genetic resistance against such diseases, the researchers said. In such a human-eat-human world, those who survived such outbreaks passed whatever genes helped in their survival to the offspring they bore -- natural selection in action -- they explained.
"What we're showing here is evidence that selection for these (disease-fighting traits) has been very widespread or happened very early in the evolution of modern humans, before human beings spread all over the planet," said lead researcher John Collinge, a neurologist at London's University College.
Mad cow disease is a fatal infection that can render the brain spongy with holes. Like other so-called "transmissible spongiform encephalopathies," or TSEs, such as scrapie in sheep, chronic wasting disease in deer and Creutzfeldt-Jakob disease in humans, mad cow disease is linked to sugary proteins known as prions found throughout the brain and body.
In victims of TSEs, prions become malformed and begin to clump together. Although prion diseases sometimes occur spontaneously, in many cases victims contract them by eating contaminated flesh.
Collinge's team first looked at kuru, a lethal prion disease that ravaged a small New Guinea tribe named the Fore. Until 50 years ago, investigations revealed the Fore cannibalized their dead at "mortuary feasts," a ritual that began around the end of the 19th century.
After the first remembered case of kuru among the Fore around 1920, which scientists suspect occurred spontaneously, the disease attacked rapidly, ultimately killing nearly one in 10 of the remote highland villagers. When cannibalism was banned, kuru eventually disappeared.
Humans normally receive two copies of every gene for each protein, one gene from each parent. Prior investigations by Collinge and others revealed people with one normal copy and one mutated copy of a prion were somehow protected against kuru and Creutzfeldt-Jakob disease. Researchers suspect having identical prions makes it easier for abnormalities to occur.
In findings appearing online April 10 in the journal Science, Collinge and colleagues in Papua New Guinea and Australia reported DNA analysis of the Fore and 1,000 representatives of ethnic groups worldwide revealed all populations carried two versions or "polymorphisms" of the prion gene.
Many gene variants improve a person's fitness more than others, with the less helpful variants becoming rare or disappearing over time. Genetic analysis of cultures worldwide, coupled with DNA comparisons among chimpanzees, suggest the mutations have survived in humans for some 500,000 years, which means they "must have undergone strong natural selection," Collinge explained.
Although such prion disease resistance could have emerged from eating contaminated meat other than from humans, Collinge noted "there is extensive anthropological evidence that cannibalism is not just some rarity that happened in New Guinea," citing telltale scratches and burn marks on Neanderthal bones and biochemical analyses of fossilized human feces.
Neuroscientist Paul Brown, a senior investigator at the National Institutes of Health in Bethesda, Md., found this interpretation "historically intriguing," though arguable.
"This kind of research has the potential to illuminate the past, and is just interesting ... by itself," he told United Press International. "It would be interesting to see if you could find out how long a time span of cannibalism you might have to maintain (to develop) the polymorphisms you see today."
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(Reported by Charles Choi, UPI Science News, in New York)
