Prion diseases are thought to be spread by eating the flesh of contaminated humans and animals in addition to being passed on from one generation to the next through genetic inheritance.
In an earlier study, members of the research team found that humans with one normal copy and one mutated copy of the prion protein were somehow protected against prion diseases, said Mead.
This mutation, or "polymorphism," is found at a certain spot in the prion gene and is known as M129V. Among the Japanese and certain populations on the Indian subcontinent and East Asia, a similar polymorphism called E219K also shows resistance to prion diseases.
Tim White, a paleoanthropologist at the University of California at Berkeley, who is an expert on the study of human cannibalism, said that the genetic link to cannibalism "is permissible given the wide body of evidence for prehistoric human cannibalism."
Fore and Kuru
Mead and colleagues began their investigation by looking at the Fore, an isolated indigenous people who live in the mountains of Papua New Guinea. The Fore, according to oral history, had a custom of consuming their dead at mortuary feasts.
The tradition is believed to have begun at the end of the 19th century and persisted until Western settlers abolished the practice in the 1950s. Men ate the best meat, the muscle, while women and children munched the brain.
From approximately 1920 through the 1960s an epidemic of the prion disease kuru swept through the Fore, killing upwards of 200 people a year. One symptom of infection was uncontrollable laughter, which gave the affliction the nickname "laughing disease."
Women and children were most vulnerable to the disease because they ate the most contaminated parts of the bodythe brains. The scientists report that kuru has not been found in any Fore born after the late 1950s.
Mead and colleagues studied Fore women who had participated in the mortuary feasts and found that 23 out of the 30 women they tested had one normal copy of the prion gene and one with the M129V polymorphism, suggesting that those who survived the kuru epidemic had a genetic resistance to the disease.
The researchers then sequenced and analyzed the prion protein gene in more than 2,000 chromosome samples selected to represent worldwide genetic diversity. They found either the M129K or E219K polymorphism in every population.
Based on additional comparisons across cultures and with DNA from chimpanzees, Mead and colleagues estimate the polymorphisms arose more than 500,000 years ago, suggesting that prion diseases were widespread in early human history. The cause of these diseases is unknown, but the researchers suggest cannibalism as a likely candidate.
"That's just one logical possibility," said Mead. "We know that the Fore practiced cannibalism and had a strong selection pressure in their lifetime. There is no evidence to support an alternative conclusion such as resistance to infectious disease."
The notion that prehistoric human cultures were cannibalistic, however, has its skeptics. Among them is William Arens, an anthropologist at Stony Brook University in New York who argued against the practice in his 1979 book The Man Eating Myth and continues to stand by his argument.
"There is limited evidence for the possibility of cannibalism in prehistoric times," he said. "It is two or three instances and all suggest they were isolated cannibalism, if it took place at all."
Arens says that the evidence for cannibalism is likely related to isolated starvation situations similar to the Donner Party, whose members spent the winter of 1846-47 in North America's Sierra Nevada and ate the dead in order to survive.
Additionally, Arens says that no anthropologists have witnessed cannibalism, not even the anthropologists who report its relatively recent practice among the Fore in Papua New Guinea. From an evolutionary standpoint, he is not surprised at the lack of evidence.
"You would think in terms of evolution that if people ate each other we wouldn't be around," he said. "It's not a good survival strategy, not a way for a species to proceed."
White, who in 1992 published evidence for cannibalism 800 years ago in southwest Colorado in the book Prehistoric Cannibalism at Mancos, refutes Arens' suggestion that cannibalism was not widespread.
"I think that whereas his politically correct devil's advocate position might have been tenable when he published his book, the burgeoning body of evidence for past cannibalism makes it untenable today," said White.
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