April 11, 2003
By NICHOLAS WADE
Deep in the recesses of the
human heart, lurking guiltily beneath the threshold of consciousness, there
may lie a depraved
craving — for the forbidden taste of human flesh. The basis for this morbid accusation, made by a team of researchers in
London, is a genetic signature, found almost worldwide, that points to a long history of cannibalism.
The signature is one that protects the bearer from infection by prions, proteins that can be transmitted in infected meat and attack the nerve cells of the brain. Prions can be acquired from eating infected animals, as in the case of the mad cow disease that in 1996 spread to people in England, but they spread even more easily through eating infected humans.
This fact is known from study of the Fore, a tribe in the eastern highlands
of Papua New Guinea that started to practice ritual
cannibalism at the end of the 19th century. Dr. D. Carleton Gajdusek, who later received a Nobel Prize for his work, noticed
that the Fore were being devastated by a neurodegenerative disease known as kuru. He linked it with their practice of eating the brains of their dead in mortuary feasts. When the feasts were banned by Australian authorities in the mid-1950's, the incidence of kuru declined, and no cases have appeared in anyone born after that time.
The Fore became of particular interest to researchers studying prion diseases at University College, London. In a report today in the journal Science, Dr. Simon Mead, Dr. John Collinge and colleagues say they analyzed DNA from 30 elderly Fore women who had participated in many cannibal feasts before they were banned. Most of these survivors of the kuru epidemic had a particular genetic signature, which was much less common in the younger population, indicating that it conferred substantial protection against the disease.
In support of this assumption, none of the patients who have contracted
the human version of mad cow disease in Britain carry
the protective signature.
The researchers then examined DNA from various ethnic groups around
the world and found that all but one, the Japanese,
carried the protective signature to some degree, and that the Japanese are protected by a different signature in the same gene.
Various genetic tests showed that the protective
genes could not be there by chance, but were a result of natural selection.
implies that human populations in the past must have been exposed to some form of prion disease, the researchers say.
They contend that that prion disease was probably spread by cannibalism.
Besides the example of the Fore, there is "strong
evidence for widespread cannibalistic practices in many prehistoric populations," the researchers say. Frequent epidemics of
prion disease caused by cannibalism in ancient populations would explain the existence of the protective genetic signature in
people today, they conclude.
About half of today's English population has the
protective signature, which may be one among several reasons why so few
people — only 134 in a population of more than 50 million — have contracted the human form of mad cow disease, Dr. Mead
An alternative explanation for the protective signature could be that
early human populations were exposed to prion disease by
eating infectious animals. Dr. Mead said he could not rule out that explanation but added that cannibalism seemed more likely.
The epidemic of kuru among the Fore showed how quickly the protective signature could be selected. Also, "there is no animal
prion disease that appears to cross the species barrier so easily and dramatically," he said.
Dr. Mead, a neurologist, said he was studying the kuru epidemic among
the Fore to learn the incubation time of the disease, a
critical factor in predicting how large the cow-derived prion disease epidemic is likely to be in England. Because of the long
latency time, some elderly Fore continue to come down with kuru.
Dr. Mead's genetic evidence pointing to widespread cannibalism is likely
to influence a longstanding debate among social
scientists. The subject is controversial because many anthropologists, led by Dr. William Arens of the State University of New
York at Stony Brook, say cannibalism is very rarely practiced, but is much more commonly cited as a slur against enemies. The
few isolated occurrences in the archaeological record, Dr. Arens said, could have been instances of survival cannibalism, which
occurs as an alternative to starvation.
The genetic signature found by Dr. Mead and his
colleagues occurs in the gene that makes the prion protein. An abnormal
of the prion protein can make the normal proteins abnormal too, the process that leads to the disease.
The normal gene itself exists in two versions. People who inherit both versions, one from each parent, are protected against prion disease, whereas people who have two copies of either version are susceptible. Usually natural selection will favor one version of a gene over another. The favoring of both versions together is called a balancing selection.
Dr. David Goldstein, a population geneticist and co-author of the report,
said the genetic evidence alone could not determine
whether the balancing selection in the prion gene occurred just once, in the ancestral human population before it left Africa, or
whether it arose more recently, and many times independently, in each of the world's major population groups.