Natural Selection, Cannibalism, and Mary Tyler Moore

Natural Selection. Cannibalism. Mary Tyler Moore. Only two of these topics will be discussed in this post.

Sorry, Ms. Moore.

While the topics of natural selection and cannibalism may appear to be as unrelated to each other as they are to M. T. Moore, they actually share a surprising connection through something called Kuru. Kuru is a prion disease that occurs when a mutated glycoprotein is introduced to the body. The mutation in the protein, which is more specifically known as a prion, causes it to be misshapen and therefore unfunctional. While a single malformed protein would be relatively harmless, Kuru prions seem to be capable of somehow changing the shape of other glycoproteins within the brain to match their own, creating more prions. Scientists aren't exactly sure how this process happens. They just don't know.

They are fools grasping at straws.

Overtime, as more and more prion proteins become mutated, brain function becomes impaired as motor and cognitive functions continually deteriorate, which eventually degrade to the point where the disease becomes fatal.

Thankfully, Kuru is extremely rare. This is due to a combination of several facts: First off, the Kuru prion cannot survive outside of the body. Secondly, humans are the only known species that can possess and transfer the disease. Last but possibly not least depending on how you look at it is the fact that the only way to get infected with Kuru is by consuming a mutated prion.

Put all those facts together and what do you get??? Cannibalism! Kuru is spread between individuals when the brain of a person infected with the disease is eaten by his fellow homo sapiens. While this fact means that Kuru is not a threat to most communities, it also led to an epidemic in the Fore Tribe of Papua New Guinea in the 1950s and 60s due to their practice of endocannibalism. This tribe would consume the body of loved ones after their death as a way of honoring them, believing that it was better for a person to be eaten by those who cared about them than for the deceased to be eaten by bloatflies and other decomposers (a belief which makes a surprising amount of sense to me).

The tribe ceased their cannibalistic practices as soon as the mode of transmission for the disease was identified, but by this time the Kuru epidemic had already ravished the community, killing approximately 2% of the population per year at its height. The disease also disproportionately affected rural communities within the tribe as they participated in endocannibalistic traditions more often than their urban, western-influenced counterparts, with the South Fore region of the tribe estimated to have lost as much as an eighth of their population to the epidemic.

Let's fast forward 50 or so years, when scientists compared the genomes of dead victims of the Kuru epidemic with the genotypes of living Fore, including about 560 individuals who had participated in cannibalism prior to it being banned yet never contracted the disease. Genetic testing revealed that 51 of these individuals and their descendants possessed a mutated allele seen nowhere else in the world. The corresponding gene for this allele is responsible for the formation of prions, and further testing revealed that the mutated allele prevents the conformational shape in proteins caused by Kuru prions from occurring, rendering those with the mutation immune to the disease.

This is a map showing the distribution of the mutant allele within different regions of the Fore Tribe. It is a truly beautiful map. Look at it and marvel at its glory.

This mutation did not evolve because of the Kuru epidemic, but was likely present in the population prior to the epidemic in low numbers. When the disease spread amongst the Fore, the allele was selected for, because carriers were resistant to the Kuru prion and thus had a greater chance of surviving to pass their genes on to the next generation. The cessation of cannibalism ended the epidemic, making the mutant allele neutral instead of advantageous and ending the overrepresentation of the allele in future generations, but scientists theorize that if the tribe never ended their cannibalistic practices mutant individuals would eventually make up the vast majority of the population. Because of this, the effects of the Kuru epidemic on allele frequencies of the Fore tribe is seen as one of the clearest modern examples of evolution by natural selection within the human species.

While the increased distribution of the kuru-resistant allele after the epidemic has important implications, the mere existence of this allele is significant by itself. The reason for this is that Kuru is not the only disease caused by infectious prions; rather, it belongs to a group of disorders known as transmissible spongiform encephalopathies (TSEs). Probably the best known prion disease seen in humans is variant Creutzfeldt-Jakob Disease, which is often erroneously referred to as "mad cow disease." Since the mutant gene variant seen in the Fore population prevents alterations to the shape of prions within the brain, the allele not only offers protection against kuru but to other TSEs as well. Because of this, researchers believe that studying the effects of the mutant allele can help them develop treatments for people who have these diseases, which currently have no treatments that have been proven effective against them and are always fatal.