In this context a polymorphism is a naturally occurring variation
in the normal nucleotide sequence of the genome within
individuals in a population, and an allele is one of two or more
forms of a given gene that control a particular characteristic,
with the alternative forms occupying corresponding loci on
homologous chromosomes. Variations are denoted as polymorphisms
only if they cannot be accounted for by recurrent mutation and
occur with a frequency of at least about 1%.
In general, a heterozygote is a diploid or polyploid individual
that has inherited different alleles at one or more loci and
therefore does not breed true.

In genetics, a "balanced polymorphism" is a genetic polymorphism
maintained in a population because the heterozygotes for the
alleles under consideration have a higher adaptive value than
either homozygote.
Prions are a class of poorly understood proteins implicated in a
number of exotic human neurological diseases and in some common
animal diseases such as sheep scrapie and bovine spongiform
encephalopathy in cattle ("mad cow disease"). Spongiform
encephalopathies are a type of brain disease found in humans and
animals and are characterized by macroscopic vacancies produced
by the disease process (the brain has a sponge-like appearance).
What is remarkable about prions is that they behave as infectious
agents, but they are 100 times smaller than viruses and their
mechanism of replication is unclear.

One human disease in which
prions have been strongly implicated is Creutzfeldt-Jakob disease
(CJD), which appears to have a genetic basis in about 15% of the
cases. All the prion diseases are apparently associated with the
accumulation in the brain of an abnormal protease-resistant
isoform of the prion protein. In other words, an abnormal variant
of the normal prion protein is somehow copied or produced by the
disease process, which can be initiated by introducing infectious
prion into the system.
Of interest in this context, is the disease "Kuru". This disease
is similar to Creutzfeldt-Jakob disease, and is a human
spongiform encephalopathy. Kuru occurs only in the eastern
highlands of New Guinea, occurs more frequently in women than in
men, which apparently coincides with the customs surrounding
cannibalism in a society where the remains of dead relatives are
handled and eaten primarily by children and women. After
cannibalism was outlawed, the incidence of the disease decreased,
and the current consensus is that cannibalism was the primary
mode of transmission of the pathological agent.

The following points are made by John F. Brookfield (Current
Biology 2003 13:R592):
1) The patterns of DNA sequence variation in human populations
are the result of migration, selection and random genetic drift
operating, as a function of the effective population size, on
human populations over the last million years. Given this, the
possibility exists of using the structure of the human
variability to infer something about human migration, selection
and effective population sizes in the past. A new study of DNA
sequence variation at the prion protein locus (Mead et al:
Science 2003 300:640) has been interpreted as suggesting a
sustained heterozygote advantage created by a lifestyle of
habitual cannibalism, implying a new vision of the lifestyles of
our ancestors.

2) One of the most challenging questions about humans is to know
the extent to which natural selection operating in our
prehistoric ancestry has shaped our present phenotypes and
behavior. Evolutionary psychology tries to explain human
behavior in terms of the selective advantage conferred by
individual behavioral traits. This approach falls into the
biological tradition of trying to explain aspects of an
organism's phenotype through the fitness advantage that the
phenotype generates. With humans, however, the explanation of
behavior in adaptive terms is more difficult than in other
species,

as the societies in which we live today are immeasurably
different from those in which our gene pool evolved. While we may
be adept at hang-gliding, the genes that make us so were
unquestionably not shaped by selection against individuals who
hang-glided unsuccessfully in our evolutionary past.

3) For this reason, selective explanations of human behavior
cannot be judged on the basis of the consequences of the
behavior for modern humans, but only in terms of whether the
genes responsible would have been favored by selection during
the prehistoric eras when our genome evolved. For this reason,
many have speculated about the "environment of evolutionary
adaptedness", the hypothetical pleistocene environment to which
our genes were adapted by selection. While some insights into our
lifestyle during the time of the environment of evolutionary
adaptedness can be gleaned from studies of hunter-gatherers
living today, and from the archaeological record, this can never
be known with certainty. This is obviously a problem for the
adaptive interpretation of human behavior, as it is very easy to
hypothesize selection having operated on any arbitrary behavior
by making appropriate guesses about the nature of the environment
of evolutionary adaptedness.

4) In summary: A new study of genetic variation in the human
prion protein gene suggests that balancing selection has operated
on an amino acid sequence polymorphism in the gene during the
last five hundred thousand years, and suggests this is a legacy
of widespread cannibalism by our ancestors.

4) In summary: A new study of genetic variation in the human
prion protein gene suggests that balancing selection has operated
on an amino acid sequence polymorphism in the gene during the
last five hundred thousand years, and suggests this is a legacy
of widespread cannibalism by our ancestors.