Abstract
Among the animal species on which observations are available, humans
have a uniquely high lifetime risk to suffer from cancer - over 38%,
compared to less than 10% for all observed other species (except
species suffering from environmental pollution). Peto’s paradox shows
that this cannot simply be explained by mathematical models which view
cancer genesis as a stochastic process, with resulting risks polynomial
in lifespan and body mass - whales have a longer lifespan and about 30
times the human body mass, however their cancer risk remains constant
throughout their life rather than increasing sharply after female
reproductive age as observed in humans. Rather, it is well documented in
the literature that species-specific tumour suppression mechanisms allow
for large lifespan and body mass. Chimpanzees, being closely related to
humans, have a very low cancer risk, and hence the weakness of human
cancer defence is likely to have resulted from the specific development
of homo sapiens. As this weakness appears past the reproductive years, a
prominent hypothesis blames it to antagonistic pleiotropy. However, homo
sapiens having lived in small tribes during most of its development,
natural selection is likely to also have acted at the level of tribes,
and higher degrees of inbreeding would quite certainly have been
detrimental to a tribe. And males of high social status can attract new
reproductive partners again and again until an age that has seen several
generations grow, which in case of a not-so-large tribe would have
considerably narrowed down its genetic pool. Furthermore, lowering
tumour suppression activities might save calories and hence benefit
tribes with limited food production; and individuals suffering from
cancer after female reproductive age could still have made contributions
to parental/grandparental care, while no more being attractive as a
reproductive partner. Is creeping abandon of human cancer defences
evolutionarily favoured?