  Effective multicellularity requires both cooperation and competition between
constituent cells. Cooperation involves sacrificing individual fitness in favor
of that of the community, but excessive cooperation makes the community
susceptible to senescence and aging. Competition eliminates unfit senescent
cells via natural selection and thus slows down aging, but excessive
competition makes the community susceptible to cheaters, as exemplified by
cancer and cancer-like phenomena. These observations suggest that an optimal
level of intercellular competition in a multicellular organism maximizes
organismal vitality by delaying the inevitability of aging. We quantify this
idea using a statistical mechanical framework that leads to a generalized
replicator dynamical system for the population of cells that change their
vitality and cooperation due to somatic mutations that make them susceptible to
aging and/or cancer. By accounting for the cost of cooperation and strength of
competition in a minimal setting, we show that our model predicts an optimal
value of competition that maximizes vitality and delays the inevitability of
senescence or cancer.
