From Ouroboros, a look at where evolution has left us in the aging and longevity stakes: "An emerging theme in biogerontology is the idea that lifespan may be determined by the balance between regeneration and tumor suppression. Long-term tissue health demands that damaged and dead cells be replaced, but unlimited replicative potential poses the risk of cancer. Therefore, to prevent tumors, organisms must accept a decrease in regenerative capacity. ... How direct is this connection? In an indirect model, differentiated cells are the initiatiors of tumor growth. p53 limits stem cell proliferation, which in turn decreases the rate of production of new differentiated cells. Fewer cells available to undergo neoplastic transformation means fewer cells available to initiate tumors, with the unfortunate consequence that tissues requiring new cells to maintain homeostasis must go begging. In a direct model, tumor suppressor activity decreases the number of stem cells that might become dysregulated and transformed into cancer stem cells, a relatively new concept in cancer biology." Evolution got us this far, and now its up to us to carry the ball forward to greater longevity through advanced biotechnology.

Link: http://ouroboros.wordpress.com/2007/02/06/devils-bargain-tradeoffs-between-stem-cell-maintenance-and-tumor-suppression/