The chapter 'constraints on perfection' from 'The Extended Phenotype' by Richard Dawkins explains why immortality cannot evolve. But it need not violate any physical laws. Ignoring practical implications and whether or not it is desirable, how close are we to being able to 'beat the system' and stave off cell copying errors, lethal genes expressing later in life etc - whatever makes us decay - and, barring catastrophic accidents, live for centuries (environment permitting)? If I cut myself I can repair/replace trillions of cells worth of damage without trying and I would find physical immortality no more 'miraculous'.