Enquiry into the evolution of aging aims to explain why almost all living things weaken and die with age. There is not yet agreement in the scientific community on a single answer. The evolutionary origin of senescence remains a fundamental unsolved problem in biology.

http://en.wikipedia.org/wiki/Evolution_of_ageing

The biological mechanisms which regulate lifespan evolved several hundred million years ago.

edit: Regarding dogs: http://www.insidescience.org/content/large-dogs-age-faster-d...