The Origins of Aging
A layman's introduction to the origins of aging, and ongoing investigation into aging in bacteria once thought to be immortal, can be found at the Boston Globe: "For people, aging appears to be the result of damage that gradually accumulates in cells over a person's lifetime. Gene sequences get garbled, for example, and proteins become damaged and take on defective shapes. Cells have a more difficult time carrying out their functions and grow more and more slowly. Yet cells also have a remarkable capacity to repair themselves. They can proofread DNA and destroy defective proteins, replacing them with new ones. So why hasn't evolution favored perfect repair - in other words, immortality? ... To never get old, organisms would have to invest a huge amount of energy in repair. They'd be left with little energy to reproduce. Natural selection would instead favor other organisms that put less energy into repair and produced more offspring. A common solution to this trade-off is to set aside a special population of cells that will reproduce. Our bodies put a great deal of energy into keeping eggs and sperm from becoming damaged. They put much less care into repairing the rest of our cells."
Link: http://www.boston.com/news/science/articles/2008/06/02/aging_is_older_than_you_think/?page=full