Injury, infection, degeneration, and cancer are the 4 major causes of unnatural death. 2 of the 4 – injury and infection – regularly kill before the age of reproduction. Healing and immunity thus have tremendous survival value, and the two are intertwined, in that injury can be an invitation to infection.
By the time Homo erectus made its way out of Africa it came equipped with the heritage of a sophisticated immune system, spawned from the premise of recognizing the difference between oneself and microbial invaders. Such cellular self-recognition was a neat trick, because a tremendous number of essential microbes were along for the ride. Killing them would be killing oneself.