Science infers that the instinct for survival evolved for the same reason that science infers that Pluto orbits the sun although science has not directly observed the repetition of either event.
Because we don't know of any factors affecting biological development other than the evolutionary process, and we observe evolution hourly in thousands of labs across the globe each day, and because survivability is at the core of evolution, there is no other "place" for the survival instinct to come from.
My guess is that there are many many more data points indicating that survival instinct evolved than data points indicating that Pluto orbits the sun. I happen to know an evolutionary biologist, so I will try to confirm that suspicion, but the holidays might stand in our way. Shall I attempt to contact her on this matter?