Jeff was curious about the effect of the evolution package. But he was concerned about testing on some random person like a pizza guy or his mother. Jeff decided if he were to evolve anything or anyone he'd do it to himself.
It was more he wanted to feel the evolution happen to himself rather than see it happen to someone else. It could affect his mind but that was the chance he had to take. He was genuinely curious to see where the evolutionary path could go. Would it be biological or would it be mechanical? Jeff didn't know where the paths of evolution would lead but when he decided to evolve himself he would see where the changes went.
Jeff selected himself as the subject of the change and selected five thousand years. It would be interesting to see what was humanity like then. What path would his evolution follow?