The sin nature was inherited by humanity from Adam. The tendency to sin was within Adam and Eve as shown in the Fall. We really cannot go much beyond that. Rationally and logically, they had absolutely no compelling reason to disobey God. The Garden of Eden was literally a Paradise and they lacked absolutely nothing.
Well...I think according to the thoughts that their bodies worked exactly like ours means they lacked a lot...they lacked freedom from pain, for one. And this is with the understanding that I think: pain -bad, cut my foot and bleed-bad, have a headache - bad. Okay, I'll drop it now. It's too new of a thought to really discuss yet.