Ok … so it is 98.6 degrees F +/- ? F degrees, whatever. I could liberally interpret the question to be … What factors in the evolutionary process led to this approximate body temperature (why not 70F or 120F) and what are the mechanisms in the body, as evolved, that accomplish this “regulated” result of approximately 97F? … But then again, I’ve never even taken a biology course …
Please, Chronos, don’t start another sig fig fight. My head is still reeling from the last one. 
As for Fox’s question, a simple answer would be that human proteins work best at 98.mumble. Of course, that begs the question, but, um, hey - what’s that over there?!