How far back...
 

Does anyone here know an anthropologist that might be able to tell me how far back in history could a modern day human go and still be able to breed with human ancestors?

I don't have a time machine and plan to try this but it is a question that has rolled around in what is left of my mind for some time now.

We split from the chimp/bonobo line 5.5 million years ago. Assuming we can't still mate with chimps or bonobos (what are the experimental results of this?), perhaps that would be a good number. Although perhaps it would be possible for chimps, bonobos and us to still mate with our common ancestor??
Could humans mate with Neanderthals?

I'm certainly no anthropoligist, but the time of the rise of modern man (Cro Magnon) is set at roughly 1,000,000 years ago. But any answer to your query is going to be wildly speculative.

Many anthropoligists believe it was possible. Interbreeding between Cro Magnon and Neanderthal has been set forth as a hypothetical (and partial) explanation for the disappearance of Neanderthal, which I believe had the largest brain of any hominid in the known fossil record.

Ever been to New Yawk City?:D

Hey boss we got the piltedown man here drivign cabs here so you know where to stik your theorem.HA

I am a Piltdown Man.

oHHHHHH.you tell me when you have a meltdown then i help you.fffuuiii///

Now don't beat yourself up over it - happens all the time. How often does a guy from S. Texas get to meet a lady from Eastern Europe? It was late, the lites were dim and it had been a hot day.
Reminds me of the time the daughter I thought I'd got home with came out of the other bedroom, saw me making coffee (as my gag reflex started) went in the bedroom and yelled MOTHER!! - Then stood at the kitchen door and said "You're disgusting!" Life at the beach 101

I would not go too far back in time with that idea. The "humans" you encounter may just confuse you with prey and eat you before you can make your intentions known.