Joe is correct here. The last time that humans lived sustainably was well back in the "stone age." Even back then, however, we had some expanding tribes that got a bit too hunt-happy and wiped out (or maybe just finished killing off) some megafauna species before they figured out how to live peacefully.

I don't know if hunter-gatherer is the best we can do without environmental damage. I suspect we can do more than that, but so far, that's the only system we have evidence of that has worked on the very-long term. We'll see. Or, I guess, maybe we won't.