The premise that there is something optimal about the state we evolved in has no basis in what we understand about evolutionary processes. Those are just the conditions that we happened to not die in, not the conditions that are in any way "best" or "optimal" for us. It's like sleeping in late and missing a flight that ends up crashing and assuming that the optimal strategy is to always sleep in late.
Further, in an ever changing environment, which is what make evolution so important, "history" in a genetic sense is completely uninteresting. Things that cause one species to not die in one era can easily be the source of it's extinction in a future one. Megafauna probably had many benefits, pre-human existence, from being very large. But being large and reproducing slowly is what ultimately let to their extinction once the environment changed.