On the other hand, I can prove there is no elephant in my garden by dint of an exhaustive search of every cubic foot of it, and determining that the properties of that cubic foot are incompatible with it being part of an elephant, and that it is impossible for an elephant to occupy less than a cubic foot.
Similarly, if for the entirety of human history/prehistory, for the entirety of human range, there is evidence of humans not being environmental angels, I thing we can justifiably assume that such angels never existed, at least not to any appreciable extent.
(Managing that exhaustive search is an exercise for the archaeologist. I shall just stand back here out of throwing range, shall I? Oh, so you're using a woomera, huh? Well, one moment, I shall move even further back!)