Most animals will in all likelihood not discover knowledge humans have, and they will never know they didn't. There's a difference between humans and other species on earth, but still, it's not inconceivable to imagine there being knowledge out there humans will never discover, and never know they didn't. Considering the size and age of this galaxy, intelligent life having spawned before us, will likely have had ample time to find us if they "wanted". Also, technology a million years down human evolution, let alone alien, would probably look like magic or be unrecognisable to us. If they exist out there, didn't end themselves and or still find exploration interesting, they're probably already here a long time ago, I say.