Y"know I was thinking of extremeophiles ..the past couple months...off and on... And it was years ago...I watched a slew of documentaries on various native examples... I think it highly likely that alien stubborn species would learn to thrive in hostile conditions... More likely than perfect balanced neutral ones... And compared to humans...there is nothing more alien than a terrestrial extremeophile.. So we may very well pass by whole species or civilizations...and not realize it.