This is true.    But you're talking about universal common descent, not evolution.    Evolution is an observed phenomenon.   A fact.  Macroevolution is an observed fact.   But until we could analyze DNA, we weren't certain that common descent was true.   Even Darwin suggested that there might have been any number of originally created organisms.    We understand how evolution works.   While every new bit of evidence continues to confirm universal common descent of all life on Earth, we will never be absolutely certain.  Being 99.9% sure, is not absolute certainty.    Likewise, we can't be absolutely certain the sun will continue to shine tomorrow.    But we are very, very confident that it will.   For the same reason we are very, very confident of common descent.