Humanity's opinion has been changing over time. It was once acceptable to do tests on people of other races, other social classes, prisoners of war, slaves, etc. When it was decided otherwise, it became "more acceptable" to do tests on non-human animals. Today the prevalent debate points to it being less than acceptable and probably into the near future it will be as frowned upon as testing on humans. What next?
Today certain branches of science do tests on bacteria. Biotechnology wouldn't be as it is today without modifying bacteria to do our bidding, killing more of them than imaginable.
It was once thought that "lesser humans" don't feel pain or fear the same way. It was then thought that "animals" don't feel pain or fear the same way. Today most likely you and I will believe that bacteria don't feel pain or fear like we do. But do they?
Will this debate ever end?