The inistence that certain animals do not, and cannot, feel pain seems to me to carry about it some of the same whiff as the old insistence that the Earth is the center of the universe. “Lower” animals not feeling pain fulfills a need on our part, a need not to feel any guilt.
Sure, they look like they’re in pain.
Sure, their reaction is indistinguishable from pain.
But it can’t be pain because they don’t “need” pain? When did evolution occur because of “need”? Aren’t structures and systems inherited and modified randomly, and then nautral selection ruthlessly weeds out? What if the pain system comes from other uses, or exists as a side effect of something we do not yet understand, or is a byproduct of some other useful function (like nipples for men)? What if pain is immaterial to evolution, but (like junk DNA) hasn’t been weeded out?
Scientfically, we believe that everything we are comes from earlier ancestors, and most physical features and experiences humans have can be traced to precursors going far back into the past. Yet there are curious exceptions --always about things we’re emotionally sensnitive about, like behavior, intelligence, pain, and so on. THOSE things we always feel are exceptions, and we say they appeared suddenly, full-blown, in us.
Why then the belief that pain sprang fully-formed, like Aphrodite on the half-shell, into humans, or “higher” mammals, or whatever?
Why can’t pain have existed in “earlier” creatures? How would we know? Well, one way would be to observe them and see if they seem to be in pain. Which…is what we do seem to see. Hrm.
I’m not a scientist, but I know scientists are human too. I’ve seen them make some pretty unscientific assertions…an astronomer once told a Washington Post reporter that “It would be perverse of the universe” if his pet theory was wrong. Scientists confidently asserted women had inferior brains at one point. And scientists perform a LOT of “painful” testing on “lower” life forms – they have a considerable stake in not feeling guilty about inflicting pain.
It just seems too pat, too neat to say pain suddenly appeared without any precursors. And it seems like many of the systems we look at turn out to be much “older” than we thought.
I take their assertions that pain cannot exist in these animals with a very large grain of salt. Heck, just recently there was a story that lab mice have been singing complex songs in higher frequencies – presumably, in scientific labs around the world, for decades – and scientists just realized what had been going on.
I’m not saying that scientists are stupid in overlooking the obvious. But they’re human like everyone else. They certainly could be avoiding painful truths – there’s a history of that kind of thing.
Or maybe they’re just responding to stimulus. 
Sailboat