I think the belief that animals don't feel pain started with Descartes (or was at least popularised by him) because we're "conscious" and they're "not". Since

I think the belief that animals don't feel pain started with Descartes (or was at least popularised by him) because we're "conscious" and they're "not". Since then we've been slowly reverting that belief but probably for reasons of cognitive dissonance the onus has been on proving that each type of animal is capable of feeling pain, rather than the other way around. Personally I think the onus should be to prove that animals don't feel pain, as that seems like the more contentious statement... not that it sounds like a field of experimentation I'd be unambiguously happy about.