For a while, science didn't admit to a female orgasm, either. There was "no evidence" in the sense that no one had bothered to perform scientific observation and collect data. There was common sense, and reasonable assumption, but no studies. Then there were studies.

Common sense tells you and I that animals are conscious to some degree. But as there is no current scientific definition of what the system of consciousness actually is (and I'm speculatively calling it a system) it's difficult to say animals have it through observation. We know they respond to anesthetics just like we do. We know anesthesia can shut off consciousness, so there's some physicality to whatever consciousness is. We just don't have a classification for it yet.