>so there has to be a line somewhere.

And that line is human consciousness.

I empathize with humans and animals too(especially mammalian), of course. But I choose not to be swayed by feelings when I think about policy. Preservation of nature doesn't require empathy, and justice among humans doesn't require it after utilitarian principle is chosen.

Domesticated animals are born and live for our utility. As long as we don't consider animal pet ownership as equal to slavery, I don't see how animal experimentation is problematic.