That you have to draw a line somewhere is obvious. Speciecism however draws the line at exactly our own species. We can do better than that. To use a physiological line seems indeed a better approach. It would also be good if we would know where there is a line of feeling pain. Good luck establishing a line. It's one of the things we can do, where we can collectively become more reasonable and emphatic than we were before.

Still, you have to draw a line among living things. It's still not possible to survive without eating plants.

Like I said, our idea – that is, our presumption, short of scientific evidence – of what inner experiences an animal might share with us should be informed by our knowledge of what physiology and behavior that animal shares with us. Orcas are mammals with complex social structures who form extremely close bonds with their mothers [1]. Hence my suggestion that the burden of proof rests with those who claim that an orca would not feel grief should her child die.

Roundworms? Not so much.

[1] https://en.wikipedia.org/wiki/Killer_whale#Social_structure

I'm curious how much of our 'soul' is a product of the relationship between the bacterial biomes in our bodies, rather than just our neurons by themselves.

Even more, how do we know all neurons are equal?

edit: not arguing against your point, just woolgathering.