one species using another as a food source, typically killing it
For the answer to this question i don't know yet.
Skeletal system.
Without a proper skeleton, we would not have enough rigidity to properly move. We'd be an awkward pile of skin, muscle & bodily fluids.
Hope that helps!
Do not worry so much about the why of classification. Classification is important because it is a lens through which to see what is there. It is not intrinsically true in and of itself. Distinctions are important only as vehicles for a common base of communication. All things have symmetry. A reflection has symmetry. Symmetry means organized life. Organized life is what happens when RNA replicates. It is the difference between chaos and order
