I generally agree with Hamster King’s post #2 above, along with Dr. Love’s addendum that the capacity for humans to assign value to things is a result of natural selection.
I would just like to add that the distinction isn’t simply one of humans versus non-humans, it is a hierarchy of value that begins at the individual and extends outwards. Personally, I value my wife and son the most, probably myself next, then close friends and family, more distant friends and acquaintances next, and so on. Then we have, say, locals versus foreigners. By the time we get to poor starving Africans and other third world miserables, I hate to admit, their value becomes largely theoretical. In fact, I think a large division in the hierarchy is that between real, physical people we’ve seen and interacted with and those we only know of second-hand. As the value tree spreads, we assign value to animals, plants, inanimate objects, etc.
A lot of arguments are caused simply by two people having wildly different hierarchies of value; either their value trees have elements out of order, or one tree is ‘squished’ or ‘stretched’ relative to the other. So we have, for example, racists who believe blacks are subhuman beasts of burden on one extreme, and PETA and their ‘sea kittens’ on the other. I’m not sure there is any way to resolve the inevitable disputes, except for aiming at a reasonable middle path the majority can follow.