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This is where strict materialists paint themselves into an absurd corner.
They need to decide whether man is just part of nature (merely an animal), or whether he is in some way really different.
On the one hand, if nature is all there is - if man is only one animal among others - by what reasoning can we speak of any of our activities as especially un-natural? Why should clear cutting in the Amazon basin or strip-mining in Appalachia be considered less natural than dam-building among beavers or the swarming of locusts? It's just the naked ape obeying his instincts and making maximum use of that big brain and opposable thumb. According to this view there may be any number of rational arguments about the usefulness of certain courses of action (hunting a species to extinction could be seen as imprudent), but there can be no moral objection at all.
The thing is, most materialists can't really live with the meaninglessness of their own philosophy, so meaning is smuggled into their world view in all kinds of invisible ways. People like the idea of rights, but the truth is, there can be no "rights" in a philosophy without "wrongs". Right and wrong are not scientifically proveable concepts, so there can be no material basis for saying that a certain approach to nature is either moral or immoral. If mankind were to totally destroy the earth, the most science could accurately say about the event would be along the lines of "Planet destroyed 9:27 a.m., Greenwich Mean Time."...(with a parenthetic note that contact with other planets will now be vital in order to establish continued funding).
On the other hand, if human beings are somehow outside or qualitatively different from the rest of nature - if we really are such oddballs - why would we expect that human concepts of rights would apply at all to non-human things? And man is different. How many animals do you know of that have been convicted of crimes? An animal might be destroyed as a danger, or trained with rewards and punishments, but no sensible person looks on animals as moral agents, culpable for their actions.
A bear that attacks a human may be shot, but we will generally spare it the indignity of a lecture.
This is not to say humans are properly speaking superior to animals, as if it were a contest. The whole point is that there can be no contest and no comparison. Apes are capable of neither the spiritual grandeur of humans nor the spiritual depravity of humans. They build neither cathedrals nor gas chambers.
If animals are granted human moral status in the form of being given human rights, you can be assured that the end result will not be increased dignity and respect for animals, it will be incrementally diminished respect and dignity for humans. It may be that Congress will be opened to orangutans, but any sane person would have to consider that a step down for the orangutan... and men would be more and more treated as beasts.