Moral argument for thiesm

There's a debate on The Secular Web on thiesm vs. naturalism.

Wanchick, the thiest, argues in his opening statement:
"But what makes us obliged not to mistreat humans? After all, if naturalism is true, "a human being is a biological animal," as naturalist Julian Baggini admits. But unless humans have unique moral worth not had by beasts, it seems objective moral truth wouldn't exist. It wouldn't, for instance, be immoral to rape or kill, for animals do so to each other regularly with no moral significance."

There are some major flaws in this argument.

First, Wanchick makes the incorrect assumption that animals who rape/kill each other do so with no moral significance. We can't possibly know what moral significance there is in the animal world. Some animals groups shun a killer. Other animal groups may simply don't have the capability to punish the killer. This is not to state definitively that animals have morals, it is simply that we cannot blindly assume that they do not simply because it's not obvious to us.

Second, humans have developed rules for the good of our species over time. It used to be considered moral to own slaves. Now it is not. It used to be considered moral for a husband to rape his wife. Now it is not. To imply that there has always been a moral significance to these actions (and others) just because humans are human is not backed up by historical evidence. History also undercuts the assumption that objective moral truth exists.

Third, humans have always (and continue to) rape and kill each other. Sometimes society punishes the offenders (murder) and sometimes not (war, execution). Again, this undercuts the concepts of objective moral truth and an anthrocentric moral significance.