Whether it’s circular reasoning or not, it is apparent to me that John and Doug are not using the term “human nature” in the same way. If human nature is what the naturalist says it is, then I agree with Doug - discovering any objective morality in nature is a contradiction.
A moral principle tells us how we ought to act and how we ought not to act. How we should act and how we should not act. But the naturalistic/materialistic worldview demands that animals, including human beings, act only according to their genetic makeup and the application of external forces. When an adult human male molests a child, he is acting according to his genetic makeup and whatever outside forces caused him to act as he did. That’s it.
If how we ought to act is equivalent to our human nature as the naturalist understands it, then the molester acted morally. And the human being whose genetics and other outside forces compelled him not to molest children, and in fact to believe that molesting children is unacceptable, then he is also acting morally. How we act and what we believe is always in accordance with human nature in the naturalist’s worldview. That means that the one who believes and acts as a child molester and the one who believes and acts contrary to the molester are both acting morally. That is moral relativism. There is no discovering how we should or should not act in nature, only how we do and do not act. However we act is moral by necessity for the naturalist.
On the other hand, I don’t believe that this is how many posters here are referring to human nature. The theist presupposes that human nature operates according to a metaphysic that includes God. That has all kinds of implications for how objective morality is possible, not the least of which is the existence of free will. But understand, when the naturalist states that he can discover objective morality in nature, what he is really doing is borrowing from some other metaphysic. Because he can’t consistently operate within in his own naturalistic world view and maintain such a claim.