We define human nature as behavior based on information in one’s DNA. That information is genetic and is not derived culturally. Other behavior reflects what the individual has learned after birth and is derived as part of culture.
Basing the definition of human nature on genetics gives us a scientific basis for judging whether someone’s behavior is instinctive, or due to more proximate causes. It helps us arrive at more precise judgments than do more old-fashioned efforts to define the same thing, like “nature versus nurture” and “instinctive versus rational”.
The problem is that real-life behaviors are almost never entirely black or white. All our behavior is based on a mix between our human nature and what we have learned, or been taught. We are the products of a highly successful kitchen, where diverse elements are mixed, processed, and cooked in ways that are producing some of the most fascinatingly complex flavors and textures in the galaxy.
This is just as well. If human behavior were controlled entirely by human nature, we would still be living in the early stone age. On the other hand, if our behavior patterns were entirely learned, with no input or guidelines from our DNA, we wouldn’t all be the same species and we certainly wouldn’t be human.
Learned experience has flourished in our species as in no other. Culture-based guidance starts very soon after birth, at a point in life when the baby is still acting largely on instinct. But that instinctive guidance contains specifically human instructions that encourage it to recognize other people and imitate them, to listen to speech patterns and mimic them, and in countless other ways start the learning process that leads to becoming a fully functioning human.
As the child matures it confronts increasingly complicated choices between instructions it inherited and ones it is learning. Many of these conflicts involve an instinct for self-preservation, which often opposes acting altruistically with other members of the group. Natural selection tugs one way while group selection, which depends on altruism in individual members, pulls in the opposite direction. Moral precepts are one of the several major workarounds that have evolved at the group level, to support group solidarity against individual selfishness. Without them any group would have difficulty hanging together long enough so its members would get to enjoy the benefits provided by cooperation. This is about the best explanation for the evolution of morality that I know.
I have never been a fan of St Augustine, who condemned many generations of our ancestors to a neurotic sense of guilt for harboring instinctive hopes and impulses. It’s easy to understand how the concept of original sin originated, but it’s a lot harder to sympathize with it once you see it in an evolutionary perspective.
When you look at how human society has evolved, it becomes evident that we can lead moral lives without starting with the assumption of original sin, or any of its many variations.