Human nature: The set of capacities to think, feel and act that characterizes all and only human beings, as evidenced by human history, science, art, and civilization, including many atrocities and much human misery.
That all human beings - born out of a woman, with bodies developed from human DNA - have a similar set of capacities that enables them to think, feel and act in particular ways, and not in others, seems from a naturalistic or commonsensical point of view an evident assumption or truth, and conforms to the natural presumption that natural things come in natural kinds, and that every individual that belongs to a given kind has the properties and relations that characterize all individuals of that kind, and that human beings may understand and represent by their unique gifts for language and mathematics.
Even so, it is an assumption, and an important one, since it is at the basis of much of the thinking that keeps human societies together, all of which tends to somehow acknowledge that you and I and every other human being, now and as long as we can trace back human history, have been very similar in our natural construction, needs, and intellectual and emotional reactions to very many events that may happen to us.
Where one can learn about human nature? In medicine, biology, history, psychology, philosophy, linguistics, mathematics, art, music, for it seems all of these have much to say about uniquely human properties, acts, and ideas, and the physical and social conditions of these.
Perhaps the best brief and memorable introduction are Shakespeare's Plays, with the introductions by Johnson and Hazlitt, or Montaigne's Essays, or Gibbons's or Thucydides's histories. A suitable side-reading to these are Swift's Gulliver's Travels and Chamfort's Maximes et Pensées.