From what two instincts "natural to human beings" does poetry, according to Aristotle, seem to have sprung?

"All men, by nature, desire to know." This quote, from Aristotle's Metaphysics, won't be found in the book in question, but it's a useful key to this answer and to the body of this philosopher's work.
In Section IV of the Poetics, Aristotle avers that the sources of poetry, deeply embedded in our nature, are the instinct for imitation and the instinct for rhythm and harmony. The first, he suggests, is the more influential, because it's so closely related to our vital desire to learn. We know that, from our earliest years, it is through imitation that much of our knowledge and skill are gained. Thus, he reasons, even "Objects which, in themselves, we view with pain, we delight to contemplate when reproduced with minute fidelity," suggesting the pleasure and insight to be found even in works of tragedy.
The second source, the musical instincts, were developed by those with a natural gifts until their "rude improvisations" gave birth to the rhythms of poetry.
In sum, the sources of poetry are the instincts for imitation and for the rhythms of poetic meter.
http://classics.mit.edu/Aristotle/poetics.html

In the Poetics, Aristotle argues that the impulse to poetry is universal, rather than grounded in particular cultural circumstances. In fact, for Aristotle, the universality of poetry and its grounding in universal instincts are what makes it more valuable and in a sense truer than history, as history tells of what is local and specific and poetry of universal truths about human nature.
The first of the two instincts Aristotle discusses is the instinct for imitation. Even young children enjoy and engage in forms of mimicry, and humans universally seem to find imitation (Greek: mimesis) pleasurable.
The second instinct that Aristotle notes is that for rhythm and harmony. All societies seem to enjoy some forms of music. It is important in the context to remember that Aristotle is discussing ancient Greek poetry, which was performed with musical accompaniment, unlike modern poetry which is normally read privately and silently rather than heard.