In the Sutta-nipāta, a collection of some of the oldest scriptures, one can find what is likely the original expression of the teaching of dependent arising.

Not by birth is one a Brahman, nor is one by birth no Brahman; by acts one is a Brahman, by acts one is no Brahman.

By acts one is a husbandman, by acts one is an artisan, by acts one is a merchant, by acts one is a servant.

By acts one is a thief, by acts one is a soldier, by acts one is a priest, by acts one is a king.

So the wise know these acts as they really are.
They see dependent arising and understand acts and their results.

This set of verses describes the acts of an individual as determining their state of existence in the present. While we do not know if this early thought about dependent arising was really the content of Śākyamuni’s awakening, it can be seen that at this stage there was an awareness that the suffering of existence is a result of one’s own actions.

In contrast, a more organized theory of dependent arising, called the “twelve links of dependent arising,” presents twelve items for observation: ignorance, volition, consciousness, name and form, the six-fold sphere of sense contact, contact, sensation or feeling, craving, grasping, becoming, birth, and finally old age and death. Biographical scriptures state that when attaining enlightenment, Śākyamuni observed in order the process by which suffering comes into existence. Volition arises because there is ignorance. Consciousness is born because volition arises, and so on. Śākyamuni also observed the reverse order of the process by which suffering is extinguished. Volition is extinguished because ignorance is extinguished. Consciousness is extinguished because volition is extinguished, and so on. This kind of insight is called a dharma, meaning “law” or “teaching.”
Śākyamuni tried to enter nirvāṇa while keeping the content of this awakening to himself. However, it is said that at this point he was encouraged three times by Brahmā, a god in Brahmanism, to teach his awakening. He then decided to preach and declared the following: “The door of ambrosia (deathlessness) has been opened for those who have ears; Brahmā, do away with faith. I did not teach the correct Dharma thinking that it might harm people.