Twenty-three of Samkhya's twenty-five tattvas are evolutes, things produced by something else in an ordered sequence, and every one of those twenty-three traces back to a single starting point: prakriti, the unmanifest root. Nothing produces prakriti in turn. Like purusha, it is uncaused, but unlike purusha it is not inert; it is entirely capable of action; what it lacks is consciousness, which is purusha's sole possession.
Prakriti is composed of three gunas, sattva, rajas and tamas, held in perfect balance in its unmanifest state, a condition the tradition calls avyakta. Nothing distinguishes one part of unmanifest prakriti from another because there is no differentiation yet to distinguish; the gunas are simply poised. Evolution, the production of the twenty-three tattvas that follow, begins when that poise is disturbed, and Samkhya is notably reticent about supplying a mechanism for the disturbance itself, since a first cause of the disturbance would need its own cause and the system prefers an eternally recurring process to a single dateable beginning.
Once disturbed, the unfolding proceeds in a fixed order: mahat or buddhi first, then ahamkara, then from ahamkara the eleven organs and the five tanmatras, then from the tanmatras the five gross elements. Each stage is prakriti itself in a more differentiated form, never something ontologically new, which is why Samkhya calls its worldview a form of satkaryavada, the effect pre-existing in its cause. The world, on this account, is not created from nothing; it is prakriti becoming visible.