In Samkhya evolution involves prakriti alone. The purusha remains unchanged, a mere witness to prakrti's unceasing transformations. Their mutual functioning is described using the metaphor of the lame man (purusha) being carried by the blind man (prakrti).
Most of the Samkhyan cosmology is concerned with the unfolding of the prakriti principle; or more precisely mulaprakriti or unmanifest root-nature (equivalent to the Greek concept of Hyle or formless matter). Mulaprakriti is described as "unmanifest" (avyakta), "uncreated" or "unmade" (avikriti), and "the chief one" (pradhana) [ Gerald J. Larson, Classical Samkhya, pp.160-1, (Motilal Banarsidass, Delhi, 1979)]. It is the original primordial root-nature from which everything else arises through a process of self-unfolding, triggered through the proximity of the purusha or centre of consciousness. All things, and all subsequent tattwas or evolutes, are contained within Mulaprakriti, but in a subtle or unmanifest form. Here then we have a theory of creation that begins not with the Absolute Reality itself (as in all the monistic emanationist cosmologies), but with the principle of Unmanifest "Nature". Because of that, it is probably more correct to understand the Samkhyan theory of creation in terms of an evolution or unfolding rather than an emanation.
Mulaprakriti itself contains or is made up of three primary qualities, the three constituents or strands, called gunas. In the unmanifest Mulaprakriti these exist in a state of equilibrium and balance, and so there is no manifestation. When the gunas are disturbed however through the presence of the purusha the equilibrium is destroyed, and creation, both gross and subtle, comes about. Here we have a parallel with the Chinese Yin-Yang cosmology and the emergence of the universe through the manifestation of the polar opposites of Yin and Yang from the originally unmanifest Tao.
The creation hoiwever is not simple but involves a series of 23 fuurther tattwas, the later ones being derived from or unfolding out of the earlier ones. The basic series is as follows:
From prakrti emerges mahat ("the great one"), also called buddhi. This is the subtlest form of mental activity, and the source of will and the unconscious. From buddhi evolves ahamkara, the "I-maker", which is the source of the sense of ego or individual identity. From ahamkara there is a four-fold unfolding into mind (manas), sense organs (jnanendriyas), the organs of action (karmendriyas), and the subtle elements (tanmatras). Of these the mind and senses are predominantly sattva, the organs of action rajas, and the subtle elements tamas. These latter are the source from which evolve the five gross elements of the material world.
The Samkhyan thesis of evolution has been widely adopted by other schools, usually witha strong theistic interpretation, in which God is both the source of consciousness and the material world.