We propose a cosmological scenario involving a scalar field, $��$, that is a source of Dark Matter as well as of Dark Energy. Besides $��$, the Lagrangian of the field theory envisaged in our scenario contains a second field $��$, for simplicity assumed to be a scalar, too. For fixed values of $��$, the potential term decays exponentially at large positive values of $��$. While $��$ is not coupled to Standard Model fields, $��$ is assumed to be coupled to them, and the Green functions of $��$ depend on the cosmological redshift in the expanding universe. We assume that the term in the Lagrangian coupling $��$ to $��$ is such that, at redshifts $z$ larger than some critical redshift $z_c$, $��$ is trapped near $��=0$, and oscillations of $��$ about $��=0$ describing massive scalar particles give rise to Dark Matter. At redshifts below $z_c$, the field $��$ is no longer trapped near the origin and starts to `roll' towards large field values. A homogenous component of $��$ emerges that acts as Dark Energy. Within over-dense regions, such as galaxies and galaxy clusters, the redshifting of $��$ stops, and $��$ therefore remains trapped near $��=0$ as long as $z_c$ is smaller than the redshift when structures on galactic scales decouple from the Hubble flow. Thus, at the present time, $��$ describes both Dark Energy and Dark Matter.

6 pages