Two scenarios for the early evolution of the solar nebula. In the common scenario 1, the disc is fed by the molecular cloud at relatively large radial distances, and the gas and the dust grains move inwards. In our favoured scenario 2, the disc is fed at much smaller radii, and the dust grains are dragged outward with the viscously spreading gas, which provides a natural explanation for the formation of CAIs embedded in a silicate matrix.