  We show that the various crossovers between dynamical regimes observed in
experiments and simulations of supercooled liquids can be explained in simple
terms from the existence and statistical properties of dynamical
heterogeneities. We confirm that dynamic heterogeneity is responsible for the
slowing down of glass formers at temperatures well above the dynamic
singularity T_c predicted by mode coupling theory. Our results imply that
activated processes govern the long-time dynamics even in the temperature
regime where they are neglected by mode-coupling theory. We show that
alternative interpretations based on topographic properties of the potential
energy landscape are complicated and inefficient ways of describing simple
physical features which are naturally accounted for within our approach. We
show in particular that the reported links between mode coupling and landscape
singularities do not exist.
