  We study the cosmological evolution on a brane with induced gravity within a
bulk with arbitrary matter content. We consider a Friedmann-Robertson-Walker
brane, invariantly characterized by a six-dimensional group of isometries. We
derive the effective Friedmann and Raychaudhuri equations. We show that the
Hubble expansion rate on the brane depends on the covariantly defined
integrated mass in the bulk, which determines the energy density of the
generalized dark radiation. The Friedmann equation has two branches,
distinguished by the two possible values of the parameter $\ex=\pm 1$. The
branch with $\ex=1$ is characterized by an effective cosmological constant and
accelerated expansion for low energy densities. Another remarkable feature is
that the contribution from the generalized dark radiation appears with a
negative sign. As a result, the presence of the bulk corresponds to an
effective negative energy density on the brane, without violation of the weak
energy condition. The transition from a period of domination of the matter
energy density by non-relativistic brane matter to domination by the
generalized dark radiation corresponds to a crossing of the phantom divide
$w=-1$.
