  The Friedmann law on the brane generically depends quadratically on the brane
energy density and involves a ``dark radiation'' term due to the bulk Weyl
tensor. Despite its unfamiliar form, we show how it can be derived from a
standard four-dimensional Brans-Dicke theory at low energy. In particular, the
dark radiation term is found to depend linearly on the brane energy densities.
For any equation of state on the branes, the radion evolves such as to generate
radiation-dominated cosmology. The radiation-dominated era is conventional and
consistent with nucleosynthesis.
