  It has recently been suggested that an isotropic singularity may be a generic
feature of brane cosmologies, even in the inhomogeneous case. Using the
covariant and gauge-invariant approach we present a detailed analysis of linear
perturbations of the isotropic model ${\cal F}_b$ which is a past attractor in
the phase space of homogeneous Bianchi models on the brane. We find that for
matter with an equation of state parameter $\gamma > 1$, the dimensionless
variables representing generic anisotropic and inhomogeneous perturbations
decay as $t\to 0$, showing that the model ${\cal F}_b$ is asymptotically stable
in the past. We conclude that brane universes are born with isotropy naturally
built-in, contrary to standard cosmology. The observed large-scale homogeneity
and isotropy of the universe can therefore be explained as a consequence of the
initial conditions if the brane-world paradigm represents a description of the
very early universe.
