  The non-equilibrium evolution of heavy-ion collisions is studied in the limit
of weak coupling at very high energy employing lattice simulations of the
classical Yang-Mills equations. Performing the largest classical-statistical
simulations to date, we find that the dynamics of the longitudinally expanding
plasma becomes independent of the details of the initial conditions. After a
transient regime dominated by plasma instabilities and free streaming, the
subsequent space-time evolution is governed by a nonthermal fixed point, where
the system exhibits the self-similar dynamics characteristic of wave
turbulence. This allows us to distinguish between different kinetic scenarios
in the classical regime. Within the accuracy of our simulations, the scaling
behavior found is consistent with the ``bottom-up" thermalization scenario.
