  We consider cosmological solutions of string and M-theory compactified to
four dimensions by giving a general prescription to construct four-dimensional
modular cosmologies with two commuting Killing vectors from vacuum solutions.
By lifting these solutions to higher dimensions we analyze the existence of
cosmological singularities and find that, in the case of non-closed
Friedmann-Robertson-Walker universes, singularities can be removed from the
higher-dimensional model when only one of the extra dimensions is time-varying.
By studying the moduli space of compactifications of M-theory resulting in
homogeneous cosmologies in four dimensions we show that U-duality
transformations map singular cosmologies into non-singular ones.
