  We introduce a microscopic model for electron doped manganites that explains
the mechanism of the observed transition from $G$-type antiferromagnetic
($G$-AF) to $C$-type antiferromagnetic ($C$-AF) order under increasing doping
by double exchange mechanism. The model unravels the crucial role played by
$e_g$ orbital degrees of freedom and explains the observed metal-to-insulator
transition by a dimensional crossover at the magnetic phase transition. The
specific heat and the spin canting angle found for the $G$-AF phase agree with
the experimental findings. As a surprising outcome of the theory we find that
spin canting is suppressed in the $C$-AF phase, in agreement with the
experiment, due to the Fermi surface topology.
