  We argue that a high precision measurement of the neutrino mixing parameter
$\theta_{13}$, within a three neutrino seesaw framework can throw important
light on the question of whether the quarks and leptons unify into a single
matter at high scales. Based on a number of examples, we conclude that a value
of $\theta_{13} \leq {\frac{\Delta m^2_{\odot}}{\Delta m^2_A}}\simeq 0.04$
would require at the minimum a pure leptonic interchange symmetry between $\mu$
and $\tau$ generations for its natural theoretical understanding and will
disfavor a quark-lepton unification type theory such as those based on
$SU(4)_c$ or SO(10) whereas a bigger value would leave open the possibility of
quark lepton unification.
