  The puzzling nature of magnetic and lattice phase transitions of iron
pnictides is investigated via a first-principles Wannier function analysis of
representative parent compound LaOFeAs. A rare ferro-orbital ordering is found
to give rise to the recently observed highly anisotropic magnetic coupling, and
drive the phase transitions--without resorting to widely employed frustration
or nesting picture. The revealed necessity of the additional orbital physics
leads to a correlated electronic structure fundamentally distinct from that of
the cuprates. In particular, the strong coupling to the magnons advocates
active roles of light orbitons in spin dynamics and electron pairing in iron
pnictides.
