  Since the work of Mac-Dowell-Mansouri it is well known that gravity can be
written as a gauge theory for the de Sitter group. In this paper we consider
the coupling of this theory to the simplest gauge invariant observables that
is, Wilson lines. The dynamics of these Wilson lines is shown to reproduce
exactly the dynamics of relativistic particles coupled to gravity, the gauge
charges carried by Wilson lines being the mass and spin of the particles.
Insertion of Wilson lines breaks in a controlled manner the diffeomorphism
symmetry of the theory and the gauge degree of freedom are transmuted to
particles degree of freedom.
