In a recent publication [1] it was shown how the geometric algebra G4,1, the algebra of 5-dimensional space-time, can generate relativistic dynamics from the simple principle that only null geodesics should be allowed. The same paper showed also that Dirac equation could be derived from the condition that a function should be monogenic in that algebra; this construction of the Dirac equation allows a choice for the imaginary unit and it was suggested that different imaginary units could be assigned to the various elementary particles. An earlier paper [2] had already shown the presence of standard model gauge group symmetry in complexified G1,3, an algebra isomorphic to G4,1.