  Primordial Big-Bang Nucleosynthesis (BBN) tightly constrains the existence of
any additional relativistic degrees of freedom at that epoch. However a large
asymmetry in electron neutrino number shifts the chemical equilibrium between
the neutron and proton at neutron freeze-out and allows such additional
particle species. Moreover, the BBN itself may also prefer such an asymmetry to
reconcile predicted element abundances and observations. However, such a large
asymmetry appears to be in conflict with the observed small baryon asymmetry if
they are in sphaleron mediated equilibrium. In this paper we point out the
surprising fact that in the Standard Model, if the asymmetries in the electron
number and the muon number are equal (and opposite) and of the size required to
reconcile BBN theory with observations, a baryon asymmetry of the Universe of
the correct magnitude and sign is automatically generated within a factor of
two. This small remaining discrepancy is naturally remedied in the
supersymmetric Standard Model.
