  We consider the big bang nucleosynthesis (BBN) in the Brane world scenario,
where all matter fields are confined on our 3-brane and the radion of the Brane
evolves cosmologically. In the Einstein frame fundamental fermion masses vary
and the results of standard BBN (SBBN) are modified. We can thus use the
observational primordial element abundances to impose constraints on the
possible variations of the radion. The possibility of using the evolving radion
to resolve the discrepancies between the Wilkinson Microwave Anisotropy Probe
(WMAP) and SBBN values of the baryon-to-photon ratio ($\eta$) is also
discussed. The results and constraints presented here are applicable to other
models in which fundamental fermion masses vary.
