  Far infrared fine-structure transitions of CI and CII and rotational
transitions of CO are used to probe hypothetical variations of the
electron-to-proton mass ratio mu = m_e/m_p at the epoch of reionization (z >
6). A constraint on Delta mu/mu = (mu_obs - mu_lab)/mu_lab = (0.7 +/-
1.2)x10^-5 (1sigma) obtained at <z> = 6.31 is the most stringent up-to-date
limit on the variation of mu at such high redshift. For all available estimates
of Delta mu/mu ranging between z = 0 and z = 1100, - the epoch of
recombination, - a regression curve Delta mu/mu = k_mu (1+z)^p, with k_mu =
(1.6 +/- 0.3) x10^-8 and p = 2.00 +/- 0.03, is deduced. If confirmed, this
would imply a dynamical nature of dark matter/dark energy.
