  The cosmic microwave background (CMB) spectrum provides tight constraints on
the thermal history of the universe up to $z \sim 2\times 10^6$. At higher
redshifts thermalization processes become very efficient so that even large
energy releases do not leave visible imprints in the CMB spectrum. In this
paper we show that the consistency between the accurate determinations of the
specific entropy at primordial nucleosynthesis and at the electron-photon
decoupling implies that no more than 7.8% of the present day CMB energy density
could have been released in the post-nucleosynthesis era. As pointed out by
previous studies, primordial nucleosynthesis complements model independent
constraints provided by the CMB spectrum, extending them by two orders of
magnitude in redshift.
