  New precision Cosmic Microwave Background Radiation (CMBR) anisotropy data
are beginning to constrain physics beyond the standard model, for example in
the form of additional light particle species. These constraints are
complementary to what can be obtained from big bang nucleosynthesis (BBN)
considerations because they apply to much later times. We derive a constraint
on the equivalent number of neutrino species, N_\nu, from the presently
available data. Specifically we analyse two different CMBR data sets to test
the robustness of our results. Analyzing only CMBR data yields an upper bound
of N_\nu < 17 (95% confidence). Adding large scale structure (LSS) data from
the PSC-z survey tightens the upper bound slightly. However, the addition of
LSS data gives a non-trivial {\it lower} bound of N_\nu > 1.5/2.5 (95%
confidence) for the two data sets. This is the first independent indication of
the presence of the cosmological neutrino background which is predicted by the
standard model, and seen in big bang nucleosynthesis. The value $N_\nu = 0$ is
disfavoured at 3\sigma and 4\sigma for the two data sets respectively.
