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_ν, from the presently available data. Specifically we analyse two different

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... R data sets to test the robustness of our results. Analyzing only CMBR data yields an upper bound of N_ν < 17 (95 the PSC-z survey tightens the upper bound slightly. However, the addition of LSS data gives a non-trivial lower bound of N_ν > 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_ν = 0 is disfavoured at 3σ and 4σ for the two data sets respectively.