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The millimeter wavelength emission from GRB 991208 is the second brightestever detected, yielding a unique data set. We present here well-sampled spectraand light curves over more than two decades in frequency for a two-week period.This data set has allowed us for the first time to trace the evolution of thecharacteristic synchrotron self-absorption frequency nu_a and peak frequencynu_m, and the peak flux density F_m: we obtain nu_a \propto t^{-0.15 +- 0.12},nu_m \propto t^{-1.7 +- 0.4}, and $_m \propto t^{-0.47 +- 0.11}. From the radiodata we find that models of homogeneous or wind-generated ambient media with aspherically symmetric outflow can be ruled out. A model in which therelativistic outflow is collimated (a jet) can account for the observedevolution of the synchrotron parameters, the rapid decay at opticalwavelengths, and the observed radio to optical spectral flux distributions thatwe present here, provided that the jet transition has not been fully completedin the first two weeks after the event. These observations provide additionalevidence that rapidly decaying optical/X-ray afterglows are due to jets andthat such transitions either develop very slowly or perhaps never reach thepredicted asymptotic decay F(t) \propto t^{-p}.Comment: 15 pages, 4 figures, submitted to ApJ Letter

The Bright Gamma-Ray Burst 991208: Tight Constraints on Afterglow Models from Observations of the Early-Time Radio Evolution