Theoretical arguments and indirect observational evidence suggest that the stellar IMF may evolve with time, such that it is more weighted toward high-mass stars at higher redshift. Here we test this idea by comparing the rate of luminosity evolution ofmassive early-type galaxies in clusters at 0.02 ≤ z ≤ 0.83 to the rateof their color evolution. A combined fit to the rest-frame U — V color evolution and the previously measured evolution of the M/L_B ratio gives χ = -0.3_(-0.7)~(+0.4) for the logarithmic slope of the IMF in the region around 1 M_⊙, significantly flatter than the present-day value in the Milky Way disk of χ = 1.3 ± 0.3. The best-fitting luminosity-weighted formation redshift of the stars in massive cluster galaxies is 3.7_(-0.8)~(+2.3), and a possible interpretation is that the characteristic mass m_c had a value of ～2 M_⊙ atz ~ 4 (compared to m_c ～ 0.1 M_⊙ today), in qualitative agreement with models in which the characteristic mass is a function of the Jeans mass in molecular clouds. Such a "bottom-light" IMF for massive cluster galaxies has significant implications for the interpretation of measurements of galaxy formation and evolution. Applying a simple form of IMF evolution to literature data, we find that the volume-averaged SFR at high redshift may have been overestimated (by a factor of 3-4 at z ＞ 4), and the cosmic star formation history may have a fairly well defined peak at z ～ 1.5.The M/L_V ratios of galaxies are less affected than their SFRs, and future data on the stellar mass density at z ＞ 3 will provide further constraints on IMF evolution. The formal errors likely underestimate the uncertainties, and confirmation of these results requires a larger sample of clusters and the inclusion of redder rest-frame colors in the analysis.
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