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It has been generally realized that supermasive black holes (SMBHs) are coevolving with their host galaxies in light of the famous Magorrian relation and SMBH mass-dispersion velocity relation. We find that the SMBHs are randomly fueled by starbursts with a delayed timesacle of ≈107 years and are spinning down with cosmic time from Sloan Digital Sky Survey data. The well-known metallicity-luminsity relation in AGNs and quasars can be explained by nuclear starbursts, and compact nuclear star clusters remain after multiple episodes.