Significance The practice of weaning, the dietary transition from exclusive breastfeeding to exclusive nonmilk food, is a key aspect of development and evolution of hominins, but its study in the fossil record is hampered by a lack of unambiguous biomarkers. Ca stable isotope ratios of skeletal remains are expected to bear information about milk consumption. Here we demonstrate that modern human tooth enamel records a temporal variation of Ca isotope compositions, which is related to breastfeeding duration. Ca isotopes could be used as a biomarker for reconstruction of weaning practices in past human and fossil hominin species.