Weaning practices differ among great apes and likely diverged during the course of human evolution, but behavioral inference from the fossil record is hampered by a lack of unambiguous biomarkers. Here, we show that early-life dietary transitions are recorded in human deciduous tooth enamel as marked variations in Ca isotope ratios (δ 44/42 Ca). Using a sequential microsampling method along the enamel growth axis, we collected more than 150 enamel microsam-ples from 51 deciduous teeth of 12 different modern human individuals of known dietary histories, as well as nine enamel samples from permanent third molars. We measured and reconstructed the evolution of 44 Ca/ 42 Ca ratios in enamel from in utero development to first months of postnatal development. We show that the observed variations of δ 44/42 Ca record a transition from placental nutrition to an adult-like diet and that Ca isotopes reflect the duration of the breast-feeding period experienced by each infant. Typically, the δ 44/42 Ca values of individuals briefly or not breastfed show a systematic increase during the first 5-10 mo, whereas individuals with long breastfeeding histories display no measurable variation in δ 44/42 Ca of enamel formed during this time. The use of Ca isotope analysis in tooth enamel allows microsampling and offers an independent approach to tackle challenging questions related to past population dynamics and evolution of weaning practices in hominins. calcium isotopes | tooth enamel | dietary transitions | weaning | breast milk