Infant diet and the timing of weaning may have played an important role in the evolution of early hominins to humans. To test this hypothesis, researchers have developed a technique to analyze the layers of dentin and enamel in teeth to determine an infant’s dietary history. Using the method to analyze a young Neanderthal’s tooth from approximately 100,000 years ago, the researchers conclude that the individual fed exclusively on breast milk for about seven months, followed by another seven months of breast milk and food before being weaned entirely (Nature 2013, DOI: 10.1038/nature12169). The researchers, led by Manish Arora of the Icahn School of Medicine at Mount Sinai, report that infant dietary history can be inferred by the ratios of barium to calcium in teeth cross-sections, as measured by mass spectrometry. Infant teeth begin to grow in utero, with layers of dentin and enamel accumulating daily. Breast milk and infant formula contain barium with higher bioavailability than the barium found in many solid foods. Thus the element is deposited in infant tooth layers in higher concentrations before weaning occurs. The scientists first proved that the barium-to-calcium ratio technique could predict the onset and completion of weaning in contemporary humans and captive macaques before applying the technique to an ancient juvenile Neanderthal’s molar.