Tracking Diet Transitions During Infancy From Teeth
Austin*, C., Smith*, T.M., Bradman, A., Hinde, K., Joannes-Boyau, R., Bishop, D., Hare, D.J., Doble, P., Eskenazi, B., Arora, M.(in press) Barium distributions in teeth reveal early life dietary transitions in primates. Nature.
* These authors contributed equally to this work.
The Study in Brief
Early life dietary transitions reflect fundamental aspects of primate evolution and are important determinants of health in living human populations. Identifying these transitions in ancient hominins has been difficult due to the lack of accurate biomarkers that are not modified during fossilization. In this study we show that major dietary shifts during infancy are accurately recorded in mineralized dental tissues, which survive in fossil remains from thousands of years ago. Teeth from human children and captive macaques with known diet histories demonstrate that barium distributions reflect dietary transitions, beginning with the onset of breast feeding and continuing through the weaning process. Barium levels in dental tissues formed during nursing are higher than in tissues that mineralize before birth, during solid food supplementation, or after nursing ceases. Consumption of common commercially-available infant formula results in higher barium values than those of exclusive human breastfeeding. This approach also allows insight into ancient breastfeeding practices, as we document the first early life diet transitions in a juvenile Neanderthal, which shows a pattern of exclusive mother’s milk for seven months, followed by seven months of supplementation. After this point, barium levels returned to baseline prenatal levels, suggesting an abrupt cessation of breastfeeding at 1.2 years of age. Integration of information on the spatial distribution of elements such as barium and the precise daily record of tooth formation (Figure 1) enables novel studies of the evolution of human life history, dietary patterns in wild primates, and human health investigations through accurate reconstructions and comparisons of breastfeeding history.
One of the most remarkable features of human development is that human infants are weaned much earlier than our closest ape relatives, often by several years. Although there is some variation among human cultures, this accelerated transition to foods other than mother’s milk is thought to have emerged in our ancestral history due, in part, to more cooperative infant care and access to a more nutritious diet. Shorter lactation periods can translate into shorter inter-birth intervals and higher reproductive rates. This has likely played a part in the relatively high lifetime fertility of humans, even before the advent of modern industrial food production. However there is much debate about when accelerated weaning occurred in the hominin lineage. For the past few decades researchers have relied on tooth eruption age as a direct proxy for weaning age. Yet recent investigations of wild chimpanzees have revealed that first molar eruption occurs prior to the cessation of weaning, complicating the estimation of weaning from fossilized dentitions. Other methods are needed to understand the evolution of human weaning. In this study we show that the timing of breastfeeding and the weaning process can be uncovered in tooth crowns – down to nearly the day!
Like the rings found in tree trunks, teeth form following a regular pattern that creates permanent daily lines in enamel and dentine, which can be viewed and counted under a microscope (see examples here). This faithful record of development also includes a marked “neonatal line” formed at birth in baby teeth and the first permanent molar of monkeys and apes (including humans) (Figure 2).