A Mouthful Of Evidence

SIMILAR TO the concentric rings that provide a chronicle of environmental conditions during a tree's growth, the historical record of an infant's diet may be extracted from layers of baby tooth enamel.

Paleontologists hope to mine fossilized teeth in this way for information about early-human eating habits, such as whether we have always weaned children as young as modern humans do. Early weaning results in shorter intervals between subsequent births and could have been the foundation for rapid population growth in early humans. This is one possible explanation for our species' global primacy, says bioanthropologist Louise T. Humphrey at the Natural History Museum in London.

It turns out that the relative ratio of calcium and strontium in tooth enamel provides a dietary signature, indicating whether a baby was breast-fed or formula-fed. Because teeth are already growing in a developing fetus, these signatures can even pinpoint the time of birth, when the infant stopped being nourished through the placenta (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0711513105).

Using mass spectrometry to measure the relative concentrations of calcium and strontium in progressive layers of tooth enamel obtained from children with known dietary records, Humphrey's team "showed enamel can provide a permanent archive of a period of infant growth and development," she notes. "Every human has a mouth that is full of fossils."

As teeth develop, hydroxyapatite crystals, a major component of enamel, are initially deposited under conditions in which calcium concentrations are carefully controlled. Strontium, however, can freely diffuse in and out of the developing enamel and replace calcium in the crystal lattice. Because strontium concentrations vary in different foods, the strontium levels in subsequent layers of baby tooth enamel reflect the infant's diet since birth.

Calcium-to-strontium ratios in milk decrease as animals move up the food chain, Humphrey explains. The ratio in cow milk, for example, is higher than that in human breast milk, she says. Although humans already have low levels of strontium in their bodies, the element is even further filtered from human breast milk.

"This work opens up new research possibilities in the field of anthropology," comments Debbie Guatelli-Steinberg, an anthropologist at Ohio State University who specializes in dental fossils.

For example, Humphrey hopes to extend the technique to study the development of both infant and adult teeth in fossil records. The adult teeth could provide information about later elements of childhood development because enamel is still being deposited on some adult molars during early adolescence. "These teeth could also provide clues to illness, malnutrition, migration, and climate," she says.