Most child health experts agree that a minimum of 6 months of breastfeeding is essential for the welfare of growing babies, although how well such recommendations are carried out widely varies  across the globe. Less is known about the breastfeeding habits of other primates—and much less still about those of prehistoric humans. A research team now reports a new technique for accurately detecting when babies were weaned, using chemical signatures in their teeth. The method was successfully applied to the tooth of a Neandertal child, raising the possibility that researchers could decipher the life histories of our evolutionary cousins and even gain insights into why they went extinct.
Fossils of prehistoric humans and other primates are relatively rare because bone does not last well in most environments. Teeth, on the other hand, are hard and strong enough to survive through the ages, and they are often found at paleontological and archaeological sites. Researchers have worked diligently to extract information from ancient teeth. Paleontologists recently reported finding the teeth of the earliest apes , and archaeologists have used chemical isotopes in the teeth of early farmers to track their movements across the landscape .
A team led by Manish Arora, an environmental health dentist at the Icahn School of Medicine at Mount Sinai in New York City, hypothesized that it might be possible to detect when a child was weaned from the amount of barium in its growing teeth. Barium is an element similar to calcium and is present in water sources and many types of soils, explains co-author Christine Austin, a dental researcher at Westmead Hospital in near Sydney, Australia. Barium makes its way into both teeth and bone in small amounts. As a tooth grows, both dentine, which makes up the center of a tooth, and enamel, which forms its hard surface, are laid down in daily layers, which are clearly visible under a microscope. The teeth begin growing before birth, but while a child is still in the womb the placenta blocks most barium—but not calcium—from reaching their dentine and enamel. After birth, barium in breast milk can more easily reach the teeth; and when a human baby is switched to infant formula, even more barium enters the teeth, because both cow- and soy-based formulas contain higher levels of the element than breast milk. Then, when the child switches from formula to solid food, the barium level goes back down.
The team, which reports its findings online today in Nature, started off by looking at the ratios of barium and calcium in the teeth of human children, using an instrument that scans the teeth with a laser and detects the elements found within the dentine and enamel layers. The researchers analyzed 25 baby teeth donated by mothers in Monterey County, California, who had kept careful records of their breastfeeding and infant formula habits as part of a child health program. Most of the teeth, a total of 22, revealed markedly higher barium levels right after birth; and in nine of 13 children who had first been breastfed and then given infant formula, the team could see a transition between the lower barium levels from breast milk and the higher barium levels of the formula. (The team could also distinguish children who went straight from breastfeeding to solid food without being given formula—their barium levels went down at the transition point.)
The researchers then looked at the molars of four young macaque monkeys at the California National Primate Research Center at the University of California, Davis, and correlated the barium signatures of the dentine and enamel of these teeth with data previously collected on the breastfeeding habits of the mother and infant monkeys. Again, the barium levels closely followed the course of breastfeeding and weaning among these animals, rising after birth and then falling to lower concentrations after weaning.
Finally, the researchers focused the technique on the molar of a 100,000-year-old Neandertal child earlier found at Scladina Cave in Belgium. Laser scanning of the tooth revealed that barium levels started off high right after birth and continued to be elevated for 7 months, apparently due to exclusive breastfeeding; they then fell to intermediate levels for another 7 months, suggesting that the mother's milk was supplemented by other food sources . But after 1.2 years, the child—who died at about 8 years of age—was abruptly weaned from breastfeeding, and barium fell to very low levels. (See photos.)
The researchers caution that it's impossible to draw broad conclusions about Neandertal life histories from this one sample, such as whether Neandertals weaned their children earlier or later than modern humans who lived at the same time, or whether Neandertal children grew up faster , as some earlier studies have suggested—questions that could heavily bear on why Neandertals could not keep up with modern humans in the survival sweepstakes. But the new technique could eventually provide some answers, says co-author Tanya Smith, an anthropologist at Harvard University. "Now that we've established an accurate and precise approach, we hope to examine additional fossils to determine at what age Neandertals actually weaned their infants."
Anthropologist Shara Bailey, an expert in ancient human teeth at New York University in New York City, says that "the barium method is novel and appears to be even more powerful" than previous approaches, adding that despite small sample sizes, "the authors present a strong argument for the utility of this method for extrapolating weaning history." Julia Lee-Thorp, an archaeologist at the University of Oxford in the United Kingdom, agrees that the work with children and monkeys represents a "very solid validation" of the method, although she cautions that the amount of barium that children absorb through their guts and into their teeth could decrease as they get older and this could skew the results.
But Lee-Thorp, Bailey, and other researchers caution against reading too much into the findings from one Neandertal tooth, particularly any conclusions that Neandertals weaned their children early. "We have to keep in mind that the Scladina individual died quite early, and this might present a bias in our interpretations," Bailey says. She adds that future data from adult Neandertals might "lend more credence to any hypotheses about what Neandertals were doing on a regular basis."
Louise Humphrey, an anthropologist and tooth expert at the Natural History Museum in London, agrees, although she says that the early weaning of the Scladina child is "intriguing" because it is more than a year earlier than the nearly 30 months typical of modern human nonindustrial societies. If early weaning was typical of Neandertals, Humphrey says, it would be consistent with other evidence for a "faster pace of development" and raise the possibility that Neandertal mothers had shorter intervals between births and thus more kids on their hands at any given time.