By looking at barium levels in the fossilized molar of a Neanderthal child, researchers concluded that the child had been breast-fed exclusively for the first seven months, followed by seven months of mother’s milk supplemented by other food. Then the barium pattern in the tooth enamel “returned to baseline prenatal levels, indicating an abrupt cessation of breast-feeding at 1.2 years of age,” the scientists reported on Wednesday in the journal Nature.
While that timetable conforms with the current recommendations of the American Academy of Pediatrics — which suggests that mothers exclusively breast-feed babies for six months and continue for 12 months if possible — it represents a much shorter span of breast-feeding than practiced by apes or a vast majority of modern humans. The average age of weaning in nonindustrial populations is about 2.5 years; in chimpanzees in the wild, it is about 5.3 years. Of course, living conditions were much different for our evolutionary cousins, the Neanderthals, extinct for the last 30,000 years.
The findings, which drew strong skepticism from some scientists, were meant to highlight a method of linking barium levels in teeth to dietary changes. In the Nature report, researchers from the United States and Australia described tests among human infants and captive macaques showing that traces of the element barium in tooth enamel appeared to accurately reflect transitions from mother’s milk through weaning. The barium levels rose during breast-feeding and fell off sharply on weaning.
The researchers then decided to apply the barium test to the fossilized molar of a Neanderthal child, collected in Belgium. The tooth’s fossilization, the researchers discovered, had not destroyed the barium biomarker, as had been feared.
This is the first documentation of diet transitions in a juvenile Neanderthal, the researchers said in interviews, suggesting that the barium technique may open the way to a more rigorous exploration of early-life dietary history of fossil hominins.
“Our studies on macaques and modern human children provide strong evidence that barium patterns in teeth do accurately reflect transitions from maternal milk to weaning,” said Manish Arora, a team member from the University of Sydney. He is also affiliated with the Icahn School of Medicine at Mount Sinai Hospital in New York City, and acted as the principal spokesman for the researchers.
But Michael Richards, a specialist in ancient teeth and bones at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, noted that the examination of trace elements, like barium, in archaeological samples went out of use in the 1970s and ’80s, as scientists showed that bone and teeth incorporated elements from the soil they were buried in, not necessarily from a lifetime diet.
“Recently, perhaps as the generation that did this work retires,” Dr. Richards continued, a new generation has been “returning to these methods.” He said he was surprised that Nature published the report.
Other scientists who investigate Neanderthals and other extinct hominins were guarded in their assessment of the findings. They worried that the key element in the study was confined to only one fossil specimen.
Dr. Arora acknowledged that “it is, of course, not possible to generalize to all Neanderthals from a single sample, but our observation of the exclusive breast-feeding period” in one young Neanderthal “does extend existing concepts of Neanderthal behavior.”
Erik Trinkaus, a paleoanthropologist at Washington University in St. Louis, who is an authority on Neanderthals, said the onset of weaning in the test appeared to be too early. He also cautioned, “My impression is the physiology and chemistry of nursing is vastly more complicated, and the concentrations of barium are too low that it’s hard to get reliable data.”
Tanya Smith, an evolutionary biologist at Harvard and an author of the report, said in an e-mail that the team hoped “to examine additional fossils to determine at what age Neanderthals naturally weaned their infants.” In the report, the researchers conceded that the abrupt, possibly early weaning could not be readily explained.
“We are excited about this technique as we feel that it will allow us to look directly at weaning, an important aspect of life history, in expanded samples of Neanderthals and fossil Homo sapiens,” Dr. Smith said.
The timing of weaning can be critical in contemporary human societies. Completed too early, it can expose a child to more health problems; but shorter periods of breast-feeding lead to shorter intervals between births, which influences population growth. Human infants are often weaned earlier than close ape relatives, often by several years.
As for Neanderthals, lately, science has been getting up close and personal with them. When Neanderthals and modern humans first encountered each other in Eurasia, some paired off for interspecies sex. How frequent these dalliances were, and over how long a time, is unknown. But the presence of at least 2.5 percent of Neanderthal DNA in most humans with European roots exposes the secret of viable interbreeding in the Stone Age caves.
In concluding their report, the researchers said the barium sampling would most likely be definitive in testing hypotheses about the consequences of later or earlier weaning on Neanderthals, compared with Homo sapiens. No one ventured speculation on any possible role maternal milk played in the downfall of Neanderthals after the arrival of modern humans in Europe.
