“Got milk?” It’s not just a ubiquitous ad slogan — a primate infant’s access to breast milk has significant consequences for a species' life cycle. For the first time, researchers have been able to look back 100,000 years to understand how Neanderthal infants might have been nursed and weaned. According to a study published today in Nature, patterns of barium distribution in the fossilized teeth of a juvenile Neanderthal indicate that the individual began weaning at seven months, comparable to human infants and chimpanzees, but that weaning was completed by the age of 14 months — within the range of human patterns but significantly sooner than chimps. Weaning, the transitional period between exclusive breastfeeding and a solid food diet, has important implications both for an individual and for the species overall. Early weaning can result in negative health consequences for the infant, but also allows for faster reproduction rates within a population, since females cannot become pregnant while breastfeeding. Although there is considerable variation in the onset and duration of weaning among different industrial and nonindustrial human societies, researchers found that on average modern human children are weaned at 2.3-2.6 years. Chimpanzees, by comparison, are weaned at an average of 5.3 years. Researchers hoping to learn more about Neanderthal weaning patterns first studied the distribution of barium in the teeth of human children and captive macaques. As an infant’s teeth develop, daily growth lines in both the crowns and roots leave a record that can be used to determine the individual’s age at various developmental points as well as the teeth’s composition. Researchers focused on barium levels because the trace element in dental enamel is fairly resistant to breakdown during fossilization.
Dietary transitions in a Neanderthal permanent first molar. Top: developmental time (in days from birth) of each stress line in enamel (dark blue lines) determined from daily growth increments. Dietary regions: 1. prenatal, 2. exclusive mothers’ milk, 3. transitional period, and 4. post-weaning. Inhibited in utero by the placenta, barium levels increase immediately after birth upon the initiation of breastfeeding. The barium/calcium ratio remains high throughout the period of exclusive breastfeeding. When the weaning process begins and solid food is gradually introduced, however, barium levels change markedly, depending on its availability from the plant and animal food sources fed to the infant. When weaned is completed, barium levels return to those seen prenatally. After verifying how changes in barium distribution corresponded to changes in modern infants’ diets, the study’s authors analyzed the remains of a juvenile Neanderthal found in Belgium’s Scladina cave. Previous, unrelated research using the remains yielded the oldest mitochondrial DNA ever obtained from a Neanderthal — dated to roughly 100,000 years — suggesting the fossil was particularly well-preserved. They determined that weaning began at seven months and was completed by the age of 14 months. Although it is difficult to draw conclusions about a species' life cycle from one specimen, this relatively short weaning period suggests that Neanderthals may have had shorter inter-birth intervals than other primates, promoting faster population growth. Researchers plan to study other early hominid teeth in hopes of learning more about weaning patterns in those populations, which will impact our understanding of how those species lived and developed. Image by Ryan Somma via Flickr