Chemical analyses of teeth, possibly including fossilized ones, can provide clues about the age at which a child was weaned, a milestone in life that may reveal information about how the human species and its predecessors evolved.
Unlike living bone, which contains a substantial amount of protein such as collagen, tooth enamel is about 98 percent mineral. Therefore, atoms in the enamel — especially those deep within a tooth — don’t tend to swap with those in the environment as they fossilize, says Louise T. Humphrey, an anthropologist at the Natural History Museum in London. Studies have shown that changes in the ratios of various isotopes in teeth record evidence of nutritional stress, movement from one location to another and exposure to heavy metals such as mercury, she notes.
Recently, Humphrey and her colleagues scrutinized baby teeth that had been shed by modern-day children to see if the effects of significant changes in their diet had been preserved. Some of the children had been fed formula from birth, others had been breastfed exclusively and others transitioned from mother’s milk to formula or other foods when they reached a few weeks or months of age.
Because various foods have differing ratios of calcium, a major component of tooth enamel, and strontium, the dietary changes should show up in teeth, the researchers had speculated.
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Even though baby teeth haven’t yet erupted through a child’s gums at birth, they have been forming deep within the jaw tissue, says Humphrey. A feature that forms in the tooth around the time of birth, called the neonatal line, is a landmark that allows scientists to distinguish enamel that forms before birth from that which forms during infancy, she notes.
In two infants fed formula from birth, the strontium-to-calcium ratio in the tooth enamel that formed immediately after birth was much higher than that recorded in prenatal enamel, the researchers found. In contrast, strontium-to-calcium ratios in the postnatal enamel in three of four newborns that were breastfed exclusively were significantly lower than those found in enamel that formed before birth. In the fourth child, who was breastfed for only five weeks, no such change in isotope ratio appeared — probably because of the relatively short duration of breastfeeding, says Humphrey. She and her colleagues report their findings in an upcoming Proceedings of the National Academy of Sciences.
The same technique should work on other teeth as well. The first permanent molar, which begins forming a month or so before birth and is retained throughout life, is a likely target, says Humphrey. “There’s an archive of childhood preserved in your mouth,” she notes.
The new findings, which could be applied to fossil teeth as well, “are really exciting for anthropological research,” says Debra Guatelli-Steinberg, a physical anthropologist at OhioStateUniversity in Columbus. Gaining insight into the weaning age of ancient humans is interesting, she notes, because humans typically wean their offspring much later than other animals do.
Gary T. Schwartz, a physical anthropologist at ArizonaStateUniversity’s Institute of Human Origins, in Tempe, agrees: Determining the timing and pace of important milestones in an organism’s life, such as the age at weaning and the spacing between births, “can help scientists sketch out a roadmap of how ancient species developed.”
“This is truly an impressive piece of work,” says Wendy Dirks, an anthropologist at NewcastleUniversity in Newcastle upon Tyne, England. Because breastfeeding tends to suppress ovulation, weaning children more quickly enabled ancient humans to reproduce more frequently, thereby allowing populations to soar. “Early weaning is how humans took over the world,” she notes.