Tanya Smith reads teeth the way most people read books.
Rich details of life—from diet to disease—are etched into each of their layers. And Smith, a biological anthropologist at Griffith University in Australia, has spent more than a decade and a half poring over their chemistry and physical structure. But one detail of these stories has long been lacking: the environmental conditions in which the changes took place.
“People in human origins research have long speculated that climate change and periods of climate instability may have been key drivers in evolutionary steps during the human journey,” Smith says. But the markers used to tease out past climate—things like ice cores and pollen records—don’t give information on tight enough time spans to illuminate impacts within the lifetime of a single individual. Now that’s set to change.
A new study, published this week in the journal Science Advances, gives an unprecedented peek into the early life of two Neanderthal youngsters who lived some 250,000 years ago in what is now southeastern France. The chemistry of their teeth reveals the many challenges they faced in coping with their environment. The ancient hominins suffered winter stress and periods of lead exposure, likely tied to seasonal shifts in resources.
What's more, the researchers used oxygen isotopes to determine that one Neanderthal youngster was born in the spring. After nursing for 2.5 years, the hominin was weaned from its mother's milk in the fall.
“This study is one of the most interesting pieces of research I’ve read in a long time,” says Kristin Krueger, a paleoanthropologist from Loyola University who specializes in ancient teeth, via email. “To be honest, there were more than a few times when my jaw dropped from amazement.”
Teeth grow in a consistent pattern, somewhat like rings on a tree. “These layers just get added one after another,” explains Smith, lead author of the new study who also recently published a book titled The Tales Teeth Tell. But unlike annual tree rings, teeth form in much finer layers and allow scientists to study each day of growth in a child's early years.
For the latest study, Smith and an international team of researchers examined two teeth from two different Neanderthal children. They also compared the results to a modern human from the same site that lived there tens of thousands of years after the Neanderthals, some 5,000 years ago. (Learn about the discovery of an ancient girl whose parents were different human species.)
By cutting a thin slice from each of the teeth, the researchers gained access to the information lurking in their many layers. The team used high-powered magnification to count these daily additions and get stunningly accurate estimates foreach child's age at the point when each layer formed.
Both molars took about three years to reach maturity. One Neanderthal molar captured the time span from just before the individual was born to nearly three years of age, Smith says. But limited wear on the early molar suggests the owner didn't make it to adulthood.
The other was a second molar, which starts growing later in a child's development. This tooth likely began forming when the Neanderthal was around three years of age and continued to develop until about age six. From that point on, the tooth was no longer growing new layers but accumulating telling patterns of wear and tear.
The researchers then took the analysis even further, mapping out changes in elemental concentrations as well as the ratio of oxygen isotopes contained in the teeth. The latter is an indicator of ancient climates, which scientists could read, in this case, on a weekly scale. Food and water both contain oxygen isotopes, so as the ancient hominins ate and drank, they encoded temperature records in their teeth.
These records showed that the Neanderthal that mothered the owner of the younger tooth gave birth in the spring, as many mammals do. But in the depths of winter, the teeth of both Neanderthal children showed subtle structural disturbances, which suggest stress. “A number of different things can cause the growth of the teeth to be a little bit altered,” Smith notes, but the fact that they coincide with winter suggests that the cold likely brought challenges such as fevers, vitamin deficiency, and disease.
These weren't the only dangers of cooler weather, either. Several regions of the teeth laid down during the winter and early spring coincided with periods of lead exposure. “What they were doing to expose themselves to lead is an interesting open question,” Smith says. Natural lead deposits linger within a reasonable range for Neanderthals, she notes, so perhaps cold conditions forced them to travel to nearby caves and rely on contaminated food or water. It may have even been due to the inhalation of smoke from a fire fed by lead-contaminated materials, she notes. (Read about how Neanderthal genes could affect your health.)
The scientists also mapped changes in the element barium, giving insights into Neanderthal nursing habits. Mothers’ milk has a surprisingly high amount of the element, which is similar to calcium and can be incorporated into children's growing bones and teeth.
Though one of the studied Neanderthal teeth likely didn’t form until after the child had already moved on from its mother's milk, the other tooth had distinct signatures from nursing throughout the first 2.5 years of the child’s life.
Scientists have previously measured just one other instance of Neanderthal nursing. In 2013, Smith and her collaborators documented a Neanderthal found in present-day Belgium whose tooth indicated that it had nursed for a mere 1.2 years. But the infant’s reliance on milk ended abruptly, suggesting the child was separated from its mother or suddenly fell ill.
Because of this, it's hard to know whether the latest results extend to other individuals. But 2.5 years old is similar to the average age of weaning in non-industrial human populations, hinting that perhaps Neanderthals may have done the same.
"The identification of weaning age is fascinating," says Debbie Guatelli-Steinberg, a biological anthropologist at The Ohio State University, via email. She points out that 2.5 years is a much shorter nursing period than, for example, chimpanzees. These primates, along with bonobos, are our closest living relatives, and commonly nurse their young for up to five years. It suggests that Neanderthals may have been more like modern humans in weaning their offspring.
Smith hopes to extend this work to other Neanderthals, time periods, and environments—as well as to ancient human children. There's little understanding of how weaning age has changed through time, she explains. Some scientists have theorized that the development of soft foods and dairy products from animal milk could have helped mothers wean their children earlier. "But nobody has really been able to test that in such a precise way, and this method would help us to do that," Smith says.
The latest study adds to the increasingly complex picture of Neanderthals, Krueger says, giving researchers an astonishing window in to the daily lives of our ancient cousins. It also further dispels the common notion that Neanderthals are “shuffling, dumb brutes,” she explains. “Example: What would your reaction be if someone called you a Neanderthal? It’s not a compliment, right?”
“But these hominins were absolutely complex and complicated; they cooked their food, they exploited a wide variety of plants and animals, and even used plants for medicinal purposes,” Krueger says. “They participated in personal adornment and cave art, and buried their dead.”
The latest study tells the story of their lives in even greater detail, showing the effects of winter and additional information about how mothers cared for their young. As Krueger says, “the dividing line between 'them' and 'us' is blurring [more] every day.”