A new study on oxygen isotope ratios and heavy metals in the enamel of Neanderthals, who lived and died 250,000 years ago in southeastern France, suggests that they tolerate colder winters and more pronounced differences the seasons as the modern inhabitants of the region. The two Neanderthals studied in the study were also exposed to lead in their early years, making them the earliest known cases of this exposure.
Hard winters end
Enamel forms in thin layers, and these layers record the chemical traces of a human's early life ̵
An important clue to past environments is oxygen, which comes from the water a person has drunk or the plants they eat. The ratio of oxygen-18 isotope to oxygen-16 depends on temperature, precipitation and evaporation. In general, higher oxygen-18 ratios indicate warmer, drier, more volatile conditions.
For the Payre Neanderthals, oxygen-18 ratios increased in the summer and declined in winter in predictable seasonal cycles that Smith and her colleagues could compare from one week to the next. The data suggests harsher winters and stronger seasonal changes than today. Seasonal shift information can be combined with other details recorded in the tooth and bone to determine how the climate affects the development and life history of Neanderthals. The climate is often referred to as the driving force for hominin development, but rarely can archaeologists combine the two directly.
"This is especially important for Neanderthals who have survived extreme Eurasian environmental differences and glaciations that are mysteriously extinct in a cool interglacial phase." Smith and her colleagues wrote
The cold seasons were harsh for Neanderthal children, as both of Payre's early years had persistent signs of illness or malnutrition. This type of physiological stress affects how the body absorbs and processes minerals, including those in the enamel, and this can leave a visible line across the tooth that marks the layers of enamel added in difficult times. On a left lower molar of a child, Payre 6, the enamel layers deposited in late winter or early spring, just before the child's second birthday, show a line that marks illness or hunger for about a week. And another child, Payre 336, apparently suffered from a similar two-week episode in the winter and another week in the next fall.
Other studies have found that Neanderthal teeth often carry the lines created by these times of need; According to Smith and her colleagues, many of these episodes were likely to take place in the cold, difficult months of late winter and spring.
Oldest Indications of Lead Exposure
Heavy metals in the bloodstream eventually settle in bones and teeth, and the enamel of Payre 6 contained significant levels of lead from the time the child was about 2.5 months old. At about nine months of age, one band in winter marks a sudden peak in lead exposure-about ten times the lead in the rest of the tooth. Just over a year later (about two months after the aforementioned onset of illness) another gang of concentrated lead marks a sudden increase in exposure in late winter or early spring.
Payre 336 also appears to have been exposed to high risk once in spring and again in winter or late autumn.
This is the oldest physical evidence for the exposure of lead that archaeologists have discovered so far, and is likely a consequence of housing in caves near underground lead deposits. At least two lead mines are within 25 km of the Payre site, far from the reach of the Neanderthals. The early, permanent stress may have been caused by contaminated water or juice. Smith and her colleagues say that milk is probably not to blame because it would have left additional barium in the enamel.
At about nine months, Payre 6 may have started getting more lead from the solid foods they started eating. Exposure to lead is not related to the consequences of illness or malnutrition, and it is impossible to know how these two children might have affected their health, but Smith and her colleagues state: "Decades of research has shown that there is no safe level for lead in humans and other animals. "
A Demographical Note
Lead is not the only heavy metal that gets into the teeth. The care often leaves a higher barium level in the enamel of a child. In Payre 6, enamel, which stores in the first nine months of life, contains a high barium content, but it wears off – a testament to the moment a Neanderthal mother drags her child from milk to solid food. However, this process lasted until the child was about 2.5 years old, when the barium signature finally disappeared completely in her enamel.
Breastfeeding takes so long in some modern human societies, especially hunter-gatherer cultures whose lifestyles may be similar to those of Payre 6 and his family.
This may possibly help to fill another piece in the mystery of Neanderthal extinction. Populations keeping their children longer also tend to have lower birth rates, which means that these populations often do not grow as fast as those who wean their children earlier. For Neanderthals trying to compete against newly arrived territories Homo sapiens, population growth may have played a role, though it is too early to say for sure.
The enamel of Payre 336 was not a unique pattern of barium levels. The only other evidence of Neanderthal weaning is a 100,000-year-old tooth from Belgium, but instead of gradual changes in weaning, the barium level drops abruptly at the age of 1.2, as if the child had suddenly been separated from his mother. Smith and her colleagues say that archaeologists need more evidence to draw conclusions as to when most Neanderthals weaned their children and how it was compared to their neighbors Homo sapiens .
Science Advances 2018 DOI: 10.1126 / science.eaau9483 (About DOIs).