The integration of developmental mapping and fine-scaled whole-crown elemental quantification provides a powerful approach to clarifying the timing of elemental incorporation and diagenetic elemental intrusion, two concerns that have limited previous investigations of hominin fossils. Substantial changes in δ 18 O values appear in the enamel within a few days to weeks of the onset of nursing. Complementary experimental evidence is apparent in fig. S1, as water δ 18 O values were artificially lowered at 202 days of age and increased at 262 days of age, which are reflected in marked changes in enamel δ 18 O within a few days of these changes. We find a comparable phenomenon with barium patterns, as elevated values due to initial nursing begin to appear in the enamel no more than a few weeks before birth (fig. S8), as has been previously shown in other primate samples forming during the birth process ( 16 , 18 ). This offset between prenatal enamel and the neonatal line is due to overprinting of the prenatal region by subsequent postnatal mineral incorporation, creating a minor elemental shift relative to the timing of growth line formation during secretion. Lastly, lead exposures in the enamel and dentine of both Neanderthals also manifest nearly synchronously ( Figs. 3 and 4 ). Thus, mineralization of the innermost enamel does not appear to substantially mask or shift biogenic oxygen isotope ratios or certain trace element inputs. Reconstructions of seasonality, nursing behavior, and lead exposure can be directly related to tooth formation timing.

Our results are consistent with archeological reconstructions of the layers that yielded the Payre Neanderthal remains ( 11 , 15 ). These have posited that individuals from layer F (Payre 336) and layer G (Payre 6) experienced a cold and dry climate around the start of the marine isotope stage 7 interglacial. Previous paleoenvironmental reconstructions from the Payre Neanderthal teeth report a narrower range of δ 18 O values within and between individuals than that found in the two individuals analyzed here ( 10 , 11 ). Bulk enamel δ 18 O sampling has been interpreted to demonstrate differences in habitat exploitation across archeological layers at Payre, yet this method has limited temporal resolution, yielding samples that integrate long formation times of unknown biological age. We can now constrain physical samples to regions of the tooth secreted during approximately 1 week or less over ~3 years. Our microsampling results show repeated, intra-annual seasonal δ 18 O fluctuations of >4‰ within individuals, with marked changes occurring over time scales of months. Sequential enamel δ 18 O measurements are more likely to capture the morphology and magnitude of environmental δ 18 O variation in temperate environments where seasonal δ 18 O fluctuations are large and dominated by relatively simple winter-summer regimes ( 9 ).

Neanderthal nursing history, lead exposure, and health

These results also provide novel insight into the reproductive biology of Neanderthals, including the seasons of birth and weaning, as well as the duration of nursing—a key determinant of population growth and life history. It appears that the Payre 6 individual was born in the spring and weaned in the fall. Mammals, including humans, show seasonal birth patterns that relate to environmental cycles (24, 25). Our results for Payre 6 are consistent with the broad mammalian pattern of bearing offspring during periods of increased food availability. Sustained low barium values in the final 3 months of tooth formation indicate that this Neanderthal ceased nursing when it was 2.5 years old, which is similar to the average age of weaning in nonindustrial human populations (26). Our previous research on a ~100-ka-old Belgian Neanderthal identified an abrupt cessation of nursing at 1.2 years of age (16). It is likely that the result from Payre 6 provides a more normative weaning age for a Neanderthal, as the Belgian individual did not show the progressive decline in barium seen when humans, apes, and monkeys gradually transition from mothers’ milk to solid foods. Instead, there was a marked disruption of enamel formation at 1.2 years of age and an immediate steep decrease of barium at this point—a pattern also seen in a captive macaque that was prematurely separated from its mother before natural weaning. Additional studies are needed to establish a mean age for Neanderthal weaning, as well as whether this differs from contemporaneous modern humans or varies across the diverse Eurasian environments Neanderthals inhabited.

Repeated lead exposures during childhood in the two Neanderthals are the earliest such evidence in hominin remains. The intensity of lead signals in prominent bands exceeds levels elsewhere in the teeth by a factor of 10. These high and acute lead lines are indicative of short-term exposure from ingestion of contaminated food or water or inhalation from fires containing lead (27). Lead can also come from mothers’ milk (28), but the divergent patterns of barium and lead in Payre 6 and the acute lead bands in both individuals suggest that mothers’ milk was not the primary source of exposure. It is plausible that the lead in Payre 6 came from nonmilk liquids beginning at ~2.5 months of age, increasing with solid food consumption in the winter from 9 months of age and, again, in the late winter/early spring of the following year. At least two lead mines are located within 25 km of the site (29), consistent with estimates of routine foraging distances (11). Periods of lead exposure during the childhoods of these two French Neanderthals are remarkable, as biogenic lead bands were not apparent in the ~100-ka-old Belgian individual discussed above, and decades of research have shown that there is no safe level for lead in humans and other animals.

Lead exposures did not result in the formation of obvious developmental defects in the Payre Neanderthals’ enamel. We found a marked defect in Payre 6 coincident with a short-term barium elevation at approximately 701 days of age (Fig. 3, A and B). It appears that during the coldest time of winter, this young individual experienced heightened skeletal remineralization. Trace elements can be released into the bloodstream from skeletal stores, exemplified by the phenomenon of lead mobilization in parallel with calcium during human lactation (28). The pattern of acute barium elevation coincident with a developmental defect in Payre 6 is akin to that seen in captive rhesus macaques after they had ceased nursing (17). Several of these macaques lost weight during severe illnesses, mobilizing trace elements that had been stored in their bones, which were recorded in concurrently forming tooth enamel and dentine. While the Payre 6 individual appeared to have continued nursing throughout the disruption at ~701 days of age, the short spike in barium concentration and the presence of a strong enamel disruption are more consistent with acute illness and associated weight loss than a transient increase in maternal milk consumption.

The approach detailed here allows more robust explorations of Neanderthal paleobiology and prehistoric environmental conditions than conventional assessment of associated fauna or geological signatures (30). Broader applications may also help to clarify the purported relationship between climate variation and technological innovation in members of the genus Homo (1, 2). Although it is unclear whether and how cold stress or neurotoxicant exposure routinely affected the health of Neanderthals, scholars have noted the frequent occurrence of developmental defects in their teeth (23, 31). Several common explanations for these defects, including weaning stress and illness, can now be probed through developmentally informed barium mapping. While diagenetic modification may prohibit characterizations of teeth interred near naturally occurring barium sources, the quantification of diagenetically resistant oxygen isotopes provides complementary insights into the lives of young hominins.