For decades, carbon and nitrogen isotopes (chemical signatures preserved in bones and teeth) have been used to help reconstruct the diet of past populations. For the majority of archaeological studies, these two isotope ratios can provide a wealth of knowledge about what types of plants and animals people consumed through history. In some cases, however, particularly in especially old samples or ones from less-than-ideal burial conditions, the bone collagen is not sufficiently well preserved to accommodate such studies. This is particularly apparent in Palaeolithic remains from Iberia, where the climate has, so far, not been favourable for the analysis of nitrogen in collagen/bone protein. This is unfortunate, as Iberia is believed to be the last place Neanderthals inhabited. In this month’s ‘Science Notes’, we explore recent research that has tested whether zinc (Zn) isotopes could be used as a proxy for nitrogen in such cases.
In other parts of Europe, where conditions are more favourable for collagen-preservation, nitrogen analysis of bone and tooth collagen extracted from Neanderthal remains has suggested that they ate meat almost exclusively. It has previously been hypothesised that such a restricted diet could have ultimately led to their disappearance. It is, therefore, important to try to elucidate the diet of the last remaining Neanderthals in Iberia, but so far nitrogen has only been successfully extracted from one example of these remains. To get around this problem, a multidisciplinary team led by Klervia Jaouen from Géosciences Environnement Toulouse recently carried out some exploratory research to see if zinc (Zn) could be used instead.
To test this possibility, the team extracted samples from the tooth enamel of a Neanderthal found in the cave-site Cueva de los Moros 1 in Gabasa, Spain. They also examined tooth enamel from contemporaneous animal bones found in the cave, comprising carnivores (including lynx, wolf, fox, dhole, and cave hyena), herbivores (including Iberian ibex, chamois, red deer, rabbit, horse, and European wild ass), and an omnivore (cave bear).
The results showed that herbivores had higher Zn isotope ratios than carnivores, while the omnivorous cave bear had values more similar to those of the large herbivores and rabbits. This pattern mirrors the modern food-web evidenced from previously published research by Klervia on animals from Koobi Fora in East Africa, suggesting that the Zn results from Gabasa are indeed picking up dietary differences and are not affected by the soil in which they were buried. This is most likely due to the fact that 66Zn is depleted in the body relative to 64Zn through the consumption of herbivore muscles. As such, the ratio between 66Zn and 64Zn should be higher in herbivores than in carnivores, which is exactly the pattern seen both in the modern data from Koobi Fora and in the animals recovered at Cueva de los Moros 1.
The Neanderthal results, however, were a bit of an outlier, with the first molar exhibiting the lowest Zn values recorded, substantially lower than even the lowest carnivore. As the strontium isotope results from the Neanderthal were similar to those seen in the other animals from the cave, this result is not likely to be due to the Neanderthal being from a different geographic origin and instead reflects the individual’s diet. Interestingly, this pattern is seen in the nitrogen isotope results from Neanderthals in other parts of Europe, such as at Goyet (Belgium), Jonzac (France), and Les Cottés (France), where Neanderthals had higher 15N values than all the other carnivores from these sites. This suggests that both Zn and N isotopes are able to pick up the same distinct pattern.
What, then, is causing this unusual pattern in Neanderthals? This remains a mystery, but it could be that Neanderthals solely consumed the meat from animals and not the blood and bone marrow, which would increase 66Zn levels. More research is needed, however, to help explore this further. Overall, though, based on this small case-study, the use of zinc isotopes as a proxy for nitrogen looks promising, and it is hoped that further research will help cement this methodology.
The full results of this research were recent published in PNAS and can be read for free here: https://doi.org/10.1073/pnas.2109315119.