Zinc isotopes in dental enamel: New method for reconstructing the diet of fossil vertebrates

Ratio of zinc-66 to zinc-64 isotopes sheds light on the nutrition of prehistoric mammals

19 February 2020

Little is known for sure about the diet of animals and humans in prehistoric times. We derive our information on our ancestors' diet mainly from carbon and nitrogen isotope analyses of collagen, a structural protein in bones and dentin. Levels of nitrogen isotopes, in particular, allow us to reconstruct the consumption of animals or plants. However, as collagen like other proteins is not easily preserved, the technique is unsuitable for investigating vertebrate fossils more than 100,000 years old. Furthermore, the dry and humid climates in the tropics do their share, limiting the option for measurements to just a few thousand years. And it is precisely such regions in Africa and Asia that are considered key to human evolution and therefore of particular scientific interest. Scientists at Johannes Gutenberg University Mainz (JGU) and the Max Planck Institute for Evolutionary Anthropology in Leipzig have now successfully demonstrated a new technique to assay isotopes of zinc in the enamel of mammalian fossils. Their method can be employed to identify the diet of humans and other mammals living in the Pleistocene in the fossil record as well as to determine whether these were consuming plants or meat. The findings have been published in the Proceedings of the National Academy of Sciences (PNAS).

Zinc isotopes as markers for the type of food consumed

New methods are opening up new research horizons. "We analyzed the ratio of two different zinc isotopes in the dental enamel of mammal fossils that have only recently been discovered in a cave in Laos," reported Professor Thomas Tütken of Mainz University. The fossils date to the late Pleistocene, more precisely to about 13,500 to 38,400 years ago. In 2015, paleontologists uncovered fossils of various mammals, including water buffalos, rhinos, wild boar, deer, bears, orangutans and leopards, in the Tam Hay Marklot cave in northeastern Laos.

"The cave is in a tropical region, in which organic material such as collagen is generally poorly preserved. These fossils therefore represent an ideal opportunity for us to verify whether we can use zinc isotopes to differentiate between herbivores and carnivores," explained Professor Thomas Tütken from the Institute of Geosciences, head of the study.

Initial analysis of zinc isotopes in fossils shows preservation of food signatures

We ingest zinc with food and it is stored as an essential trace element in bioapatite, the mineral phase of tooth enamel. Zinc, therefore, has a better chance of persisting over longer periods of time than collagen-bound nitrogen. It is the ratio of zinc-66 to zinc-64 that is crucial to the technique. "The ratio allows us to determine whether an animal was herbivorous, carnivorous, or omnivorous. That means we can clearly distinguish between fossils originating from carnivores and herbivores, with omnivores lying somewhere in between," said Nicolas Bourgon from the Max Planck Institute for Evolutionary Anthropology, first author of the study and a doctoral candidate in Tütken's research group. Muscle contains more zinc-64 than plant matter. The higher an animal is in the food chain, such as jaguars and tigers, for example, the lower is the ratio of zinc-66 to zinc-64 in their tooth enamel.

Scientists aim to extend the time horizon to encompass fossils over 100,000 years old

This is the first time that the zinc isotope method has been successfully applied to fossils. "The diet-related Zn isotope values in fossil enamel from the Tam Hay Marklot cave suggest an excellent long-term preservation potential, even under tropical conditions," summarizes the paper. This means scientists will be able to employ zinc isotopes as a new tool to study the diet of humans and other mammals using their fossilized teeth. This opens the door to recording data from prehistoric and geological ages well over 100,000 years ago. Tütken is also hoping that it will be possible using the new technique to study fossils of extinct mammals and dinosaurs that are millions of years old. "Our immediate goal, however, is to reconstruct human nutrition on a time scale dating back to the Paleolithic."

The study entitled "Zinc isotopes in Late Pleistocene fossil teeth from a Southeast Asian cave setting preserve paleodietary information" was primarily funded by the German Research Foundation (DFG) and supported by an ERC Consolidator Grant awarded to Professor Thomas Tütken.