According to a study, there is still untapped potential for anthropology in fossil teeth: changes in diet over the course of the year are reflected in the composition of certain trace elements in the growth lines of tooth enamel. The researchers were able to show this in the case of teeth from early humans and orangutans that lived on the island of Java more than 700,000 years ago. It is becoming apparent that Homo erectus, as a more flexible omnivore, was less dependent on the seasonal food supply than the great ape, the scientists report.
In the past, the focus was more on the external forms of fossil bone structures and teeth. But in the meantime, high-tech has increasingly found its way into the research of primeval primates. In addition to DNA remnants, other "internal values" of the hard body structures can also provide clues: By examining certain substances in the structures of teeth, researchers have already been able to obtain fundamental information about the nutrition of living beings. Now the scientists led by Jülide Kubat from the Goethe University in Frankfurt have successfully gained even more detailed insights.
The focus of their analysis process was the so-called Retzius stripe in the teeth, which occurs when the enamel grows. This material is already formed in the womb and then forms the permanent teeth layer by layer until they later replace the milk teeth of a young person. As with all mammals, tooth enamel is accumulated in batches in microscopically fine layers. This is how the Retzius stripes are formed, which run across the tooth. It is known that certain aspects can be reflected in the distance between these stripes: the development speed of a young person can be read and illnesses can leave traces. The scientists explain that the varying chemical composition of tooth enamel can also contain finely resolved indications of changes in diet.
Targeting fine tooth structures
They have now explored the extent to which analyzes of special substance ratios in the Retzius strips can actually provide such indications. The international team examined fossil teeth from museum collections that were found on the Indonesian island of Java. They come from early humans of the species Homo erectus as well as orangutans and other animals that lived there between 1.4 million and 700,000 years ago. To analyze the enamel, the teeth were cut into wafer-thin slices 150 microns thick. A special laser then removed tooth material, which was then analyzed for specific substance contents using mass spectrometry.
The focus was primarily on the elements strontium and calcium, because their relationship to each other depends on the food, explains senior author Wolfgang Müller from the Goethe University in Frankfurt: "Strontium is gradually excreted by the body - more or less as an impurity of the essential calcium. In the food chain, this means that the strontium-calcium ratio decreases continuously from plant eaters to omnivores and meat eaters," says Müller. This was also fundamentally reflected in the results, as the scientists initially found: big cats had a low strontium-calcium ratio, pure herbivores such as rhinos, deer and hippos, on the other hand, had a high strontium-calcium ratio and Pleistocene pigs were omnivores expected to be in the middle.
nutrition throughout the year
The detailed look at the strontium-calcium ratios (Sr/Ca) over the course of the year in Homo erectus and its relative, the orangutan, which is closer to the other species, was particularly exciting. In the case of the great apes, the results reflected not only the predominantly vegetarian diet, but also a clear change in the composition of the diet in the annual rhythm, the researchers report: They determined pronounced Sr/Ca "peaks". Kubat explains: "The results indicate a rich plant food supply in the rainy season, during which many fruits were formed in the rainforest. In the dry season, the orangutans had to switch to other food sources that might include insects or eggs."
The team found that there was no sign of such a clear change in diet over the course of the year in Homo erectus: the less pronounced peaks and generally lower Sr/Ca values make it clear that he was a more pronounced omnivore or occasional meat consumer. "Our data suggest that on Java, Homo erectus maximized the available resources and was less dependent than the orangutan on fluctuations in the seasonal availability of food sources," the scientists write.
As they emphasize in conclusion, their results show above all the fundamental potential of their method for anthropology. "Overall, the analysis shows that the spatially high-resolution laser analysis of trace elements together with tooth enamel chronology can provide a remarkably detailed insight into the life history of our ancestors," says Müller.
Source: Goethe University Frankfurt am Main, specialist article: Nature Ecology and Evolution, doi: 10.1038/s41559-022-019470