Isotopic clues to diet of early hominins

‘We are what we eat’ is certainly a truism, but it is neither a trope nor a cliché. The phrase is especially appropriate when scientists examine isotopes of a variety of elements in bones or teeth. For instance the relative proportions of two stable isotopes of the metal strontium – 87Sr and 86Sr – differ from place to place in soil because 87Sr is the daughter isotope of radioactive 87Rb. The older the rock from which a soil has formed the more of the radioactive rubidium isotope will have decayed. Not only does this increase the 87Sr/ 86Sr ratio in the rock and the soil derived from it, but vegetation inherits it too. So it gets into an animal’s diet and ultimately its teeth. A human who has migrated will carry the ratio of the geology of her early home geology in her adult teeth – fully developed by about 13 years-old – to wherever she dies. Likewise, the different oxygen isotopes in rainwater, which result from climate variation, end up in teeth thanks to what a person ate before adulthood. The two ‘signatures’ together allowed archaeologists to backtrack the famous ‘Amesbury Archer’, who may have brought Bronze Age culture to Britain, back to the Alps of Central Europe. Just what a human diet comprised can be roughly assessed from the carbon and nitrogen isotopes in collagen that fossil bone sometimes preserves: the proportion of seafood relative to the meat of land herbivores and the amount of terrestrial grains, nuts and fruits. The trouble is, collagen degrades with the age of human remains and another approach is needed to assess the diets of our distant forebears.

Calcium isotope data from early hominins and some modern primates. Increasingly negative values of δ44/42Ca signify lower values of the ratio compared with a standard. (Credit: Martin et al. 2020; Fig. 1)

It turns out that calcium isotopes in teeth, which do not degrade over extremely long time spans, offer clues to diet. In particular the dental 44Ca/42Ca ratio decreases as its hosts rise in the food chain; effectively as the meat content in their diet increases. This approach has been applied to the hominin and non-human primate fauna of the Turkana Basin in Kenya (Martin, J.E. et al. 2020. Calcium isotopic ecology of Turkana Basin hominins. Nature Communications, v. 11, article 3587; DOI: 10.1038/s41467-020-17427-7). The shores of a large lake in the vicinity of modern Lake Turkana were occupied from 3.5 to about 2 Ma ago by early Homo, australopithecines, paranthropoids and baboons. Using dental Ca isotopes fails to distinguish Australopithecus anamensis and Kenyanthropus platyops, whereas carbon isotopes suggest that the first had a purely C3 plant diet – fruiting plants that thrive under cool, wet conditions, as beneath woodland canopies – whereas Kenyanthropus foraged on both these and the C4 plants – many grasses and sedges – that favour open, well-lit grassland. The 44Ca/42Ca ratios in Homo teeth span a wide range of values that point to omnivory and even a high dietary meat content: a similar isotopic pattern to those of fossil baboons and geladas. Paranthropus boisei is definitely the odd-one-out, among both ancient and modern primates, and even among paranthropoids as a whole. It most likely had a specialised diet. Its teeth show wear patterns that suggest soft plant material, which seems to rule out grasses which are abrasive. Perhaps it fed on succulent semi-aquatic plants of the lake shore. When Mary Leakey first discovered P. boisei in 1959, she and husband Louis considered that its huge molars with thick enamel indicated that it ate hard vegetable matter, hence its original nickname ‘Nutcracker Man’. It also had hands capable of precise manipulation, indeed the association of the first specimen with Oldowan-type stone tools led to speculation that it had made them. Some specimens are associated with long bones with worn ends, suggesting that they may have used them for digging.