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Roots

What did our early ancestors and related species eat? Different data seemed to give different answers. This week’s paper may have helped to solve this mystery.

Isotope data suggest that tropical grasses were a big part of the diet of the hominins Australopithecus africanus and Paranthropus robustus. These grasses have CO2-concentrating C4 photosynthesis. As a result, they have a little more of the rare carbon-13 isotope, and a little less C12, relative to most other plants. So do the fossil teeth of these early human relatives, as if they ate these grasses. But the shape of their teeth, and wear patterns, are wrong if they mostly ate grass leaves or animals that ate grass. What about roots, or underground storage organs? These are an important food for some human foragers today, especially in dry climates. If our early relatives mostly ate these “USOs?, then the isotope ratios in their teeth should be like those of other species with a similar diet. Mole rats, for example.

This week’s paper is “The isotopic ecology of African mole rats informs hypotheses on the evolution of human diet?, by Justin Yeakel and colleagues at UC Santa Cruz and the University of Pretoria, South Africa, published online in Proceedings of the Royal Society.

They collected skulls of five mole rat species in South Africa. They then analyzed their teeth for C13 and the more common isotope C12. To compare modern samples with those from the Pleistocene, they had to correct for the decrease in the percent C13 in the atmosphere since then. This is due to our burning so much C12-rich coal and oil. Only two of five mole rat species had isotope ratios like those of fossil hominins. The other three apparently didn't eat much grass (leaves or roots).

But isotope ratios in fossil mole rat teeth were similar to those of fossil hominins from the same area. The data suggest that tropical grasses (perhaps their underground storage organs) were an important part of the diet of both mole rats and hominins, but not 100% for either.

These extinct hominins weren’t our direct ancestors. But they branched off our family tree close enough to human ancestors that their diets could be similar.

I liked this paper because it’s like much of science: filling in pieces of the puzzle, rather than one breakthrough that gives a final answer. Also, it reminds me of research at my old Long-Term Research on Agricultural Systems project at UC Davis, where researchers have used isotope ratios in soil organic matter to see how much of it came from recent corn crops (more C13) versus previous alfalfa and tomatoes (less C13).

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