I’ve had on my stack for quite a long time, a short paper by Nicholas van der Merwe and colleagues, assessing the stable carbon isotope ratios in several specimens from Tanzania. These include the Homo habilis specimens OH7, OH62 and OH65, and the A. boisei specimens OH5 and the Peninj mandible.
The ratio of stable carbon-13 and carbon-12 enable an assessment of the amount of C4 versus C3 plants in the diet. I discussed the basic ideas in a longer post from 2005.
The results on the Homo specimens are not too surprising. All three specimens overlap with South African A. africanus. OH7 and OH62 in particular have values around 20% C4, which is right near the mean observed for South African Homo and A. robustus from Swartkrans. OH65 has a higher C4 percentage than the other two, but within the range observed for Sterkfontein Member 4 A. africanus, which was significantly higher than Makapansgat or the other South African samples. So it would appear that the diet of Homo habilis did not differ from earlier hominins in terms of the ultimate origin of carbon in grasses versus non-grass plants.
What is more surprising is the extremely high amount of C4-derived carbon in OH5 and Peninj. They score 77% and 81% C4, respectively. These are the only two specimens of A. boisei for which these stable isotopes are known, and they are very far from the observed range in the South African A. robustus.
The authors suggest an interesting source for this high C4 proportion – papyrus. They described a tasting tour of the wild plants of the Okavango:
Bamford and van der Merwe investigated (and ate) the edible plants of the Okavango Delta in Botswana during the dry season (July 2003), assisted by Ezaya Karesaza, a tourist guide who grew up in this extensive wetland. Among the C3 plants that are traditionally eaten raw in this region are a variety of fruits and seeds, as well as plants of which the leaves and rhizomes are eaten. The latter include Aeschynomene fluitans, a floating legumi- nous plant, of which the leaves taste like lettuce; Typha capensis, which grows in thick stands along the waters edge, of which the rhizomes have a pleasant taste; and Schoenoplectus corymbosus, a big water sedge, of which the stem is succulent at the bottom end. Among C4 plants, the rhizomes and culms of three other species of sedges are edible. These include Cyperus denudatus and C. dives, which grow in the grasslands of the floodplains. Unlike the grasses, they are green year-round, although not particularly prolific. The most common C4 sedge, by far, is Cyperus papyrus, which grows in dense thickets along the water edge. This species has culms as high as 4 m, of which the lowermost 0.5 m is frequently chewed by local people. It has a soft, white rind about 0.5 cm thick; the interior, about 2 to 3 cm in diameter, is more fibrous. It is chewy and pleasant tasting. The thick rhizome of papyrus is more fibrous and starchy than the culm, somewhat astringent, and requires considerable chewing effort. It produces a bolus in the mouth that has to be spat out at intervals.
They then reported the results of a nutritional analysis of the papyrus culm and rhizome, which have roughly the nutritional and caloric value of domestic potatos, although would require a significant gut flora to deal with the cellulosic content.
All in all, it’s very curious that A. boisei is so different in these isotopic values compared to other early hominins. The theme was picked up last year in a paper by Richard Wrangham and colleagues, who focused on the idea of “fallback foods” – the kinds of foods that an animal does not prefer, but eats when other more highly preferred foods are not available. Considering the very high C4 proportion indicated by the OH5 and Natron isotope values, it doesn’t seem likely that this reflects a fallback strategy, but possibly an initial exploitation of such resources as fallbacks facilitated a later, more developed adaptation to them.
van der Merwe NJ, Masao FT, Bamford MK. 2008. Isotopic evidence for contrasting diets of early hominins Homo habilis and Australopithecus boisei of Tanzania. S Afr J Sci 104:153-155.
Wrangham R, Cheney D, Seyfarth R, Sarmiento E. 2009. Shallow-water habitats as sources of fallback foods for hominins. Am J Phys Anthropol 140:630-642. doi:10.1002/ajpa.21122