Hominins living on the sedge.

T he evolution of C4 photosynthesis was a signal shift in the history of life on Earth, including at least 50 independent origins in 19 families of higher plants (1). Today, C4 plants account for a quarter of the primary productivity on the planet despite representing a small fraction (∼3%) of the estimated 250,000 land plant species (2). Most C4 plants are grasses (family Poaceae, 4,500 species), followed by sedges (family Cyperaceae, 1,500 species) and dicots (1,200 species). A few C4 plant species contribute significantly to the human diet, either directly as domesticated cereals (e.g., maize, sorghum, millet) and additives (e.g., cane sugar, corn syrup) or indirectly via animals raised on C4 crops and pastures. As a result, human tissues can have high δC values because C4 plants and their consumers are enriched in C (3). In recent years, this telltale signature of a C4-based diet has also beenfound in a wide range of hominin species (4), a finding that has both informed and fueled debate. Now, paleoanthropologists must contend with a unique and surprising finding. In PNAS, Lee-Thorp et al. (5) report carbon isotope data from the tooth enamel of Australopithecus bahrelghazali, a hominin species that lived in the Chad Basin ca. 3.5 million years ago. Their findings, which indicate an early dependence on C4 biomass (∼55–80% of the diet by linear extrapolation), are fascinating because they raise challenging questions concerning the diet of this species. The magnitude of C enrichment, which, among australopithecines, is eclipsed only by Paranthropus boisei (6), suggests that the carbon in their diet was derived mainly from C4 plants rather than the tissues of C4 grazing animals (5). This inference led the authors to focus on sedges, a graminoid plant that is perhaps more promising than grass as a food source for hominins. Indeed, the thickly enameled, low-cusped (bunodont) teeth of A. bahrelghazali and P. boisei would appear to be functionally incompatible with a diet of grass blades (7). Could sedges, then, bring consilience to the C4 conundrum?