Hominins, sedges, and termites: new carbon isotope data from the Sterkfontein valley and Kruger National Park

Abstract

Stable carbon isotope analyses have shown that South African australopiths did not have exclusively frugivorous diets, but also consumed significant quantities of C₄ foods such as grasses, sedges, or animals that ate these foods. Yet, these studies have had significant limitations. For example, hominin sample sizes were relatively small, leading some to question the veracity of the claim for australopith C₄ consumption. In addition, it has been difficult to determine which C₄ resources were actually utilized, which is at least partially due to a lack of stable isotope data on some purported australopith foods. Here we begin to address these lacunae by presenting carbon isotope data for 14 new hominin specimens, as well as for two potential C₄ foods (termites and sedges). The new data confirm that non-C₃ foods were heavily utilized by australopiths, making up about 40% and 35% of Australopithecus and Paranthropus diets respectively. Most termites in the savanna-woodland biome of the Kruger National Park, South Africa, have intermediate carbon isotope compositions indicating mixed C₃/C₄ diets. Only 28% of the sedges in Kruger were C₄, and few if any had well-developed rhizomes and tubers that make some sedges attractive foods. We conclude that although termites and sedges might have contributed to the C₄ signal in South African australopiths, other C₄ foods were also important. Lastly, we suggest that the consumption of C₄ foods is a fundamental hominin trait that, along with bipedalism, allowed australopiths to pioneer increasingly open and seasonal environments.

Introduction

Over the past decade, stable carbon isotope analysis of tooth enamel has been used to study the diets of early hominins in South Africa (Lee-Thorp et al., 1994, Sponheimer and Lee-Thorp, 1999a, van der Merwe et al., 2003). The premise of these studies is that you are what you eat, and that the stable carbon isotope composition of your food is ultimately reflected in your tooth enamel, even at several million years remove. Previous research, using non-isotopic techniques, had suggested that australopiths ate diets dominated by fleshy fruits or hard objects most likely originating from trees or bushes (e.g., Kay, 1985, Grine, 1986, Grine and Kay, 1988). These types of plants use the C₃ photosynthetic pathway which discriminates markedly against ¹³C, leading to very depleted ¹³C/¹²C ratios (about −27‰). In contrast, plants that utilize the C₄ photosynthetic pathway, such as tropical grasses and some sedges, discriminate less against ¹³C and are consequently less depleted (about −12‰) (Smith and Epstein, 1971). These distinct isotopic signatures are passed down into the tissues of animals that eat these plants. For instance, the tissues of zebra, which eat C₄ grass, are more enriched in ¹³C than the tissues of giraffe, which eat leaves from C₃ trees. Consequently, it was expected that early hominins, like the modern frugivorous chimpanzee, would have a C₃ isotopic signature (Schoeninger et al., 1999, Carter, 2001). Unexpectedly, however, all of the carbon isotope studies to date have shown australopiths to be rather enriched in ¹³C, suggesting that foods other than fruits were also important dietary components (Lee-Thorp et al., 1994, Sponheimer and Lee-Thorp, 1999a, van der Merwe et al., 2003).

While these isotopic studies were significant in providing evidence that our understanding of australopith diets was too narrow, they had limitations. First, the number of individual hominins analyzed was relatively small. While this has been substantially remedied through recent publication (van der Merwe et al., 2003), the total number of published hominin carbon isotope ratios is still small compared to the number from C₃- and C₄-consuming fauna to which they are statistically compared (Sponheimer and Lee-Thorp, 2003). This limitation in sample size, together with concerns that diagenesis may have affected some results, has led some to question whether or not australopiths really differed from their C₃ plant consuming coevals or modern chimpanzees (Schoeninger et al., 2001).

Another limitation of these studies was that, although they suggested that non-C₃ foods were consumed in significant quantities, they were not able to identify what these foods might have been. Thus, C₄ grasses, C₄ sedges, and animals that ate these foods were all offered as possible australopith foods (Lee-Thorp et al., 1994, Sponheimer and Lee-Thorp, 1999a, van der Merwe et al., 2003, Peters and Vogel, in press). Amongst the reasons for this inability to identify the C₄ dietary source was a lack of data available on the carbon isotope compositions of potential C₄ foods other than grasses and large vertebrates. For instance, a recent investigation of bone tools at Swartkrans suggested that they have wear formed by digging in termite mounds, leading the researchers to hypothesize that consumption of C₄ grass-eating termites might explain the ¹³C-enriched isotopic signature of the australopiths (Backwell and d'Errico, 2001). Although this possibility was both intriguing and plausible given that some termite taxa (e.g., Trinervitermes, Hodotermes) consume grass, it remained speculative as there were no published data on the carbon isotope compositions of African savanna termites excepting a single species Macrotermes michaelseni (Boutton et al., 1983).

We encountered a similar problem with sedges. Conklin-Brittain et al. (2002) recently proposed that underground storage organs (USOs) in wetlands and river margins, such as the starchy rhizomes of some sedges, were important foods for australopiths; and since many sedges utilize C₄ photosynthesis, sedges represent a potential source of the non-C₃ signal observed in early hominins (Sponheimer and Lee-Thorp, 1999a, Sponheimer and Lee-Thorp, 2003, Lee-Thorp et al., 2003, van der Merwe et al., 2003). Unfortunately, however, although an estimated 33% of the world's sedges utilize C₄ photosynthesis (Sage et al., 1999), relatively little is known about the isotopic compositions of sedges in modern South African environments that are most similar to those associated with australopiths (but see Stock et al., 2004). Thus, it was difficult to evaluate the likelihood that C₄ sedges were even available for consumption by South African hominins.

The aim of this paper is to begin to address these limitations in two ways. Firstly, we provide new carbon isotope data from 14 australopith specimens that greatly increase the previously published sample size. Secondly, we proffer novel carbon isotope data from a study of modern termites and sedges in Kruger National Park, South Africa. As Kruger contains a variety of environments that may be similar to those inhabited by australopiths (Reed, 1997, Sponheimer et al., 1999, Sponheimer et al., 2001), we believe that these data represent a reasonable first step towards understanding the isotopic compositions of their potential foods.