Early Hominin Paleoecology

8. Hominin Ecology from Hard-Tissue Biogeochemistry

281 8 Hominin Ecology from Hard-Tissue Biogeochemistry Julia A. Lee-Thorp and Matt Sponheimer Hominin dietary ecology has been the subject of lively debate for many years, beginning with the discoveries of the first australopiths in Africa, in what seemed to be unlikely habitats for great apes (e.g., Dart 1926, 1957; Robinson 1954). There is good reason for this interest. Large primates spend much of their time searching for or consuming food (e.g., Altmann and Altmann 1970; Teleki 1981; Goodall 1986) and diet is considered one of the most important factors underlying behavioral and ecological differences among extant primates (Ungar 1998; Fleagle 1999). Similarly, the habitats in which they preferred to live are of considerable interest because they essentially form the ecological framework in which various food types may or may not be available at different times of the annual cycle. Thus dietary ecology and environments are closely linked, and both are amenable to investigation by biogeochemical approaches. Information about past diet and ecology can be gleaned from many sources, but in the case of early hominins they are likely to provide only very partial glimpses, and each approach has distinct constraints. The first potential hominins precede the earliest archaeological traces by millions of years, so dietary information from conventional archaeological evidence —butchered animal bones and stone tools—is unavailable except for the more recent periods. Even then this approach encourages a strong emphasis on animal foods at the expense of plants. A good deal of DOI: 10.5876/9781607322252:c08 julia a. lee-thorp and matt sponheimer 282 the information about early hominin diet has come from the fossils themselves, and comparisons with extant primates. Comparative morphology and allometry of the dental “equipment” used for processing and consuming foods have received by far the most attention (e.g., Kay 1985). But there are limitations, as some extant primates are, apparently, poorly equipped for their day-to-day diets. For instance, the relatively large incisors and bunodont molars in modern Papio are consistent with frugivory (Hylander 1975; Fleagle 1999; Ungar 1998), but many modern populations also consume quantities of grass (Altmann and Altmann 1970; Harding 1976; Dunbar 1983; Strum 1987) for which they are less well-equipped. Attempts to explain such apparent contradictions in the fossil record have argued that the dental morphology of extant apes and early hominins is most informative about their “fallback”or stress-season foods than about their “typical” or preferred foods (Ungar 2004). That may be the case, although on closer inspection some of the evidence cited does not support this argument (see below). The dietary problem can be summed up by the observation that most primates are equipped to eat a wide range of foods, and this is especially likely to have been the case for early hominins. Two techniques based on the chemical composition of bones and teeth have been developed as alternative, or rather, complementary, approaches to paleoecology .They are both based on the principle that “you are what you eat,”or that the chemical composition of food is ultimately traceable in the tissues. In this chapter, we outline how trace elements and stable light isotope biogeochemistry has contributed to our understanding of early hominin diets, and also of the ecology of the associated animal assemblages. We begin with a discussion of how trace element abundances in hard tissues are related to diet,as they were the first biogeochemical data to be published for early hominins (Boaz and Hampel 1978). We then discuss the application of stable-isotope data, mostly carbon isotopes . Last, we discuss the ways that stable isotopes can contribute to the ongoing dialogue about the environments with which early hominins were associated. Trace elemenTs Principles There is a long history of using alkaline earth metal concentrations in ancient hard tissues to investigate diet, which began with the work of Toots and Voorhies in 1965, following on from studies of the pathways of radioactive strontium-90 in the human foodweb (Toots and Voorhies 1965). The approach is based on the well-known principle that mammals discriminate against the alkaline earth metals, strontium (Sr) and barium (Ba), with respect to calcium hominin ecology from hard-tissue biogeochemistry 283 (Ca) in the digestive tract and kidneys in a process known as biopurification of calcium (Spencer et al. 1973). The result, as famously demonstrated for a North American ecosystem (Elias et al. 1982), is that herbivore tissues have...