For the first million years of their existence, the early members of our genus, Homo, shared the African landscape with another group of hominids, the robust australopithecines. Although the two groups were closely related, there were striking differences between them. Perhaps most notably, the robusts had giant molars, thick tooth enamel and a bony crest atop the skull that anchored huge chewing muscles. Paleoanthropologists have long believed that the robusts used their elaborate headgear to process tough plant foods. But the results of a new study suggest otherwise.

Received ecological wisdom holds that two closely related species cannot live side by side unless they differ significantly in the way they use local resources. To explain how early Homo coexisted with the robusts for so long--and under the difficult circumstances of a global drying trend that replaced food-rich forests with grasslands--experts concluded that whereas Homo developed a large brain and tool-making capabilities that enabled it to pursue a diet rich in meat, the robust australopithecines became dedicated vegetarians, evolving the anatomical equivalent of a Cuisinart to grind up nuts, fruits, seeds or tubers, or some combination thereof.

But in recent years the various vegetarian scenarios have come under fire, in large part because of findings from studies of carbon isotopes. These isotopes derive from the food an animal eats and become incorporated into its tissues over time. In the case of the robusts, the ratio of carbon 13 to carbon 12 in their teeth is higher than would be expected of animals that ate mostly fruit and nuts but lower than that of creatures that subsisted on grass seeds. This discrepancy prompted experts to wonder whether the robusts were instead omnivores. The problem is, their anatomy is not what one would expect to see in an equal-opportunity eater.

At the annual meeting of the Society of Vertebrate Paleontology in Mesa, Ariz., last fall, graduate student Alan B. Shabel of the University of California, Berkeley, offered up a possible solution to the riddle of the robusts. He proposed that they were built for eating not tough plant stuffs but hard-shelled invertebrates. Shabel noted that although eastern and southern Africa, where the vast majority of the continent's hominid fossils have turned up, were indeed undergoing desiccation at the time of early Homo and the robusts, wetlands still abounded, as they do today. And like modern wetlands, these ancient lakes, rivers and marshes would have been rich in crabs and mollusks, such as giant land snails.

Looking at the contemporary African animals that specialize in eating these prey--namely, the Cape clawless otter and the marsh mongoose--Shabel found that they have the same suite of peculiar skull features seen in the robusts. They use their powerful chewing apparatus to crunch through the shells. If, as current evidence indicates, the robusts lived in wetland areas, perhaps they, too, evolved to exploit these armored invertebrates.

To test that hypothesis, he performed carbon isotope analyses on the otter and the mongoose and their quarry. Their chemical signatures were comparable to those known for the robusts. According to Shabel, that makes his theory of what the robusts were dining on the only one that is consistent with all the available data.

Other researchers find the idea hard to swallow. To be sure, the robusts had the occasional crab cocktail, surmises paleoanthropologist Donald C. Johanson of Arizona State University. But he doubts that they were specialized to do so: "It is much more likely that [they] subsisted predominantly on a low-quality vegetarian diet, which required significant masticatory preparation."

Shabel, meanwhile, is conducting a much more extensive isotope analysis, which will include more than 1,000 samples of bone, hair and enamel from all the small- and medium- size carnivores found in southern Africa. This will reveal whether the chemical signatures in the tissues of shellfish eaters are indeed unique. His results will no doubt give paleoanthropologists plenty to chew on.