Mmmm, that's crunchy.

Analysis of isotopes in the teeth of otters and mongooses from Africa have led one paleontologist to suggest that some of humanity's ancient kin shared those modern animals' preference for shelled prey such as freshwater crabs and snails.

The eating habits of ancient hominids known as robust australopithecines have been a matter of debate for decades. These creatures, which are typically lumped into the genus Paranthropus, roamed the African landscape side by side with human ancestors for more than 1 million years--a sign that the two lineages were exploiting different resources, says Alan B. Shabel of the University of California, Berkeley.

Researchers previously suggested that the two lineages could coexist because early Homo species were increasingly eating meat, while robust australopithecines were vegetarians. The teeth of the latter creatures were apparently well suited for chewing plant material. Their molars had a cross section the size of a nickel, about four times the size of modern human molars, says Shabel.

Various scientists have speculated that the teeth enabled robust australopithecines to eat tough foods, such as nuts, hard fruits, and the seeds of grasses. Later research showed that the ratio of carbon isotopes in the creatures' teeth falls between that found in seed-eating animals and that found in fruit-and-nut eaters. That led some scientists to conjecture that Paranthropus had a varied vegetarian diet. But there may be another explanation, says Shabel.

Wetland ecosystems were common in Africa between 2.5 million and 1.5 million years ago, when robust australopithecines thrived. Those regions would have been rich in freshwater crabs and large snails, just as they are today, says Shabel. When he measured the carbon-isotope ratios in the teeth of modern African predators that prefer such shelled prey, he found that those ratios are in the range of that of Paranthropus.

A diet of wetland prey would explain why robust australopithecines needed large molars. Even if the creatures used tools to crush their prey, they'd have required robust teeth to chew shell-ridden morsels.

Future analyses, such as comparisons of the tooth-wear patterns in robust australopithecines with those of modern shell eaters, such as mongooses, may bolster the notion that Paranthropus had a taste for shelled invertebrates long before modern humans developed small wooden hammers and crab picks.