The demise of "Nutcracker Man".

M ore than 50 y since the first announcement of Zinjanthropus boisei at Olduvai Gorge (1, 2), the diet of Paranthropus boisei, as it is now known, has continued to be the focus of study and debate. In part, this is because it has such distinctive features, shared in some degree with the South African australopith, Paranthropus robustus: a dished, buttressed face, strongly developed muscle attachments, huge molars and premolars and diminutive anterior teeth, and thick enamel caps, all pointing to a diet requiring heavy, forceful oral processing (from whence the term “Nutcracker Man”). Additionally, P. boisei shared the East African landscape with the earliest members of the genus Homo for more than a half million years, which strongly suggests that they occupied distinct dietary niches. What was P. boisei eating that required such heavy-duty “equipment,” and how did its diet differ from that of early Homo? Until recently, we thought we knew the answers: according to conventional interpretations of the craniodental morphology, they were specialists in small, hard fruits and nuts (3). However, recent research, culminating in the stable isotope study by Cerling et al. in PNAS (4), shows this interpretation to be seriously off the mark. Their study demonstrates firmly and unequivocally that the bulk of P. boisei’s diet was from C4 plant sources. These data, combined with the results of microwear texture studies showing no evidence for hard object feeding (5, 6), place a rather different slant on the diet of this hominin. Equally important perhaps, the results suggest that we need to exercise more caution in interpreting the morphological data in terms of paleodietary ecology. The results are not entirely new, or even completely unexpected, as a preceding study by van der Merwe et al. (7) gave essentially the same result, but it was difficult to know how much reliance to place on data from just two P. boisei specimens. Both studies now show that P. boisei tooth enamel δC values average −1.3 ± 0.9‰. A simple calculation yields a C4 contribution to the diet of approximately 75% to 80%. An important contribution of the study of Cerling et al. (4) is that it allows a good deal of confidence to be placed in the results, based as it is on a decent sample size (N = 24 from 22 individuals) drawn from several sites and periods. Given the sample size and the tight clustering of data toward the positive, C4 end of the spectrum, there can be no equivocation about the quality of the data and what the results mean.