COMMENTARY

The demise of “Nutcracker Man”

Julia Lee-Thorp1 Research Laboratory for Archaeology, University of Oxford, Oxford OX1 3QY, United Kingdom

ore than 50 y since the first tight clustering of data toward the positive, foods—they must be regarded as staples, announcement of Zinjan- C4 end of the spectrum, there can be no and moreover they seem to have formed M thropus boisei at Olduvai equivocation about the quality of the a remarkably consistent part of the diet Gorge (1, 2), the diet of data and what the results mean. for at least a half million years (4). Paranthropus boisei, as it is now known, has continued to be the focus of study and What Do These Data Tell Us About the Grasses or Sedges? P. boisei debate. In part, this is because it has Dietary Ecology of ? Is there any evidence to favor either such distinctive features, shared in some The carbon isotope composition of tooth grasses or sedges? On present evidence, degree with the South African austral- enamel reflects all sources of carbon in the there is little to choose between them. opith, Paranthropus robustus: a dished, diet, with the primary difference coming Cerling et al. weigh both options but are buttressed face, strongly developed muscle from isotopically distinct C3 or C4 plants unable to draw a conclusion. The outcome attachments, huge molars and premolars at the base of the foodweb. Since in most carries important implications for the and diminutive anterior teeth, and thick tropical African environments, trees, ecology, biogeography, and social struc- enamel caps, all pointing to a diet re- shrubs, rootstocks, and herbs follow the C3 ture of P. boisei, so it is worth pursuing quiring heavy, forceful oral processing photosynthetic pathway, whereas almost further. Both grasses and sedges are (from whence the term “Nutcracker low-quality foods, that is, they are low in Man”). Additionally, P. boisei shared the The consistently positive protein and high in cellulose (9). Grasses East African landscape with the earliest 13 are widely and fairly homogeneously dis- members of the genus Homo for more tooth enamel δ C values tributed in African savannas, and they than a half million years, which strongly can support a relatively high biomass, as suggests that they occupied distinct dietary for P. boisei indicate a Cerling et al. point out (4). However, the niches. What was P. boisei eating that re- more nutritious period of new growth quired such heavy-duty “equipment,” and direct reliance on C4 and seed production is seasonally re- how did its diet differ from that of early stricted to the wet season; in the dry Homo? Until recently, we thought we plants. season, grass blades quickly become des- knew the answers: according to conven- iccated, leaving the rhizomes as the re- tional interpretations of the craniodental maining grassy resource. Modern grass- all grasses and many sedges are C4, the morphology, they were specialists in δ13C dietary distinction is really about eating suids (i.e., warthogs) use the full small, hard fruits and nuts (3). However, whether the carbon source is from tropical suite of grass resources, as do modern recent research, culminating in the stable grasses or sedges versus the edible parts Theropithecus gelada, which utilize grass isotope study by Cerling et al. in PNAS of all other plants. Cerling et al. (4) make rhizomes and herbs when their favored (4), shows this interpretation to be seri- a compelling argument that the consis- grass blades are unavailable (10). P. boisei ously off the mark. Their study demon- tently positive tooth enamel δ13C values could possibly have followed a similar fi strates rmly and unequivocally that the for P. boisei indicate a direct reliance on strategy, as Jolly originally suggested bulk of P. boisei’s diet was from C plant (10). However, the idea of seed- and 4 C4 plants—grasses or sedges—because sources. These data, combined with the rhizome-gathering as a viable strategy indirect consumption via C4-consuming results of microwear texture studies animals would require impossible amounts has been regarded with some skepticism, showing no evidence for hard object of animal food. That does not rule out possibly because it requires large energy expenditure for relatively poor energy re- feeding (5, 6), place a rather different C4-consuming animals in the diet, but turns. C sedges, on the contrary, are slant on the diet of this hominin. Equally plants must form the bulk of the C4 carbon 4 important perhaps, the results suggest contribution. Leaving aside the source of perennials available throughout the year, that we need to exercise more caution in carbon for the moment, it is worth em- but they are patchily distributed because interpreting the morphological data in phasizing that such a strong reliance on most sedges prefer permanently damp terms of paleodietary ecology. C4 resources in a higher primate is well- ground or wetland (12). van der Merwe The results are not entirely new, or even nigh unique. It is rivaled only by the iso- et al. favored exploitation of C4 sedges completely unexpected, as a preceding topic composition of the extinct Gelada as a plausible explanation for the high 13 study by van der Merwe et al. (7) gave baboon, Theropithecus oswaldi (figure 1 δ C values observed in P. boisei, pointing essentially the same result, but it was in ref. 4). That P. boisei was even more out that the raw culms and large, fleshy fi dif cult to know how much reliance to heavily reliant on C4 resources than this rhizomes of the common, highly pro- place on data from just two P. boisei grass-eating baboon is remarkable. ductive C4 sedge, Cyperus papyrus, are specimens. Both studies now show that We can also infer with some confi- good sources of carbohydrates still used δ13 P. boisei tooth enamel C values average dence that these C4 foods were reliable by people in the Okavango Delta (7). −1.3 ± 0.9‰. A simple calculation yields throughout the year because, although Can other, independent evidence be aC4 contribution to the diet of approxi- no intratooth measurements were made, brought to bear on this question? Analyses mately 75% to 80%. An important con- δ13C values near zero allow little room of preferred habitat associations produce tribution of the study of Cerling et al. (4) for much seasonal variation. This obser- is that it allows a good deal of confidence vation stands in contrast with the high to be placed in the results, based as it is variability seen in the diets of the South Author contributions: J. L.-T. wrote the paper. on a decent sample size (N = 24 from African australopith, P. robustus (7). Such The author declares no conflict of interest. 22 individuals) drawn from several sites a strong emphasis also precludes notions See companion article on page 9337. 1 and periods. Given the sample size and the that the C4 resources formed “fall-back” E-mail: [email protected].

www.pnas.org/cgi/doi/10.1073/pnas.1105808108 PNAS | June 7, 2011 | vol. 108 | no. 23 | 9319–9320 Downloaded by guest on September 28, 2021 highly variable results (13, 14) and are thus means that internal water in their plant which P. boisei focuses on C4 grasses an- not very helpful to address the question. foods has relatively low δ18O values (15). d/or sedges whereas P. robustus continues Microwear texture analysis on P. boisei It could be speculated that the low P. to follow a more traditional hominoid δ18 specimens are reported to show similar boisei O values indicate more than just mixed diet expanded to include some C4 complexity profiles—and dissimilar di- a requirement for drinking water; might resources. This explanation is less than fi — rectionality pro les as T. oswaldi (6), their preferred plant foods have low values satisfactory given the timing. P. boisei suggesting that the mechanical properties too? If that is the case, sedges would fi fi sensu stricto possesses more derived mor- were similar despite differing direc- best t the pro le, as they are not subject phological features than P. robustus, and tionality imposed by tooth morphology. to the same pressures of evapotranspira- yet appears earlier in East Africa (14). However, similar complexity profiles, tion as purely terrestrial plants. However, That would imply that the form with the and indeed steep macrowear profiles, in at present, we lack supporting evidence, P. boisei could just as easily reflect re- as there are no published δ18O values for more derived set of dentognathic features petitive forceful chewing of fibrous sedge sedges, or sedge-consumers, and little in- evolved before the less derived form, culms and rhizomes. One of the problems formation on the precise distribution of which makes little evolutionary sense. The important question, now given added faced by methods that investigate wear C4 sedges in East African wetlands. We fabrics is the requirement for appropriate also lack δ18O and δ13C data for East impetus by the demonstration of the comparative models among primates. African T. oswaldi, to provide a helpful distinct dietary ecologies, is whether Yet, in the case of P. boisei, there are terrestrial C4 grass forager primate com- the two forms evolved from a common none, bar perhaps Theropithecus. parison for P. boisei. These gaps require ancestor or independently in the two re- One other clue may reside in the enamel further research. gions. Certainly there are suitable candi- oxygen isotope (δ18O) composition. The dates for precursors in both regions— δ18O values reported for P. boisei are low What Are the Implications for P. boisei P. robustus Paranthropus aethiopicus in East Africa (−1.3 ± 1.3‰) compared with all other and Monophyly? and Australopithecus africanus in South fauna except the hippopotamus (4). The Both stable carbon isotope and microwear Africa. Independent origins would consti- authors suggest that this observation in- data have shown that the diets of P. boisei tute, as noted by Wood and Constantino dicates high water dependency for P. boi- in East Africa and P. robustus in South (ref. 14, p. 118), “a striking case of ho- sei, which is almost certainly the case. Africa are fundamentally different despite moplasy with both lineages exhibiting However, these values are lower than the apparent similarities in their craniofa- a morphological trend towards mas- those of the other obligate-drinking, grass- cial morphology. This result alone sug- ticatory hypertrophy and extreme post- eating animals, such as the suids and gests that we should be more cautious in ” equids (figure 2 in ref. 4). The low δ18O transferring observations between the two canine megadontia, and further, all in values universally observed in hippo- forms and look more carefully at the dif- the absence of close similarity in dietary potamus are linked to a combination of ferences. There are no obvious external ecology. several factors: in addition to their re- explanations for the stark difference in Certainly the new isotope data for quirements for copious amounts of drink- diet, and indeed δ13C values for early P. boisei has unleashed many intriguing ing water, their semiaquatic lifestyle Homo in Swartkrans (16) and Olduvai questions and raised new avenues for regulates body temperature without loss (7) remain broadly similar. Cerling et al. future research. That is the way it of water vapor, and nocturnal feeding (4) suggest an adaptive divergence in should be!

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