Abstract
The diet of early human ancestors has received renewed theoretical interest since the discovery of elevated δ13C values in the enamel of Australopithecus africanus and Paranthropus robustus. As a result, the hominin diet is hypothesized to have included C4 grass or the tissues of animals which themselves consumed C4 grass. On mechanical grounds, such a diet is incompatible with the dental morphology and dental microwear of early hominins. Most inferences, particularly for Paranthropus, favor a diet of hard or mechanically resistant foods. This discrepancy has invigorated the longstanding hypothesis that hominins consumed plant underground storage organs (USOs). Plant USOs are attractive candidate foods because many bulbous grasses and cormous sedges use C4 photosynthesis. Yet mechanical data for USOs—or any putative hominin food—are scarcely known. To fill this empirical void we measured the mechanical properties of USOs from 98 plant species from across sub-Saharan Africa. We found that rhizomes were the most resistant to deformation and fracture, followed by tubers, corms, and bulbs. An important result of this study is that corms exhibited low toughness values (mean = 265.0 J m−2) and relatively high Young’s modulus values (mean = 4.9 MPa). This combination of properties fits many descriptions of the hominin diet as consisting of hard-brittle objects. When compared to corms, bulbs are tougher (mean = 325.0 J m−2) and less stiff (mean = 2.5 MPa). Again, this combination of traits resembles dietary inferences, especially for Australopithecus, which is predicted to have consumed soft-tough foods. Lastly, we observed the roasting behavior of Hadza hunter-gatherers and measured the effects of roasting on the toughness on undomesticated tubers. Our results support assumptions that roasting lessens the work of mastication, and, by inference, the cost of digestion. Together these findings provide the first mechanical basis for discussing the adaptive advantages of roasting tubers and the plausibility of USOs in the diet of early hominins.
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Acknowledgements
We are grateful to Nigel C. Bennett, M. Casper Bonyongo, Erin E. Butler, Georges Chuyong, Maricela Constantino, the Duckitt family (Jeanette, John, Michael, Richard, Susan, and Wilferd), Nick Georgiadis, Gudo, Johannes and Lene Kleppe, Alicia Krige, Annelise LeRoux, Lomojo, Sarah L. McCabe, George H. Perry, Eric Philander, Frank W. Marlowe, Mustaffa, Kerry Outram, Moses Sainge, Bongani Sethebe, Marietjie Smit, Shirley C. Strum, Duncan Thomas, Carel P. van Schaik, Dirk Wolters, and Bernard Wood. Research permission was granted by CapeNature (permit no. AAA005-00055-0028), the Henry Oppenheimer Okavango Research Center, the Mpala Research Center, and the Northern Cape Department of Nature and Environmental Conservation (permit no. 030/2006). Research funding was received from the American Philosophical Society, the National Science Foundation (BCS-0643122 and IGERT 9987590), the University of California Santa Cruz (grants from the Committee on Research and the Division of Social Sciences), and the Wenner-Gren Foundation.
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Appendix
Appendix
Mechanical properties of plant underground storage organs
USO form, genus, and species | Family | Collection locality | Fracture toughness (J m−2) | Young’s modulus (MPa) |
|---|---|---|---|---|
Bulbs | ||||
Albuca canadensis | Hyacinthaceae | WF | 184.0 | 3.3 |
Albuca cooperi | Hyacinthaceae | WF | 329.0 | 0.8 |
Albuca juncifolia | Hyacinthaceae | WF | 350.0 | 5.0 |
Albuca maxima | Hyacinthaceae | K | 40.0 | 1.4 |
Albuca setosa | Hyacinthaceae | WF | 219.0 | 3.1 |
Albuca spiralis | Hyacinthaceae | K | 87.0 | 1.7 |
Amaryllis belladonna | Amaryllidaceae | WF | 600.0 | 0.3 |
Boophane disticha | Amaryllidaceae | Maun | 161.0 | 3.2 |
Brunsvigia orientalis | Amaryllidaceae | WF | 2293.0 | 2.9 |
Brunsvigia sp. | Amaryllidaceae | K | 260.0 | 1.0 |
Crinum foetidum | Amaryllidaceae | Maun | 126.0 | 1.1 |
Crinum sp. | Amaryllidaceae | HOORC | 101.0 | 2.8 |
Dipcadi crispum | Hyacinthaceae | WF | 451.0 | 0.8 |
Gethyllis affra | Amaryllidaceae | WF | 313.0 | 2.4 |
Haemanthus coccineus | Amaryllidaceae | WF | 560.0 | 4.7 |
Haemanthus crispus | Amaryllidaceae | K | 247.0 | 2.1 |
Hessea chaplinii | Amaryllidaceae | WF | 77.0 | 0.3 |
Lachenalia carnosa | Hyacinthaceae | K | 150.0 | 2.7 |
Lachenalia mutabilis | Hyacinthaceae | WF | 126.0 | 3.1 |
Lachenalia unifolia | Hyacinthaceae | WF | 100.0 | 1.2 |
Ledebouria cooperi | Hyacinthaceae | D | 120.0 | 3.2 |
Ornithogalum thyrsoides | Hyacinthaceae | WF | 202.0 | 1.9 |
Oxalis hirta var. tenuicaulis | Oxalidaceae | WF | 324.0 | 1.5 |
Oxalis obliquifolia | Oxalidaceae | D | 183.0 | – |
Oxalis purpurea | Oxalidaceae | WF | 437.0 | 1.3 |
Oxalis pusilla | Oxalidaceae | WF | 683.0 | 1.7 |
Oxalis versicolor | Oxalidaceae | WF | 606.0 | 2.1 |
Oxalis sp. A | Oxalidaceae | K | 135.0 | 1.8 |
Oxalis sp. B | Oxalidaceae | K | 221.0 | 3.6 |
Scilla dracomontana | Hyacinthaceae | D | 336.0 | 3.9 |
Tulbaghia capensis | Alliaceae | WF | 519.0 | 8.0 |
Veltheimia glauca | Hyacinthaceae | WF | 420.0 | 4.1 |
Corms | ||||
Babiana ambigua | Iridaceae | WF | 194.0 | 7.2 |
Babiana scariosa | Iridaceae | K | 362.0 | 7.1 |
Chlorophytum triflorum | Anthericaceae | WF | 180.0 | 3.1 |
Cyperus alatus | Cyperaceae | MRC | 288.0 | 8.8 |
Cyperus cristatus | Cyperaceae | MRC | 117.0 | 4.7 |
Empodium veratrifolium | Hypoxidaceae | WF | 234.0 | 2.3 |
Ferraria uncinata | Iridaceae | K | 325.0 | 8.8 |
Gladiolus carinatus | Iridaceae | WF | 100.0 | 6.2 |
Gladiolus gracilis | Iridaceae | WF | 220.0 | 2.4 |
Hesperantha falcata | Iridaceae | WF | 634.0 | 4.6 |
Ixia maculata | Iridaceae | WF | 291.0 | 5.8 |
Ixia monodelphia | Iridaceae | WF | 426.0 | 4.4 |
Lapeirousia jacquinii | Iridaceae | WF | 86.0 | 3.8 |
Lapeirousia silenoides | Iridaceae | K | 487.0 | 5.7 |
Melasphaerula ramosa | Iridaceae | WF | 261.0 | 3.1 |
Moraea fugax | Iridaceae | WF | 158.0 | 1.8 |
Moraea miniata | Iridaceae | K | 241.0 | 3.6 |
Moraea tricolor | Iridaceae | WF | 299.0 | 3.5 |
Romulea flava | Iridaceae | WF | 232.0 | 3.4 |
Romulea cf. tabularis | Iridaceae | WF | 292.0 | 5.7 |
Sparaxis bulbifera | Iridaceae | WF | 269.0 | 12.0 |
Spiloxene ovata | Hypoxidaceae | WF | 245.0 | 3.9 |
Wachendorffia paniculata | Haemodoraceae | WF | 100.0 | 3.5 |
Watsonia coccinea | Iridaceae | WF | 328.0 | 2.3 |
Rhizomes | ||||
Bulbinella triquetra | Asphodelaceae | WF | 3645.0 | 2.5 |
Cynodon dactylon | Poaceae | MRC | 3770.0 | 14.0 |
Cyperus dives | Cyperaceae | HOORC | 2379.0 | 13.7 |
Ficinia lateralis | Cyperaceae | WF | 7967.0 | 13.6 |
Nymphea lotus | Nymphaeaceae | HOORC | 414.0 | – |
Phragmites australis | Poaceae | HOORC | 451.0 | 6.2 |
Schoenoplectus corymbosus | Cyperaceae | HOORC | 4743.0 | 13.9 |
Willdenowia incurvata | Restionaceae | WF | 25468.0 | 18.7 |
Zantedeschia aethiopica | Araceae | WF | 193.0 | 5.5 |
Root tubers | ||||
Acanthosicyos naudinianus | Cucurbitaceae | H | 979.0 | 8.0 |
Arctopus echinatus | Apiaceae | WF | 2758.0 | 2.7 |
Asparagus asparagoides | Liliaceae | WF | 114.0 | 2.8 |
Asparagus exuvialis | Liliaceae | Maun | 143.0 | 2.2 |
Asparagus rubicundus | Liliaceae | WF | 296.0 | 1.5 |
Cissampelos capensis | Menispermaceae | WF | 3484.0 | 9.2 |
Coccinea aurantiaca | Cucurbitaceae | Mangola | 399.0 | 3.5 |
Conicosia elongata | Aizoaceae | K | 874.0 | 4.8 |
Cucumis africanus | Cucurbitaceae | H | 1397.0 | 8.3 |
Dioscorea sp. | Dioscoreaceae | Korup | 5955.0 | 5.3 |
Eriospermum capense | Rusaceae | WF | 466.0 | 5.4 |
Eriospermum nanum | Rusaceae | WF | 1089.0 | 3.8 |
Eriospermum sp. | Rusaceae | K | 205.0 | 2.6 |
Euphorbia tuberosa | Euphorbiaceae | WF | 2080.0 | 3.2 |
Helichrysum cf. cochleariforme | Asteraceae | WF | 916.0 | 2.9 |
Hypoxis argenta | Hypoxidaceae | D | 825.0 | 0.8 |
Hypoxis hemerocallidea | Hypoxidaceae | Pretoria | 1290.0 | 7.8 |
Monsonia longipes | Geroniaceae | MRC | 1243.0 | 5.6 |
Nymphea lotus | Nymphaeaceae | HOORC | 1139.0 | 5.5 |
Nymphea nouchali | Nymphaeaceae | HOORC | 1064.0 | 9.4 |
Pelargonium seneciodes | Geraniaceae | WF | 902.0 | 8.2 |
Pelargonium triste | Geraniaceae | WF | 742.0 | 4.6 |
Pergularia daemia | Asclepiadaceae | Maun | 2303.0 | 5.1 |
Pteronia divaricata | Asteraceae | WF | 754.0 | 7.6 |
Rumex lativalvis | Polygalaceae | WF | 735.0 | 5.9 |
Vatovaea pseudolablab | Fabaceae | Mangola | 448.0 | – |
Vigna frutescens | Fabaceae | Mangola | 4859.0 | – |
Vigna macrorhyncha | Fabaceae | Mangola | 543.0 | 4.2 |
Vigna sp. A | Fabaceae | Mangola | 848.0 | – |
Unidentified no 1 | Apiaceae | MRC | 1081.0 | 5.5 |
Unidentified no 2 | Apiaceae | MRC | 679.0 | 4.2 |
Unidentified legume no 1 | Leguminosae | WF | 2114.0 | 7.0 |
Unidentified legume no 2 | Leguminosae | WF | 318.0 | 1.4 |
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Dominy, N.J., Vogel, E.R., Yeakel, J.D. et al. Mechanical Properties of Plant Underground Storage Organs and Implications for Dietary Models of Early Hominins. Evol Biol 35, 159–175 (2008). https://doi.org/10.1007/s11692-008-9026-7
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DOI: https://doi.org/10.1007/s11692-008-9026-7