Cooked starchy rhizomes in Africa 170 thousand years ago

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Science  03 Jan 2020:
Vol. 367, Issue 6473, pp. 87-91
DOI: 10.1126/science.aaz5926

Middle Stone Age cooking

Early evidence of cooked starchy plant food is sparse, yet the consumption of starchy roots is likely to have been a key innovation in the human diet. Wadley et al. report the identification of whole, charred rhizomes of plants of the genus Hypoxis from Border Cave, South Africa, dated up to 170,000 years ago. These archaeobotanical remains represent the earliest direct evidence for the cooking of underground storage organs. The edible Hypoxis rhizomes appear to have been cooked and consumed in the cave by the Middle Stone Age humans at the site. Hypoxis has a wide geographical distribution, suggesting that the rhizomes could have been a ready and reliable carbohydrate source for Homo sapiens in Africa, perhaps facilitating the mobility of human populations.

Science, this issue p. 87


Plant carbohydrates were undoubtedly consumed in antiquity, yet starchy geophytes were seldom preserved archaeologically. We report evidence for geophyte exploitation by early humans from at least 170,000 years ago. Charred rhizomes from Border Cave, South Africa, were identified to the genus Hypoxis L. by comparing the morphology and anatomy of ancient and modern rhizomes. Hypoxis angustifolia Lam., the likely taxon, proliferates in relatively well-watered areas of sub-Saharan Africa and in Yemen, Arabia. In those areas and possibly farther north during moist periods, Hypoxis rhizomes would have provided reliable and familiar carbohydrate sources for mobile groups.

Ancient hunting strategies receive more attention from archaeologists than plant collecting, because plant preservation is often poor in archaeological sites. A plant diet, though sometimes invisible, must have contributed substantially to food security in the past, as reported for hunter-gatherers in Africa during the last century (1). African venison is especially lean in the dry season; thus, human populations able to supplement meat diets with carbohydrate or fat avoided malnourishment (2, 3). Geophytes (corms, bulbs, tubers, and rhizomes) store starch in their underground organs, and these underground portions become sources of carbohydrate for humans and those animals able to excavate them. Modern collecting of edible geophytes in South Africa demonstrates that a gatherer’s daily caloric requirement can be met within 2 hours (4), and such foods may become dietary staples. Cooking increases digestibility of meat and plant food, reduces toxicity, and, in the case of geophytes, has a considerable softening effect (5), eases peeling and chewing, and enhances glucose availability (6, 7).

Before the use of fire, hominins may have eaten geophytes raw, especially Cyperaceae and aquatic plants, though some of these first required pulverizing (8). No geophyte remains have been recovered from early, pre-fire sites. Instead, circumstantial evidence is from sources such as isotopes, DNA, or dental calculus (914). Seeds from several edible geophytes and aquatic plants were found in samples from 780 thousand years (ky) ago at Gesher Benot Ya‘aqov in Israel (15); the samples contained seeds rather than the geophytes themselves, perhaps because no trace remains after consumption of geophytes. At Klasies River, South Africa, 120-ky-old charred parenchyma fragments from unidentified geophytes imply cooking (16). Holocene layers of Bushman Rock Shelter and Melkhoutboom, South Africa, yielded geophytes including Hypoxis spp. (17, 18). Here, we present earlier evidence, from at least 170 ky ago, for the cooking of identified edible rhizomes from Border Cave, northern KwaZulu-Natal, South Africa (19) (fig. S1).

Border Cave formed in a Lebombo Mountain cliff facing west across eSwatini (Fig. 1A). The cave was extensively excavated from early in the 1900s (20), but little attention was given to its botanical remains, apart from preliminary studies of 40-ky-old seeds and leaves (21) and chemical identifications of plant poison and resin (22). The cave has alternating brown sand (BS) and white ash (WA) stratigraphic members (21), and rhizomes were recovered from the 5 Brown Sand (5 BS) and 4 White Ash (4 WA) samples. Discrete layers are discernible in each member (Fig. 1B). The majority of identified 4 WA rhizomes come from White 8 to 5 (Table 1), which are combustion features near the base of the member. Only humans could have transported whole rhizomes from the field to the cave. The Border Cave specimens were preserved because they were charred and presumably because they were lost while roasting in the ashes, from which they were recovered archaeologically (Fig. 2). Nothing indicates that the rhizomes were pulverized (23), but circumstantial evidence for cooking is compelling. The spatial context of the rhizomes in ash rather than adjacent sediment is significant. Further support for cooking comes from amylase gene analysis results, which indicate that a high starch diet, possibly involving processing and/or cooking of carbohydrate-rich geophytes by early humans, was already in place by the Middle Pleistocene (24). Cooking enables dietary diversity (25), and transporting geophytes to a home base like Border Cave facilitates both food processing and sharing.

Fig. 1 Border Cave entrance and stratigraphy in Members 5 BS and 4 WA.

(A) Border Cave perched on a cliff in the Lebombo Mountains. (B) Border Cave stratigraphy, excavated from 2015 to 2018, from Members 5 BS and 4 WA in Squares N108 E113 and N109 E113. Note the stacked combustion features in 4 WA. Scale bar, 30 cm.

Table 1 Border Cave whole rhizome frequencies in Members 5 BS and 4 WA.

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Fig. 2 Two whole Border Cave charred rhizomes.

(A) Three views of Border Cave rhizome BC 17 from Square N108 E114, Member 4 WA, layer White 6.1 Idaho (#9577). The proximal end (top left) has been abraded, exposing radial splits caused by charring the fresh organ. Scale bar, 2 mm. (B) Three views of Border Cave rhizome BC 6 from Square N108 E114, Member 4 WA, layer White 6 Idaho (#9433). The rhizome proximal end (left) has a marked central depression and cavities, some of which are root cavities on the circumference. The rhizome profile (center) has a few raised ring scars, and the distal end (right) has a wrinkled, convex base. Scale bar, 2 mm.

The early Middle Stone Age occupations at Border Cave were dated by electron spin resonance on tooth enamel to 161 ± 10 and 144 ± 11 ky ago for 5 BS and 168 ± 5, 115 ± 8, and 113 ± 5 ky ago for 4 WA (means ± standard errors) (26). Bayesian analysis to obtain 95.4% probability ranges for boundaries between members indicated that Member 5 BS occupations may have commenced around 177 ky ago, while those in 4 WA began about 150 ky ago and terminated no later than 99 ky ago (27).

The term rhizome is used here (supplementary text), following the terminology of Wiland-Szymańska and Adamski (28) and Singh (29).The Border Cave 5 BS and 4 WA rhizomes comprise 55 complete, charred specimens (Table 1) that seem to belong to the same taxon, as well as fragments of charred parenchyma that cannot be securely identified (table S1). They are typically globose with a convex or slightly conical base, a depressed center on the proximal surface, radial splits in the ground tissue, rings from leaf scars or fiber on the outer circumference (Fig. 2), and root traces emerging from fiber sheaths within the cortex (Fig. 3B). High magnification using scanning electron microscopy (SEM) reveals distinctive anatomical features, notwithstanding the deterioration caused by charring and mechanical abrasion by cave sediment (supplementary text). Vascular bundles mostly comprise elongated clusters of xylem vessels (12 to >20) with thickened walls of scalariform tissue (Fig. 3D). Parenchyma cells, when not fused into a solid carbon mass, have geometric or oval shapes. The outer ground tissue (cortex) contains many bundles of needle-like raphides of calcium oxalate (Fig. 3F) that are also present in lower frequencies in the inner ground tissue (pith). All of these morphological and anatomical features match those of modern Hypoxis L. spp. rhizomes (Fig. 3, A, C, and E; figs. S2 and S3). Attributes of geophytes from other families appropriate to the area did not match the Border Cave specimens (supplementary text; table S2).

Fig. 3 Selected anatomical features of modern charred Hypoxis rhizomes compared with Border Cave rhizomes.

(A) Modern charred Hypoxis rhizome #55 with rhizodermal sheath. Transverse section (TS) scale, 500 μm. (B) Border Cave rhizome BC 17 with rhizodermal sheath. TS scale, 200 μm. (C) Modern charred Hypoxis rhizome #27, cluster of xylem vessels with walls of scalariform tissue. TS scale, 30 μm. (D) Border Cave BC 6, cluster of xylem vessels with walls of scalariform tissue. TS scale, 30 μm. (E) Modern, charred Hypoxis rhizome #27 with raphide bundle. TS scale, 30 μm. (F) Border Cave rhizome BC 30, raphide bundle. TS scale, 30 μm. X, xylem; P, parenchyma; RB, raphide bundle; RS, rhizodermal sheath.

The morphology of the Border Cave rhizomes is suggestive of Hypoxis angustifolia Lam. (Fig. 4, A, B, and C) rather than the tiny H. filiformis Baker or large H. hemerocallidea Fisch., C.A.Mey. & Avé-Lall. The relatively small, slender-leafed Hypoxis taxa, such as H. angustifolia, H. argentea Harv. ex Baker, and H. filiformis, have rhizomes with white (Fig. 4) or pale yellow flesh. They are more palatable than the orange-fleshed rhizomes from plants such as H. hemerocallidea (29) and are therefore favored as food. While they are edible raw, Hypoxis rhizomes have high fracture toughness until they are cooked (5). The rhizomes are nutritious with an energy value of approximately 500 KJ/100 g, supplemented by essential vitamins and minerals (17). H. hemerocallidea rhizomes have constant carbohydrate composition year-round, though soluble sugars are slightly reduced during the resting season (30).

Fig. 4 Modern Hypoxis angustifolia field appearance, morphology, anatomy, and distribution.

(A) H. angustifolia habit. (B) H. angustifolia fresh rhizome, cut transversely to reveal white flesh. Scale bar, 500 μm. (C) H. angustifolia mature fresh rhizome (left) with developing offshoot (right). Scale bar, 10 mm. (D) Simplified distribution of H. angustifolia through Africa, Yemen, and Indian Ocean islands. (E) H. angustifolia fresh rhizome cut transversely to expose parenchyma cells filled with starch grains. Scale bar, 200 μm.

H. angustifolia, H. argentea, H. filiformis, and H. hemerocallidea grow in the Border Cave area, but recent overexploitation has reduced their populations. H. angustifolia is gregarious, propagates readily from rhizome side shoots (Fig. 4C), and retains visibility year-round, unlike deciduous Hypoxis species. It thrives in a variety of modern habitats and is thus likely to have had a wide distribution in the past as it does today. It occurs in sub-Saharan Africa, Sudan (to about 13°N), some Indian Ocean islands, and as far afield as Yemen (Fig. 4D) (28, 31). Its modern occurrence in Yemen may indicate wider distribution of the rhizome during previous periods of humid conditions.

The Border Cave discovery is early evidence of cooked starchy plant food. The wide distribution of Hypoxis, particularly the small, palatable Hypoxis angustifolia rhizome that grows gregariously in many habitats, implies that it could have provided a reliable, familiar staple food source for early humans moving within or out of Africa. Monocotyledons also provided food security when people moved through southern Europe during the Palaeolithic (32). Certain raw materials found in the Middle Stone Age site of Olorgesaile, East Africa, had distant origins by ≥295 to ~320 ky ago, implying that mobile networks were longstanding (33). Border Cave is remote from dispersal routes out of Africa, yet the site contributes data (19) for assessing the ease with which early Homo sapiens could travel within the continent.

Supplementary Materials

Materials and Methods

Supplementary Text

Figs. S1 to S3

Tables S1 and S2

References (3459)

References and Notes

  1. Materials and methods are available as supplementary materials.
Acknowledgments: We thank M. Bamford, N. Crouch, S. Simm, H. Glen, R. Glen, and L. Kubiak-Martens for comments and botanical assistance and two anonymous reviewers for useful suggestions. Border Cave was excavated with permit #SAH 15/7645 (to L.B.) from Amafa. The comparative plant collection was made with permit #OP4367/2017 (to C.S.) from Ezemvelo KZN Wildlife. G. Pettit supplied some geophytes. The Evolutionary Studies Institute provided resources. A. Kruger produced the drone image. A. Millard provided data from his 2006 Bayesian analysis of ESR dates. Funding: L.B. received funding from the National Geographic Society (NGS-54810R-19) and Centre of Excellence (OP2015/04LB). The Research Council of Norway Centres of Excellence, SFF Centre for Early Sapiens Behavior (SapienCE) (262618) partly funded F.D. Author contributions: The excavation was conceived by L.B. and F.D. and largely funded by L.B. Botanical investigation, formal analysis, and project conceptualization were undertaken by L.W. and C.S. Writing was by L.W. with contributions from C.S., L.B., and F.D. All authors reviewed and edited the manuscript. Competing interests: The authors declare no competing interests. Data and materials availability: All data and materials are included in the text and supplementary materials. The Border Cave excavated material (by L.B.) is housed in the Evolutionary Studies Institute, University of the Witwatersrand, Johannesburg, South Africa (Border Cave 2015/2019).
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