Phytoliths as an indicator of early modern humans plant gathering strategies, fire fuel and site occupation intensity during the Middle Stone Age at Pinnacle Point 5-6 (south coast, South Africa)

Abstract

The study of plant remains in archaeological sites, along with a better understanding of the use of plants by prehistoric populations, can help us shed light on changes in survival strategies of hunter-gatherers and consequent impacts on modern human cognition, social organization, and technology. The archaeological locality of Pinnacle Point (Mossel Bay, South Africa) includes a series of coastal caves, rock-shelters, and open-air sites with human occupations spanning the Acheulian through Middle Stone Age (MSA) and Later Stone Age (LSA). These sites have provided some of the earliest evidence for complex human behaviour and technology during the MSA. We used phytoliths-amorphous silica particles that are deposited in cells of plants-as a proxy for the reconstruction of past human plant foraging strategies on the south coast of South Africa during the Middle and Late Pleistocene, emphasizing the use and control of fire as well as other possible plant uses. We analysed sediment samples from the different occupation periods at the rock shelter Pinnacle Point 5-6 North (PP5-6N). We also present an overview of the taphonomic processes affecting phytolith preservation in this site that will be critical to conduct a more reliable interpretation of the original plant use in the rock shelter. Our study reports the first evidence of the intentional gathering and introduction into living areas of plants from the Restionaceae family by MSA hunter-gatherers inhabiting the south coast of South Africa. We suggest that humans inhabiting Pinnacle Point during short-term occupation events during Marine Isotope Stage (MIS) 5 built fast fires using mainly grasses with some wood from trees and/or shrubs for specific purposes, perhaps for shellfish cooking. With the onset of MIS 4 we observed a change in the plant gathering strategies towards the intentional and intensive exploitation of dry wood to improve, we hypothesise, combustion for heating silcrete. This human behaviour is associated with changes in stone tool technology, site occupation intensity and climate change.

Fig 1. Pinnacle Point geographical location.

Map of South Africa and the location of Pinnacle Point and an aerial and panoramic photograph of cave Pinnacle Point 13B and rock shelter Pinnacle Point 5-6N.

Fig 2. Phytolith samples location and concentration.

Long Section stratigraphic silhouette showing the stratigraphic aggregates and sample location (bottom) giving distribution of phytolith presence and preservation (modified after 31, with the BBCSR and BAS sediment profiles, phytolith sample location and concentration information added). This figure is similar, but not identical, to the original image and is therefore for illustrative purposes only.

Fig 3. Layers of an ideal combustion feature.

Schematic drawing of an ideal combustion feature showing the three combustion layers, these being the white (blank), black (horizontal lines) and red (dots) layers, and the sample location of the control samples from above (1) and outside (3) hearth.

Fig 4. Sample mineral composition.

Representative FTIR spectra of sediment samples from different StratAggs and sample types (hearth facies). a) white layer (162466) showing clay absorption peak at 1038 cm^-1 characteristic of burned clay; b) white layer showing clay absorption peak at 1047 cm^-1 characteristic of clay exposed to high temperatures; c) white layer showing three calcite absorption peaks at 1420, 874 and 712 cm^-1.

Fig 5. Plant type and plant parts distribution along PP5-6N.

Box-plots showing the plant types and plant parts identified as significantly different among the different StratAggs at PP5-6N. The median (mid-line), standard error ± (box), standard deviation (whiskers), outliers extended beyond the whiskers, and the trend line (or line of best fit) showing the confidence region for the fitted line are given for each of the plant groups.

Fig 6. Common phytolith morphotypes.

Microphotographs of common phytolith morphotypes identified in samples from different StratAggs of PP5-6N. a-c) irregular morphologies from samples 162467, 162548 and 46682 from SADBS; d-f) grass silica short cells (GSSCs): d- GSSC rondel from sample 162749 from LBSR, e- GSSC rondel tall from sample 162781 from LBSR and f- GSSC oblong tabular sinuate from sample 356455 from LBSR; g-l) restio phytoliths: g,h- sample 602414 from BAS, i- sample 356490 from SADBS, j,k- samples 162782 and 157209 from LBSR, l- sample 388612 from YBSR. Pictures taken at 400x. Scale bar represents 10 mm.

Fig 7. Dicot leaf phytoliths.

Microphotographs of articulated phytoliths from dicotyledonous leaves showing different outlines of the cell walls from different StratAggs of PP5-6N. a) indeterminate outlines from ALBS (357374); b) indeterminate outlines from LBSR (357366); c) polyhedral outlines from LBSR (357368); d) indeterminate outlines from LBSR (357368); e) showing polyhedral outlines from LBSR (356474); f) polyhedral outlines from LBSR (162782); g) sinuate outlines from LBSR (357365); h) sinuate outlines from LBSR (162778); i) sinuate outlines from LBSR (162549). Pictures taken at 400x. Scale bar represents 10 mm.

Fig 8. Comparison with PP13B.

Box-Plots showing the distribution of grass, elongate without decoration margins, dicot leaf and wood/bark phytoliths among the different StratAggs from PP13B (Albert and Marean, 2012) (DB Sand 4c, Upper Roof Spall, Shelly Brown Sand and DB Sand 3 StratAggs) and PP5-6N (YBSR, LBSR, ALBS, SADBS and BBCSR StratAggs) sites. The median (mid-line), standard error ± (box), standard deviation (whiskers), outliers extended beyond the whiskers, and the trend line (or line of best fit) showing the confidence region for the fitted line are given for the four plant types.