Date Published: March 29, 2017
Publisher: Public Library of Science
Author(s): Jayne Wilkins, Kyle S. Brown, Simen Oestmo, Telmo Pereira, Kathryn L. Ranhorn, Benjamin J. Schoville, Curtis W. Marean, Michael D. Petraglia.
There are multiple hypotheses for human responses to glacial cycling in the Late Pleistocene, including changes in population size, interconnectedness, and mobility. Lithic technological analysis informs us of human responses to environmental change because lithic assemblage characteristics are a reflection of raw material transport, reduction, and discard behaviors that depend on hunter-gatherer social and economic decisions. Pinnacle Point Site 5–6 (PP5-6), Western Cape, South Africa is an ideal locality for examining the influence of glacial cycling on early modern human behaviors because it preserves a long sequence spanning marine isotope stages (MIS) 5, 4, and 3 and is associated with robust records of paleoenvironmental change. The analysis presented here addresses the question, what, if any, lithic assemblage traits at PP5-6 represent changing behavioral responses to the MIS 5-4-3 interglacial-glacial cycle? It statistically evaluates changes in 93 traits with no a priori assumptions about which traits may significantly associate with MIS. In contrast to other studies that claim that there is little relationship between broad-scale patterns of climate change and lithic technology, we identified the following characteristics that are associated with MIS 4: increased use of quartz, increased evidence for outcrop sources of quartzite and silcrete, increased evidence for earlier stages of reduction in silcrete, evidence for increased flaking efficiency in all raw material types, and changes in tool types and function for silcrete. Based on these results, we suggest that foragers responded to MIS 4 glacial environmental conditions at PP5-6 with increased population or group sizes, ‘place provisioning’, longer and/or more intense site occupations, and decreased residential mobility. Several other traits, including silcrete frequency, do not exhibit an association with MIS. Backed pieces, once they appear in the PP5-6 record during MIS 4, persist through MIS 3. Changing paleoenvironments explain some, but not all temporal technological variability at PP5-6.
Humans are unique for their extreme behavioral plasticity and their exaggerated dependence on social learning [1–7]. Given that the modern human lineage originates in Africa, it then follows that the roots for these traits have their origins in Africa, but the origin point or points are still unknown. Humans with modern physical characteristics first appeared in East Africa ~190–150 ka [8–10]. Evidence for behaviors linked to modern behavioral capacities–symbolism, complex tools, sophisticated foraging strategies–first begin appearing in South Africa at about the same time ~ 160 ka [11, 12], but become more common and widespread starting ~100 ka [13–32].
Multiple scales of analysis are possible for the PP5-6 lithic artifacts. Analysis can occur at the level of MIS, technocomplex, geological StratAgg, the more specific Sub-aggregate, or smaller, at the level of the plotted artifact within a Stratigraphic Unit. The maximum amount of behavioral information will come from conducting analyses at each of these multiple scales [118, 123, 145]. This first overall analysis of the assemblage examines variation mainly at the largest scale–MIS.
Descriptive and comparative statistics for all recorded traits are available in Tables A-O in S1 Dataset. Correspondence plots are also presented in Figs A-D in S2 File. Results are summarized here in Figs 1–5, and Tables 3–5. Additional intra-MIS variability plots are available in the Figs E-K in S3 File.
This study analyzed the lithic technological record at PP5-6 between ~90 and 50 ka with a focus on change over time at the scale of MIS. Here we focused on identifying traits that exhibit a pattern consistent with an effect from glacial cycling; they are more alike before and after MIS 4 than during it. A summary of the results is presented in Table 5. A large number of the traits that vary with glacial-interglacial cycles involve how foragers were accessing and processing silcrete and how they were knapping it. Compared to the interglacial occupations at PP5-6, MIS 4 occupants were making increased use of outcrop sources of silcrete. Based on the higher frequency of silcrete pieces that lack visible luster and exhibit cortex, they were preparing nodules by removing detachments prior to heat treatment onsite at levels higher than during MIS 5 and 3. The silcrete core reduction methods utilized during MIS 4 emphasized blade production and resulted in a significantly higher cutting edge to mass ratio. These methods produced an assemblage with increased frequency of flakes and blades with uni- or bidirectional scars relative to flakes with radial or subradial scars. Discarded cores more commonly exhibit removals parallel to a plane of intersection between two faces, and were reduced to a smaller size and with smaller final removals. Many of the core reduction method traits just described relate to maximizing the amount of useable cutting edge (i.e., flaking efficiency, see further discussion below). Preferential selection for silcrete blades as blanks for retouched pieces was more pronounced during MIS 4 than MIS 5 and 3. Retouched piece frequency on silcrete is associated with the interglacial-glacial cycle at PP5-6, with higher frequencies associated with MIS 4. Glacial conditions at PP5-6 triggered changes in the nearby environment, and early human foragers using PP5-6 increased their focus on outcrop silcrete, increased the frequency with which they discarded earlier stage silcrete products at the site, and employed more efficient blade-focused reduction methods for that silcrete.
The analysis presented here identified lithic technological traits that may reflect a response to Late Pleistocene glacial cycling with no a priori assumptions about what those traits will be or how the data are structured. The following qualities of the lithic assemblage are associated with glacial cycling because they are unique to the MIS 4 occupation at PP5-6 compared to the MIS 5 and MIS 3 occupations and they are expressed by a relatively large number of proxy traits: increased quartz, increased evidence for outcrop sources of quartzite and silcrete, increased evidence for earlier stages of reduction in silcrete, evidence for increased flaking efficiency in all raw material types, and changes in tool types and function for silcrete and quartz. Based on these results, it is hypothesized that humans responded to MIS 4 glacial environmental conditions at PP5-6 with increased population or group sizes, ‘place provisioning’, longer and/or more intense site occupations, and decreased residential mobility. Evidence at PP5-6 is not consistent with a climate-driven reduction in population size, shorter and less intense site occupations, or increased residential mobility during glacial conditions.