Biocultural evidence of precise manual activities in an Early Holocene individual of the high‐altitude Peruvian Andes

Abstract

Objectives: Cuncaicha, a rockshelter site in the southern Peruvian Andes, has yielded archaeological evidence for human occupation at high elevation (4,480 masl) during the Terminal Pleistocene (12,500-11,200 cal BP), Early Holocene (9,500-9,000 cal BP), and later periods. One of the excavated human burials (Feature 15-06), corresponding to a middle-aged female dated to~8,500 cal BP, exhibits skeletal osteoarthritic lesions previously proposed to reflect habitual loading and specialized crafting labor. Three small tools found in association with this burial are hypothesized to be associated with precise manual dexterity. Materials and methods: Here, we tested this functional hypothesis through the application of a novel multivariate methodology for the three-dimensional analysis of muscle attachment surfaces (entheses). This original approach has been recently validated on both lifelong-documented anthropological samples as well as experimental studies in nonhuman laboratory samples. Additionally, we analyzed the threedimensional entheseal shape and resulting moment arms for muscle opponens pollicis. Results: Results show that Cuncaicha individual 15-06 shows a distinctive entheseal pattern associated with habitual precision grasping via thumb-index finger coordination, which is shared exclusively with documented long-term precision workers from recent historical collections. The separate geometric morphometric analysis revealed that the individual's opponens pollicis enthesis presents a highly projecting morphology, which was found to strongly correlate with long joint moment arms (a fundamental component of force-producing capacity), closely resembling the form of Paleolithic hunter-gatherers from diverse geo-chronological contexts of Eurasia and North Africa. Discussion: Overall, our findings provide the first biocultural evidence to confirm that the lifestyle of some of the earliest Andean inhabitants relied on habitual and forceful precision grasping tasks.

Biomechanics of the human thumb and the evolution of dexterity

“…On this basis, the likelihood that these structures were also functionally equivalent in extinct hominins is very high, offering the necessary scientific framework for meaningful comparisons and functional interpretations across species. In fact, the entheses of m. opponens pollicis have been frequently analyzed in past anthropological research, 39 , 40 , 62 , 63 likely due to their high distinctiveness and morphological variability across and within hominin species. In contrast, given that m. flexor pollicis brevis and m. abductor pollicis brevis tend to insert into the same broader tubercle of the proximal phalangeal base, 41 an accurate distinction of each muscle’s attachment area on the fossil remains of extinct species would be challenging.…”

“…Moreover, we would argue that the reliability of any attempted (mathematical or geometrical) reconstruction of the tubercle’s missing landmark points would likely be extensively undermined by the very high morphological variability of hand muscle attachment sites (e.g., Karakostis et al. 62 ), in combination with the fact that the complete trapezium morphology of this 4-million-year-old species of Australopithecus is entirely unknown. 4 …”

“…This technique comprises a standard step for size-adjustment in landmark-based geometric morphometrics. 9 , 62 …”

“…The developed models focused on the calculation of joint torque based on three landmark locations on the elevated bone area of the metacarpal muscle attachment ( Figure 1 C; Table S4 ). In order to further address variability in bone projection across the entire entheseal surface, we analyzed this area using the highly repeatable 3D geometric morphometric approach introduced in previous research 62 , 63 The entire process was carried out using the Geomorph package (version 3.3.1) of the R software. 53 That previous study focused on a sample from the same recent modern human collection (Basel-Spitalfriedhof collection), identifying a primary principal component associated with proportional elevation across the 3D entheseal surface (also see Karakostis et al.…”

“…This broader area encompasses both the distalo-lateral roughened area as well as a large amount of surface that does not typically present distinctive alterations on dry bone. 62 , 75 Moreover, this might not consistently be the case for the insertions of chimpanzees (e.g., see Jacofsky et al. 76 ), despite the fact that their m. opponens pollicis also broadly attaches in the lateral metacarpal shaft.…”