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University of Wollongong researchers have experimentally confirmed that changes in hammer strike angle significantly affect the fracture path and form of stone flakes produced by Neanderthals during the Middle Paleolithic.

Published in Archaeological and Anthropological Sciences, contradict a widely cited fracture model that credited rock core geometry and stiffness with flaking patterns and predicted that hammer strike angle would have minimal effect on flake formation. Results suggest a greater degree of cognitive control by early human tool makers than previously recognized.

Middle Paleolithic stone tool technology is defined by deliberate core preparation to produce flakes of predetermined size and shape. First appearing in the archaeological record between 200,000 and 400,000 years ago, the Levallois method is a hallmark of Neanderthal tool making in this period.

Flakes produced by this method exhibit structured reduction patterns associated with advanced cognitive capacities such as foresight, planning, and long-term memory. Some scholars have argued that consistent flake morphology signals the presence of linguistic and active teaching capacities among hominin toolmakers, as the methods persisted with great consistency for over a hundred thousand years.

Core morphology is widely believed to control flake shape. One standard model describes a bifacial core with distinct striking and debitage surfaces structured around a plane of intersection. Studies have emphasized how lateral and distal convexities and ridge organization guide fracture paths. Much of the literature has focused on the preparation and maintenance of these surfaces to produce distinct flake forms.

Conchoidal fracture is typically described as a wave or plane propagating in the direction of applied force. One influential model credits the stiffness of the developing flake with maintaining fracture trajectory through self-correction, predicting little effect from hammer strike angle.

In contrast, a growing body of experimental research reports systematic changes in flake size, shape, and interior platform angle when the hammer angle varies. Perpendicular strikes tend to produce flakes that are larger and heavier, while more oblique blows yield smaller, thinner forms. This suggests that flake characteristics result from the decisions and techniques of the toolmaker, not simply from the physical properties of the stone.

Determining if hammer strike angle influences flake propagation in prepared cores may clarify the degree of control Neanderthal knappers exercised in stone tool production.

In their study, "Controlling Levallois: the effect of hammer angle of blow on Levallois flake morphology and fracture trajectory," the researchers conducted a controlled experimental study to evaluate the influence of hammer angle on Levallois flake outcomes.

A total of 20 flakes were produced from standardized soda-lime glass cores that replicated the morphology of real Levallois stone cores. Soda-lime glass was selected because it is homogeneous, cost-effective to prepare, and fractures conchoidally in ways similar to common stone tool raw materials. The core design was derived from an experimentally knapped specimen and replicated using a four-axis automated milling machine to produce archaeologically realistic Levallois cores under controlled and repeatable conditions.

Controlled hammer blows were delivered at three strike angles—0°, 10°, and 20°—while platform depth and applied force were also measured. Platform surfaces were adjusted to 75° ±1° using a diamond trim saw to ensure consistency. Each core was mounted in a custom 3D-printed holder and struck by a beveled steel hammer tip powered by a pneumatic system.

Measurements included flake weight, dimensions, fracture trajectory, and detachment force. The analysis relied on 3D scanning, cross-sectional profiling, and general linear modeling to examine relationships among strike angle, platform variables, and flake morphology.

Flakes produced at lower strike angles were consistently larger, heavier, and thicker than those produced at more oblique angles. At 0°, flakes reached fracture trajectory angles between 107° and 116° (median = 116°). At 10°, angles ranged from 105° to 112° (median = 109°), and at 20°, from 102° to 106° (median = 103°). Fracture trajectory angle was found to correlate positively with flake length.

General linear models showed that platform depth and strike angle independently affected flake weight, length, and width. Flake thickness showed a significant interaction between platform depth and strike angle. The force required to detach flakes correlated strongly with flake weight, and this relationship remained stable across all strike angles.

Shape differences were also observed. Flakes detached at higher strike angles were narrower and more pointed distally, while those produced at lower angles tended to be wider and more evenly thickened. Outward-tilting fracture paths at higher angles caused earlier exits from the core surface, resulting in shorter flakes. Fracture trajectories at lower angles propagated deeper into the core, allowing interaction with distal and lateral convexities.

Findings demonstrate that hammer strike angle directly influences flake morphology and fracture trajectory in Levallois core reduction. Lower strike angles produced larger, thicker flakes with deeper fracture paths that intersected prepared convexities more fully. These outcomes contradict predictions from fracture models that attribute flake shape solely to core stiffness and geometry.

Neanderthal knappers may have manipulated strike angle deliberately as part of a skilled knapping process aimed at controlling flake size, shape, and termination. Detaching larger flakes at low angles required greater force and involved increased risk, suggesting that strike angle selection reflected active decisions balancing effort and outcome. Results support the view that Levallois flake production involved knowledge of both spatial planning and real-time adjustment of motor technique.

Controlled variation in strike angle reveals that Neanderthal tool production involved more than core preparation. The findings point to a behavioral repertoire that includes physical coordination, decision-making under risk, and responsive control of force. These capacities expand current models of early human cognition by situating Levallois flake production within a broader context of motor skills and learned techniques.

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