Late Pleistocene Adhesives and the Relevance of Birch Tar

The high profile of adhesive technology and birch tar manufacture in discussions about Neandertals is problematic given the so few well-characterized and dated archeological finds. The earliest known evidence of birch tar adhesives dates to a minimum age of 191 ka and consists of 2 unretouched flakes partly covered in birch bark tar from Campitello, Italy (14). At Königsaue, Germany, 2 birch bark tar objects were found dating to >48 ka and >43 ka calBP (15). Other unambiguous MP adhesive evidence consists of bitumen in Syria and pine resin in Italy applied to stone tools for hafting (5, 16, 17) (Fig. 1 and Table 1).

Adhesives also developed in southern Africa. Here residues were observed on Middle Stone Age tools dating to at least 100 to 80 ka (22). They consist of conifer (Podocarpus) resin and tar (22, 23) (Table 1). Authorship of the African adhesives cannot be reliably determined because of the survival of late archaic forms and the limited number of associated taxonomically diagnostic fossils (25, 26). Nevertheless, adhesive technology was used in both Africa and Eurasia by varied hominin populations, and it may be a shared behavior among highly encephalized Pleistocene populations.

The production of adhesives is considered complex when the process is multistepped and requires forward planning, knowledge of materials, and abstraction (27, 28), such as when combining disparate ingredients or synthesizing a new material. For example, Neandertals mixed pine resin with beeswax (5) and bitumen with quartz and gypsum (16) and distilled tar from birch bark. Similarly, African humans combined resin with quartz and ochre (22, 29) and made Podocarpus tar (23). Whereas compound adhesives are made through an additive process, destructive distillation is transformative and concealed. The latter is only observed again with the invention of pottery and, later still, metallurgy. The complex procedural character of tar distillation, combined with recent experimental and archaeological finds, make birch tar a unique window into the development and maintenance of complex technology.

The Zandmotor Find

Geological Setting and Paleoenvironmental Context.

The artifact was found in 2016 by W. van Wingerden on the Zandmotor beach, The Netherlands (SI Appendix, Fig. S1). This beach was constructed in 2011 using dredged sands from 2 permit areas (Q16F and H), located 9 to 13 km offshore (Fig. 2). Here a wide range of archeological and paleontological remains from the Late Pleistocene and the Holocene were brought to the surface (30, 31). The provenance of the sands is documented in the dredging ships’ logs and by the Dutch Ministry of Infrastructure and Water Management.

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Fig. 2.

Paleogeography for the Rhine-Meuse-Scheldt Valley and surroundings during the Last Glacial (after ref. 33). Black dots indicate the relevant find locations: Zandmotor (tar find location, B4 depletion); Q16 F, H (dredging site for the Zandmotor beach); MV2 (Rotterdam Maasvlakte 2, find location MP artifacts, B4 sand depletion); ZR (Zeeland Ridges, find location Neandertal skull fragment, B4 outcrop).

The Zandmotor dredging exploited medium- to coarse-grained sands, deposited on the Last Glacial Rhine-Meuse braid plain. Composing the majority of the dredged interval in permit area Q16 are medium- to coarse-grained fluvial sands of the Rhine-Meuse valley, Units B2 and B4, dating to 70 to 30 ka (32). The full thickness of Unit B4 was mined, including reworked portions of Unit B2. The source bed stratigraphy is confirmed by the Zandmotor malacological and paleontological find assemblage (Fig. 2 and SI Appendix, Fig. S2; SI Appendix provides geological details).

Permit area Q16 is located at the northern rim of the MIS 3 Rhine-Meuse valley. Unit B4 stretches 40 km south (32, 33). Unit B4 is a source bed for Late Pleistocene mammal fauna and MP finds, including bifaces, and a Neandertal skull fragment (1, 30, 31, 34). The Zandmotor find is part of the same archaeological-paleontological complex, firmly situating it in an MP context (SI Appendix, Fig. S3).

Description of the Find.

The find has maximum dimensions of 39 × 35 × 14 mm and weighs 12 g (Fig. 1 and SI Appendix, Fig. S5). The flake is made of a relatively fine-grained grayish flint. It originates from Saalian gravely outwashes, situated close to the findspot (Fig. 2 and SI Appendix, Fig. S2). The flake is unretouched and roughly oval in shape, with a sharp convex side. Located opposite the portion covered in adhesive, the convex side is interpreted as the tool’s working edge. Approximately 40% of the dorsal surface is cortical. The cortex is almost completely covered by tar, possibly providing better adhesion owing to its rough texture (38). As a simple flake, the find cannot be assigned to a particular MP culture/industry.

No traces of extensive rounding are evident, and the surface of the flint appears relatively fresh, suggesting that the find derives from a primary context. The postdepositional microscopic polish that covers the flint surface obscures any wear traces, and although the shape of the lateral edge is suitable for scraping and cutting, no conclusive use traces were found.

The adhesive has a total volume of 1,990 mm³. It has been folded and pressed over the dorsal side of the flake and the dull lateral edge (Figs. 1A and 3). The contact surface between the tar and the flake covers approximately one-third of the flint. The tar has a rough, rounded outer surface that protrudes 10.2 mm from the flake edge and shows a slight concavity. The protrusion might be the remainder of a simple tar handle.

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Fig. 3.

Micro-CT cross-section scans. (A) Weathered surface coating the tar and penetrating along an open crack. (B) Veins of highly attenuating matter following cracks in the tar. (C) Possible charcoal fragments.

The tar has a heterogeneous microstructure (39). Its outer surface consists of a layered coating 0.5 mm thick (Fig. 3A). The coating is tentatively attributed to weathering. Cracks through the tar present similar signs of weathering. Thin veins of highly attenuating material run along the interface of the flint and the tar and penetrate throughout the tar (Fig. 3B). Where the veins outcrop on the tar surface, they have an orange rust color, suggesting that they consist of iron oxide. The veins may result from preferential weathering along cracks and ancient flow lines from when the tar was in a molten state during production. A few dark elongated inclusions likely represent charcoal fragments (Fig. 3C).

Middle Paleolithic Tar Production

To date, 4 methods of tar production, increasing in procedural complexity, have been successfully trialed: condensation, ash mound, pit and vessel, and a raised structure composed of an earthen mound containing a vessel and screen (8, 40). Increasing procedural complexity directly relates to increased tar yield efficiency (SI Appendix, Table S1 and Fig. S6). In single attempts, these experimental methods produced tar volumes of approximately 646, 877, 1,579, and 13,772 mm³, respectively. To make the amount of tar found at the Zandmotor is feasible with each method, but the simple methods would take considerably more time and energy. The simple methods, and the condensation method in particular (8), provide an excellent explanation for the origin and discovery of birch tar and offer suitable methods of producing small quantities of tar when birch resources are plentiful. However, the latter technique would require 40 times as much bark as the raised structure and would take roughly 10 h to produce the Zandmotor tar (8, 40). Similarly, in a Late Pleistocene open woodland (41), compared with the most complex method, the ash mound requires nearly twice as long to collect the firewood and 10 times as much birch bark, which takes 10 times longer to distill (40, 42) (SI Appendix, Table S1). The size of the Zandmotor tar also falls within the range of the other Neandertal birch tar finds, which measure (maximum dimensions in mm, excluding flint) 33 × 21 × 14 (Zandmotor), 42 × 33 × 18 (Campitello Quarry), 27 × 20 × 12 (Königsaue A), and 23 × 14 × 6 (Königsaue B). Thus, the production of these amounts of MP tar represents a considerable technological investment in terms of resources.

Moreover, looking at production temperatures, it is likely that the most complex method was used. Temperatures inside the bark roll for the most successful ash mound experiment reached a maximum of ∼260 °C. In the most successful raised structure experiment, temperatures reached between 310 °C (inside the bark roll) and 360 °C (inside the reaction chamber) (40). Based on the abundance of betulin and lupeol and the absence of degradation markers, the Zandmotor tar may have been produced in the range of 350 to 400 °C. Similarly, the Königsaue betulin content shows that it was also produced at temperatures below 400 °C (15).

Contaminants can be a by-product of the production process, and the soil and bark products in the tar vary based on the production method (40). Micro-computed tomography (CT) scans show a fine-grained contaminate of similar molecular weight to quartz sand or iron oxide, as well as some charcoal distributed throughout the adhesive matrix (Fig. 3C). The homogeneity of the fine-grained Zandmotor contaminants indicate that they were present when the tar was in a molten state and were mixed in thoroughly. Of the experimental production methods, only the intermediate and complex methods made a tar with sufficiently low viscosity to readily mix with contaminant particles. Tar produced by the simple methods has more charcoal and bark fibers and less sediment contaminants, while tar made by the complex production methods has higher concentrations of sand and lower concentrations of charcoal and bark fiber (40). The latter pattern is similar to what we see in the Zandmotor tar. The amount of time and energy required to collect the materials, the temperatures achieved during production, and the contaminants in the Zandmotor find all point to the use of a more complex high-yield tar production method.

Procedural Complexity and Hafting Practices

The qualities that make a technology complex are often unspecified. Although Neandertal single-component tools sometimes exhibit elaborate production sequences (43), the most complex hunter-gatherer technology is represented by hierarchically organized composite facilities and tools and multiple-state tools (i.e., tools with moving parts). The development of composite technology is often seen as a hallmark of cognitive sophistication and demonstrates expert cognition, comparable to that in contemporary populations (28). Adhesive finds represent composite tools that require significantly more cognitive resources to produce and use than single-component tools (28, 44). Further to the use of tar in a multicomponent tool, the production of tar itself represents a 3-level hierarchically organized facility, with different components made to function together (40, 44) (SI Appendix, Fig. S6). In addition, the use of a separate object to collect the produced tar also reflects a degree of mechanical complexity.

Many ideas on the development of composite tool technologies are based on microscopic use-wear, macrofractures (6, 45), and the shape of tools (e.g., the presence of tangs, basal thinning). Yet the functional significance of such morphological features is not always clear (46). The exact hafting configurations and functioning of hafted tools are also debated (47, 48), while variability in methods of hafting is almost completely unexplored (22, 27, 45, 49). Finds from Zandmotor, Campitello, and Fossellone demonstrate that Neandertals repeatedly hafted unmodified, typologically undiagnostic flakes (5, 14), not only Levallois products and retouched tools. This underscores that morphological tool features alone are not a good indication of the presence of hafting technology.

Hafted artifacts are generally envisaged as a stone tool connected to an organic handle (16, 47). The presence of folds, creases, and, in some instances, imprints indicate that all MP tar finds were thick and viscous when applied. The lumps are all folded and pressed over the prehensile portion of the flakes, opposite to the working edge. In addition, the Zandmotor and Campitello finds show no clear evidence of an organic handle (14). This suggests that the tar might not have affixed the flakes to a separate handle, but rather acted as a handle or backing material itself. Reconstructions of the lithic artifact originally embedded in the tar at Königsaue A also suggest the lump was directly attached to a retouched bifacial knife (SI Appendix, Fig. S7A). This is comparable with the Levallois flakes from Syria, in which bitumen functions as a backing material (17). Similar objects are also found ethnographically, such as Australian aboriginal “leiliras” with Spinifex resin handles (50) (SI Appendix, Fig. S7B). This pattern demonstrates the need for nuanced thinking about the roles of adhesives in hafting in the Pleistocene.

We argue that the evidence for hafting and procedural complexity shown here represents a taphonomic exception that provides a window into Neandertal normality. We demonstrate that significant technological investment was expended even on the simple Zandmotor flake, mirroring the Campitello situation. This confirms the routine production of relatively large quantities of tar.

Behavioral Implications

Evidence for Neandertal complex behavior is steadily accumulating. Potential indications for symbolic behavior include cave art (51, 52) and personal ornaments from >115 ka (52, 53). More frequent and continuously exhibited complex behaviors are technological in character, including adhesive production, multicomponent tool technology (5, 6), technological decisions based on a deep understanding of material properties (54), and pyrotechnology (3, 4). The shared nature of multicomponent tools and adhesive technology among Neandertals and African humans suggests that the propensity for such behaviors stems from a common ancestor.

The processes enabling the accumulation and maintenance of complex (technological) behaviors are underevaluated, however. The use of complex technology has been proposed to depend on social group size (9) and to be negatively correlated with residential mobility (11). Archeological and genetic evidence demonstrates that Neandertals lived in very small social groups (55, 56). Due to their lower limb anatomy, these groups had relatively small territory sizes, likely exploited using a system of high residential mobility (57, 58). These modeled effects are supported by archaeological evidence, including limited site structures and shorter raw material transport distances compared with modern humans (59, 60), stable isotope evidence of relatively small territory size (61), and high femoral robusticity pointing to higher degrees of habitual mobility than seen in preindustrial hunter-gatherers (62). These effects must have been most pressing in the northern part of their range, where extreme residential mobility is expected (63). This means that small population size and high residential mobility did not constrain Neandertals from developing and maintaining highly complex (e.g., birch tar) technology. In a similar vein, the development and maintenance of complex behaviors in southern Africa has been attributed to an increased population density (10), but careful scrutiny of the evidence appears to not support this (64).

To warrant the considerable technological investment exhibited by tar production, the development and use of this technology had to confer fitness benefits on the users (65, 66). Complex tools and technological procedures are not exhibited under all conditions, not even by sufficiently cognitively equipped populations (44). Moreover, fitness benefits do not necessarily increase with increasing investment in complex behavior, and the technological investment must be worth the trouble (cf. ref. 67). Generally, as climates get colder, technological complexity increases (44, 68). During MIS 4 and 3, Neandertals at the northern edge of their distribution faced severe ecological risk (63, 66), and the North Sea fauna and vegetation confirm cold, inhospitable conditions for the Zandmotor find (1, 33, 41). The mitigation of ecological risk is one likely explanation for the development and use of complex procedures and technology. Neandertals who operated at the limits of their ecological tolerance (i.e., in conditions where they faced a high risk of resource failure) had to maintain highly complex technological routines. Similarly, in southern Africa, ecological risk also better explains behavioral changes than demography (69, 70). The maintenance of complex procedures can be aided through task specialization. There are ethnographic cases in which the maintenance of technology in general, and adhesive application in particular, are exclusively female domains (71, 72). Neandertal hafting of “domestic” undiagnostic flakes may suggest a higher degree of task specialization than previously considered (cf. refs. 12 and 73). The substantial technological investment into small domestic tools, as testified here, demonstrates that Neandertals used complex behavioral strategies to insulate themselves from the inclement conditions they experienced during MIS 4 and 3.