Hand morphology, manipulation, and tool use in Neandertals and early modern humans of the Near East

Between 140,000 and 50,000 years ago, both Neandertals and early modern humans periodically inhabited the coastal woodlands and inland steppe of the Near East. The archeological remnants of the behavior of these two groups—mostly in the form of stone tools and animal bones—are so similar that were it not for the fact that both groups occasionally buried their dead in the caves in which they lived, we would not have known that they derived from more than one population. Although morphologically and taxonomically distinct, both types of human are associated with Middle Paleolithic (Mousterian) stone-tool assemblages that cannot be differentiated clearly on the basis of technological and formal typological attributes (1, 2). But although the archeological record indicates behavioral similarity, and by inference adaptive similarity, between Neandertals and early modern humans at this time, functional morphological studies of the human skeletons suggest quite a different story. Detailed study of parts of the skeleton that alter their material and geometric properties during life in response to activity or that show degenerative changes from wear and tear suggest important behavioral differences between these groups (3, 4). This situation raises perplexing questions. If both tool assemblages and the malleable aspects of the skeletons of the humans who made, used, and discarded those tools are monitoring prehistoric behavior (5), why do these two data sets produce such contrasting interpretations of that behavior? And if behavioral contrasts did exist, did these contrasts serve to give one group a competitive advantage? This later question is important, because many see the Near Eastern early modern humans as the source population from which modern humans migrated into Europe approximately 36,000 years ago—leading to the extinction of the Neandertals soon after. Evolutionary models that posit a single center of origins for modern humans (such as Africa) with subsequent geographic expansion of modern humans and replacement of indigenous archaic populations (such as the Neandertals) rely on the notion of adaptive superiority of the invading modern humans. Might there then be indicators of a competitive edge to early modern human adaptive strategies in the Near East? By focusing on skeletal morphology that reflects manipulative behavior, the paper by Niewoehner (6) in this issue of PNAS provides an important connection between morphology and the production and use of technology. Although many questions yet remain, this study represents an important step in resolving contradictions in our behavioral interpretations of the fossil and archeological records of the Near Eastern Middle Paleolithic and in delineating the adaptive characteristics of both groups of Mousterian humans.

On the basis of a multivariate analysis of three-dimensional landmark data from carpal and metacarpal joint surfaces of Neandertals and Near Eastern early modern humans, Niewoehner (6) concludes that important manipulative differences existed between them—differences not in dexterity but in grip strength and the ability to resist forces incurred in certain grip positions. These differences indicate contrasts in the habitual manipulative repertoires of the two groups. Specifically, details of the form of the base of the thumb, index, and middle finger in the modern humans from Skhūl and Qafzeh Caves indicate a hand that was, like ours, well adapted to withstand forces acting on the hand during oblique power gripping (as in gripping a hammer handle). Neandertals, in contrast, had hands well suited to forceful transverse power grips (as when gripping a hammerstone), as indicated by the greater leverage of their thumbs, enlarged crests for the muscles of finger flexion, broad finger tips, and lack of specializations in the midhand (carpometacarpal joints) to resist obliquely oriented reaction forces (Fig. 1).

Niewoehner concludes that important manipulative differences existed between Neandertals and Near Eastern modern humans—differences not in dexterity but in grip strength …

Figure 1.

Fossil hands of Near Eastern Mousterian humans. (Left) Palmar view of the left hand of the Shanidar 4 Neandertal. (Right) Palmar view of the right hand metacarpal and phalangeal skeleton of the Qafzeh 9 early modern human The two hands are not to the same scale. (Reprinted with permission from E. Trinkaus and Israel Department of Antiquities).

The small joints between the bones of the hand must withstand both violent forces (such as impact forces from striking a hammerstone on a flint core) and repetitive loading. For these reasons, articular cartilage in the wrist and hand is prone to degeneration, and osteoarthritis is found often in the hands of foraging people (7, 8). The debilitating pain of osteoarthritis and reduced mobility of the thumb and fingers certainly would have interfered with an individual's ability to manufacture tools and procure edible resources. The associated reduction in fitness creates selection for joint configurations that best limit damaging forces in the context of commonly used grips and manipulative activities. For these reasons, the results reported by Niewoehner are likely to reflect real and meaningful differences in manipulation between groups. But what is the significance of these differences?

Living humans commonly use oblique power grips when using tools set in handles. Hafting of lithic tools into wooden or bone handles was likely a part of the technological repertoire of early modern humans. Mastic (gum) was used as a hafting adhesive during the Middle Paleolithic at Apollo 11 Cave in Namibia (9), which is precisely the region of the world in which modern human anatomy and many aspects of modern behavior are thought to have arisen first (10). However, hafting seems to have been part of the Neandertal repertoire as well. Haft polish has been observed on lithics from the European Mousterian (before the arrival of modern humans; ref. 11), and solidified pieces of resin bearing imprints of the stone tools and wooden hafts have been recovered from the Mousterian of Germany (12). Shea (13) also reported edge wear attributable to hafting in roughly equal frequencies on Mousterian lithics from Kebara (with associated Neandertal fossils) and Qafzeh (with associated modern human fossils) caves in Israel (some of what is identified as haft wear, however, may be the result of prehension with gritty fingers, and thus differences in the frequency of hafting still may have existed between groups). What may have differed between these groups then was not the ability to produce composite tools but rather the importance of this technology. Given the increased mechanical advantage of hafted over hand-held tools in butchery, wood working, hide preparation, and other tasks, such a difference may reflect contrasts in the relative amount of muscular effort that each group devoted to economic and technological activities. A more profitable use of leverage by the Skhūl/Qafzeh modern humans would be consistent with aspects of their upper-limb morphology that denote reduced mechanical loads relative to, and arm positions different from, the Neandertals (4, 14).

Hafting seems to have been relatively common during Mousterian times in the Near East. Traces of bitumen adhesive on tools from Hummal and Umm El Tlel (Syria; ref. 15), as well as the recovery of a broken Levallois point embedded in a cervical vertebra of a wild ass at Umm El Tlel (16) and the edge wear analyses reported by Shea, attest to a level of reliance on hafted tools sufficient to have made this behavior archeologically visible. Unfortunately, without direct evidence in the form of human fossils, we cannot know which group left these remains, and evaluating the importance of hafting in the two groups remains difficult.

Two indirect lines of archeological evidence hint that hafting may have played a more important role in the economic and technological realms of early modern humans in the Levant. Although wear attributed to hafting occurs with equal frequency at Kebara and Qafzeh (13), there is a marked contrast between the sites in the extent of retouch on the used tools. Only about 20% of the used tools from Kebara exhibited retouch, compared with 64% of those from Qafzeh (13). Sharpening retouch is to be expected with hafted tools, because it's easier usually to resharpen an edge than to replace the lithic. However, retouch on the tools from Qafzeh seems to have been used more to shape the lithics than to resharpen them (13). Although the lack of sharpening retouch remains enigmatic, the greater attention that modern humans paid to shaping the tool initially may reflect a concern for fitting the lithic into a prefabricated handle.

The Levallois point/lithic core ratio also has been argued to differ systematically between Neandertal and early modern human Mousterian sites in the Near East (17). Assuming this ratio reflects the intensity of point production relative to production of all tools, higher point/core ratios in Neandertal-associated assemblages have been interpreted as reflecting greater hunting intensity by them than by modern humans (17). Variation in these ratios, however, can result from multiple factors including differential loss of points from residential sites (18). The use of foreshafts on hunting spears, for example, may allow for the quick replacement (and discard) of damaged points in the field, whereas points hafted directly to the spear may have been transported more often back to camp before being removed and discarded. This example highlights the complicating effects that hafting and retooling can have on the archeological record (19) and illustrates the complexity inherent in trying to interpret behavior from lithic assemblages.

Perhaps it is just my bias as a paleontologist, but I feel strongly that functional-morphological interpretations of Near Eastern Mousterian human behavior, such as the work of Niewoehner (6), rest on epistemologically firmer ground than do inferences derived from typological and technological analyses of lithic assemblages (20, 21). If recent suggestions (22) that Neandertals and early modern humans are associated uniquely with different variants of the Levantine Mousterian (Tabun B and Tabun C, respectively) can be supported, then traditional modes of addressing variation in lithic assemblages may yet reveal adaptive differences between groups. Until then, approaches that focus on adaptively meaningful and generally more subtle aspects of the lithic record, such as the work discussed above, hold more promise for generating testable inferences about prehistoric behavior. Although differences in retouch frequencies and point/core ratios are admittedly weak lines of evidence for differences in the use of hafting between groups, they do constitute archeological signatures of behavioral differences (although we are still far from confidently identifying those differences).

A hand better suited to oblique power grips as found by Niewoehner (6), and a greater use of hafting, are not in themselves adaptive innovations sufficient to have given modern humans a competitive edge over indigenous archaic populations. Rather, these differences are likely part of an emerging modern human adaptive system that involved greater use of task-specific tools, more complex composite tools, greater planning depth and logistical complexity to foraging, and increased social complexity (10).