Mary W. Marzke M. Steven Shackley

Department of Anthropology, Arizona State University, Tempe, Arizona 8.5287, U.S.A.

Received 11 April 1986

Revision received 14 November 1986 and accepted 28 January 1987

Publication date June 1987

Keywords: Experimental tool making, tool use, hominid hand morphology, grips, manipulative behavior.

Hominid Hand Use in the Pliocene and Pleistocene: Evidence from Experimental Archaeology and Comparative Morphology

Grips and hand movements were observed during the manufacture and use of Paleolithic tools by archaeologists. Positioning and movements of the fingers and palm were noted for each grip and activity, and regions of the hand stressed by the activities were identified. Derived morphological features in the modern human hand were assessed with regard to their contribution to the effectiveness of the grips and their potential for bolstering the hand against the accompanying internal and external forces. The potential range of effective gripping postures and movements and accompanying stresses were then estimated for fossil hominid hands.

It was found that the control of most Paleolithic stone tools requires two categories of grips. One combines passive support by the palm with movements of the stone by the thumb and fingers. The other involves the thumb and the sides or palmar surface of the second and third fingers. "Precision" grips exclusively by the thumb and fingertips and a "power" grip that locks tools into the palm of the hand rarely occurred in the experiments, yet these are the ones usually discussed in connection with the evolution of hominid hands and tool use.

The proportions and joint morphology of the Australopithecus afarensis hands are compatible with a variety of grips that should have facilitated habitual and effective manipulation of unmodified stones as tools. Derived human features in the Olduvai hand should have enhanced the control of stones by the thumb and fingers in the same grips. The robusticity of Neandertal hands is consistent with internal forces that are associated with intrinsic muscle control of finger position and with the large, repetitive extrinsic impulsive forces which accompany the wielding of hand-held stone tools.

Previous classifications of grips are modified and extended, to facilitate communication about the evolution of hominid hand structure and function.

Journal of Human Evolution (1986) 15, 439-460

Introduction

R e c e n t efforts to i m p r o v e the d e s i g n a n d p e r f o r m a n c e of r o b o t h a n d s h a v e b r o u g h t i n to

re l i e f the s i g n i f i c a n c e o f m e c h a n i c a l f e a tu r e s u n d e r l y i n g t he ab i l i ty of the h u m a n h a n d to

b o t h s t ab i l i ze a n d p rec i s e ly m a n e u v e r tools w i t h v a r i e d g r a s p i n g c o n f i g u r a t i o n s ( S a l i s b u r y ,

1984). T h e u n i q u e p a t t e r n o f t he se f ea tu r e s in h u m a n s h a s evo lved o v e r s eve ra l m i l l i on

years , d u r i n g w h i c h t i m e the use o f h a n d - h e l d s tone tools b e c a m e a n i n c r e a s i n g l y

i m p o r t a n t c o m p o n e n t o f f o r a g i n g b e h a v i o r .

A few e l e m e n t s o f th i s u n i q u e p a t t e r n a re p r e s e n t in the h a n d s of t he ea r l i e s t k n o w n

h o m i n i d species , Australopithecus afarensis, a n d a n a t t e m p t to assess p o t e n t i a l g r ips a n d

m a n i p u l a t i v e ac t iv i t i e s f r o m a p r e l i m i n a r y a n a l y s i s of h a n d m o r p h o l o g y in th i s species

( M a r z k e , 1983) h a s g r o w n in to th is s t u d y o f gr ips a n d t h e i r m o r p h o l o g i c a l c o r r e l a t e s in

s tone tool use. W e f o u n d a t the o u t s e t t h a t t h e r e is v e r y l i t t le i n f o r m a t i o n a v a i l a b l e , e i t h e r

a b o u t the k i n d s o f g r ips a n d m o v e m e n t s n e c e s s a r y to effect ively use a n d m a n u f a c t u r e

h a n d - h e l d s t o n e tools , or a b o u t s t resses on the h a n d s t h a t a c c o m p a n y s tone tool use.

W i t h o u t th i s i n f o r m a t i o n it is i m p o s s i b l e to j u d g e e i t h e r the e x t e n t to w h i c h m o r p h o l o g y of

0047-2484/86/060439 + 22 $03.00/0 �9 1986 Academic Press Inc. (London) Limited

440 M. W . M A R Z K E A N D M. S. S H A C K L E Y

fossil hands is compatible with the effective manipulation of stone tools, or the connections between evolutionary changes in hand morphology and changes in Paleolithic technology. Accordingly, we have experimented with the use and replication of stone tools, and have identified grips that facilitate their manipulation. Joint movements associated with these grips have been investigated, and regions of the hand skeleton targeted by internal and external forces have been located. With the results of our study, it has become possible to make some predictions about potential grips and tool-using capabilities from the presence of features in fossil hands that reflect stresses and facilitate joint movements commensurate with grasping configurations.

There has been one previous at tempt to apply experimental use and manufacture of Paleolithic tools to the interpretation of hominid hand structure (Krantz, 1960). This was very limited in scope. Other functional analyses of hominid hand morphology have focused on the manipulation of modern human tools or unspecified objects of various shapes (Napier, 1962a, b; Lewis, 1977), but modern tools do not necessarily stress the hands and require the same range of hand postures and movements as unmodified stones and Paleolithic tools. Experimental tool-making by Odell & Odell-Vereeken (1980) was aimed at identifying clues to hand postures in the wear patterns of the tools, rather than in the fossil hands themselves.

Our study is limited to a consideration of the capacity of hands to effectivcly hold, maneuver and wield stonc tools. Our research indicates, however, that there is an important interaction between the evolution of the hominid hand and the use of stone tools, and this conclusion has interdisciplinary ramifications. The effectiveness with which tools are used and manufactured clearly depends upon the level of intelligence and advancement in sensory and motor control of the hand by the brain, and on the size, shape and raw materials of the tools, as well as on the mechanical capacities examined here. Parker & Gibson (1977, 1979) and Wynn (1979, 1981, 1985) are investigating the intellectual corrclates of tool-use and tool-making, and Jones (1980, 1981) and Toth ( 1982, 1985) have explored the relation of tool material and design to the effectiveness with which stones may bc used in foraging activities. Further progress in all appropriate disciplines is necessary in order to gain insight into the interplay ofthesc factors in the evolution of hominid tool use. A principal aim of" this report is thus to improve communication between disciplines, by clarifying terminology and by drawing attention to grips and manipulative behavior that should be considered in future functional analyses of fossil hominid hands and Paleolithic tools.

Materials and Methods

Experimenlal Use and Man@cture of Stone Tools Two sets of experiments are reported here. One was performed by the second author, and was devoted exclusively to the reproduction of cores and flakes found at Olduvai, Beds I and II , and at Koobi Fora. Some of the sessions were filmed. The other was a videotaped session by F. Bordes, J . Tixier and D. Crabtree, who demonstrated techniques in the manufacture of late Acheulean handaxes, Levallois flakes, and Upper Paleolithic bifaces at Arizona State University in 1969.

All core reduction in the authors' experiment was accomplished with spherical and oval basalt hammerstones of two sizes (569 gm and 449 gm). These were selected following a

H O M I N I D H A N D U S E 441

long period of experimentation during which a variety of hammerstone sizes and shapes were tried. The hammerstones selected proved to be similar in size and dimensions to the one used by Toth (1982) in his experimental manufacture of tools replicating those at Koobi Fora. Every effort was made to choose raw materials that were chemically similar to the Olduvai and Koobi Fora materials used in the production of Oldowan tools (Hay, 1976). The materials selected for the experiments included basalt, silicic ignimbrites, and rhyolite (see Table 1).

Table 1 Oldowan tool production data

Lab Raw Mass F ina l Manufacture F i n i s h e d Decrease no. material (g) form time mass in mass*

4 Waterworn 569 Unifacial 22 s 382 0"33 basalt chopper

14 Waterworn 831 Bifacial 45 s 536 0"36 basalt chopper

6 Waterworn 746 Core 1'41 min 533 0'29 basalt scraper

17 Silicic 2527 Mega-chopper 3 min 1625 0"36 ignimbrite

10 Rhyolite 1100 Unifacial 3'5 rain 533 0"52 pick/hand-axe

* (Mass - finished mass)/mass.

There is evidence at both East African sites of evolution in tool forms, and at Olduvai there is also change in the selection of raw materials (Leakey, 1971). Forms and raw materials used here were selected to represent stages in this evolution.

Natural spherical and oval stones, flakes, and "core choppers" manufactured in tile experiments, as well as unretouched flakes resulting from the process, were used to shell walnuts, hazelnuts, and mungongo nuts, cut vegetation, dig for roots and cut skin, muscle, sinews, and bones of a chicken. (The last, easily procurable resource provided a test for the effectiveness of some tools in cutting these tissues.)

At the time of the experiments, Shackley was unfamiliar with the literature on hominid hand morphology and manipulative behavior. He used grips that comfortably accommodated the tools, and movements that suited the tasks, without purposely at tempting to simulate postures that have been postulated for early hominids. While he hand-held most of the cores and flakes during knapping, it is recognized that many aboriginal knappers remove flakes from large cores set on the ground (see Binford & O'Connell , 1984, and Toth, 1982). However, small artifacts are knapped in the hands. Modern humans performing these experiments do not necessarily recreate the full range of grips and hand movements used by earlier hominids, since our hand structure and joint mobility differ somewhat from theirs. This is a limitation implicit in all functional analyses of morphology in fossils, using modern species as analogs. We also cannot be certain of all the sociocultural factors that may have biased the selection of manipulative motor patterns in early hominid groups (Kleindienst & Keller, 1976). However, by comparing modern humans and apes in the range of effective grips permitted by their hand morphology, it should be possible to at least identify limits to the potential range of effective grips by fossil hands.

There is always concern that observations of a small number of modern individuals may underestimate potential individual variability in the behavior of fossil species. It is unlikely

442 M. W . M A R Z K E A N D M. S. S H A C K L E Y

in this case, however, that there was a wide range of variability in grips and hand movements for manipulation of each type of tool. The principal observations underlying this assertion are as follows. The number of manipulative patterns that can be used comfortably, effectively, and repeatedly is limited by the constraints of joint movement range, fatigue associated with certain orientations of the musculature and ligaments, locations of nerves and blood vessels, size and shape of the materials being manipulated, and potential injury to the hand. Designers of modern tools take these factors into consideration, often with the advice of kinesiologists (see, for example, Soderberg, 1986). Toth (1982, pp. 247-248) found that several Dassenich volunteers, who experimented with the replication of prehistoric tools, arrived at similar patterns of execution without previous training, after a few hours of experimentation. The same was true of studcnts in a flint-knapping seminar directed by Shackley. And the manipulative patterns used by Shackley were observed in comparable activities by the stone-knappers in the videotapes. I t is possible that other knappers would reduce the material in different ways, but the postures and forces presented here would still occur in the reduction process and thus the essential techniques of tool use by individuals with varying hand size and hand preference will be basically the same. In this connection, it is hoped that archaeologists experimenting with tool-making will record their manipulative patterns in the future, and those of aboriginal knappers, so that we may better estimate the range of variability in effective grips.

Films. Eight-millimeter films were made of two of Shackley's sessions. Film speed was 18 frames per second. The tools successfully made during the filming are listed in Table 1, along with measurements of the tools, the stones from which they were formed, identification of the raw material used, and the time required for their manufacture. Four kinds of information were sought with fYame-by-frame examination of the films: (1) position of the thumb, fingers and palm as they gripped and moved the stones, (2) the direction of blows on the hand incurred during hammering in the manufacture of tools and during pounding, cutting, and digging with the hand-held tools, (3) the relative force of these blows, inferred from the length and arc of the hand and arm swings, (4) the relative precision of hand movements required for each kind of tool manufacture and use and (5) relative frequencies of grips. Much of this information was obtained without use of film, through close observation of repeated experiments and monitoring by the experimenter of stresses felt by the hand and changes in precision needed for the various tasks. The film provided a permanent record of the experiments and allowed us to discern subtleties of postures and movements that went unnoticed during the experiments. Use of higher-speed filming, against a background grid for precise recording of hand kinematics in three dimensions, was not called for in this study.

Functional analysis of hand structure. Identification of the morphological pattern which distinguishes the hand of modern humans fi~om hands of nonhuman primates, and the functional correlates of this pattern, are based on dissections of 40 hands from 14 anthropoid species, and examination of 100 hand skeletons from 42 anthropoid primate species by the senior author. (All species, with the exception of one Alouatta, one Lagothrix and three Ateles specimens examined at the U.S. National Museum and in the collection of Erikson, are listed in Marzke & Marzke, in press). The Australopithecus afarensis hands and

HOMINID H A N D USE 443

casts of hand bones from Sterkfontein, Olduvai and Swartkrans were also examined. Internal and external forces on the hand associated with tool use, and joint mobility and muscle activity required for the grips and movements observed, were assessed through biomechanical analysis (Marzke & Marzke, in press) and reports of electromyographic investigations (Long et al., 1970; Long, 1981).

Definition of grips. Napier 's (1956) classification of prehensile movements into two categories (power and precision grips) was a landmark in the medical and anthropological literature, forming the basis for most discussions of human manipulative behavior and comparison of species in prehensile movements (see, for example, Campbell, 1974; Day, 1978; Landsmeer, 1962; Lewis, 1977; Long, 1981; Napier, 1962b, 1965, 1980; Susman & Creel, 1979; and Tuttle, 1981). His categorization of grips was valuable in distinguishing between those which allow fine manipulation of objects by the fingers and thumb (precision grips) and those which employ the palm in stabilizing objects (power grips). However, when the categories were applied to discussions of nonhuman primates and fossil hominids, there was an emphasis on precision grips involving the thumb and all four fingertips and on power grips requiring clamping of the tool in the palm (Napier, 1960, 1962a,b). Unfortunately, fimctional interpretations of derived morphological features in the modern human hand have been channeled by this emphasis, and as a result structural potential for controlled manipulation of tools by early hominid fossil hands may have been underestimated.

Since the present discussion is focused upon hand morphology underlying grips and hand movements in tool use, we provide in Table 2 and Figures 1-3 descriptions and illustrations of grips categorized according to the relative positions and involvement of the fingers and palm. Our terminology is an extension and adaptation of the one presented by Long el al. (1970) and Long (1981). Precision and power categories are not used, because many of the grips we observed incorporate elements of both.* Grips employing the palm are distinguished on the basis of whether the palm passively supports the object or actively secures it in place.

The functional importance of the distinction between finger grips and active palm grips is that objects held by the former grip may be positioned by movements at the finger joints as well as at the wrist, elbow and shoulder joints. (Landsmeer, 1962, introduced the term "precision handling" for this dynamic aspect of thumb/finger grips.) These finger joint movements lend precision to the orientation of tools and leverage of the fingers to movements of the tools. The active palm grip does not exploit finger movements, but has the advantage of securely positioning tools like the modern hammer to be used as an extension of the hand. Passive palm grips combine both dynamic and stabilizing elements, buttressing tools while still allowing their precise maneuvering by the fingers.

* An excellent discussion of the subtle blend of power and precision in the dextrous manipulation of modern tools may be found in Elllott & Connolly (1984). However, their classification of manipulative hand movements was made with reference to the analysis of impaired function in modern human hands, and is not applicable to the present discussion. Although a single classification would seem to be desirable for communication across disciplines, varying applications of classifications seem to be better met by classifications specific to those applications.

444

Table 2

M. W. MARZKE AND M, S. SHACKLEY

Grips frequently employed in the manipulation of tools*

Number of fingers in grip

Finger/passive Finger/active Finger grips palm grips palm grip

Thumb/finger Thumb/finger, Thumb/finger, position pahn position palm position

2~aw chuck

3-jaw chuck

Tip-to-tip (I/II)

Pad-to-pad (I/II)

Pad-to-side (I/side DP II)*

Pad-to-pad (I / I I - - I I I )

Buttressed pad-to-side (I/side II, buttressed by III--V; tool rests against MclI, buttressed by McI)

4-and 5~awchuck

4 fingers

Thumb-to-fingers (I/full palmar surface II-III)

Pad-to-pad (I/II-IV,V)

Thumb-to-fingers (I/full palmar surface II-IV, V)

Hook (II Vflexed)

Extended 3-jaw chuck (I, lI, side IiI; tool rests against McII, buttressed and maneuvered by McI)

Cradle (1/full palmar surface II IV, V; tool rests diagonally against MclI--V)

Digitopalmar (II-Vdistal pads/McII V; McImay beincluded)

Squeeze (I/palmar surface II-V; IV-V strongly flexed; tool clamped diagonally across Mcl I-V, McV flexed, supinated, McI adducted)

* M: metacarpal; DP: distal phalanx; Roman numeral: number of ray; used alone it identifies a finger. "~ In great apes, the tool is usually held against the more proximal phalanges of II.

Resul t s

Experimental Tool Manufacture Early Oldowan Tools. Hammerstoncs used in core reduction were held exclusively by a three-jaw chuck thumb/finger grip in all filmed and videotaped reduction sequences. The larger harnmerstones were secured and maneuvered by the palmar aspects of the thumb, index and third fingers. Slightly smaller ones were buttressed by the radial side of the third finger. An advantage of this grip fbr hammering is that it exploits the leverage of the thumb

HOMINID HAND USE 445

Figure 1. Finger grips, (a) Two-jaw chuck tip-to-tip; (b) Two-jaw chuck pad-to-pad; (c) Two-jaw chuck pad-to-side; (d) Three-jaw chuck thumb- to-finger.

a) (b)

(c) (d)

446 M. W . M A R Z K E A N D M. S. S Y I A C K L E Y

Figure 2. Finger/passive palm grips. (a) Buttressed pad-to-side; (b) Cradle; (c) Digitopalmar.

(a)

(c)

HOMINID HAND USE z}47

Figure 3. Finger/active palm squeeze grip.

and fingers to precisely direct blows. For removal of large flakes unifacially, the right arm was swung with considerable fbrce directly downward, striking the edge of the core with the hammerstone and continuing beyond until the elbow was almost extended. The index and third finger were always separated, with the palmar aspect of the index finger in full contact with the surface of the tool. The blow was struck with the end of the hammerstone directly opposite the apex of the angle between the two fingers. The reaction force therefore was directed toward the region of the second and third metacarpal heads, which are connected by the deep transverse metacarpal ligament. This three has the potential of hyperextending the metacarpals and displacing their bases, unless there are stabilizing structures in this region (Marzke & Marzke, in press). Friction by the distal thumb and finger pads is an important component of the grip (ibid.).

Preforms were held by a succession of grips which adapted to progressive changes in the size and shape of the stone during reduction. The largest ones in some cases were steadied against the thigh during flake removal until they could be held firmly by the hand. Large preforms were usually held by the cradle grip, as we hold a large book. As the preforms became smaller in size during flaking, frequently only the thumb and palmar surfaces of the fingers or fingertips remained in contact with the stone, often in a thumb/four finger posture. All these grips require flexion of the ulnar fingers to resist displacement of the perform with hammering, but neither appears to involve significant rotation of the hypothcnar area toward the thumb. Alternatives to the thumb/finger grips of the smaller objects were the buttressed pad-to-side and digitopalmar grips. A strong pinch was required to maintain the pad-to-side grip when strikes by the hammerstone rotated the preform over the index finger. The digitopalmar grip required friction and strong flexion of the distal phalanges to resist blows by the hammerstone.

Preforms were constantly reoriented for flaking by movements at the wrist, elbow and shoulder joints, and shifted slightly in the hand by movements of the thumb and fingers. The position of the core was maintained in the transverse plane during removal of flakes

448 M . W . M A R Z K E AND M . S . S H A C K L E Y

with relatively little displacement. From the length and speed of the swing of the right arm it was obvious that the extrinsic and instrinsic wrist and linger muscles of.'the left hand must be contracting forcefully to resist the blows of the hammcrstone on the preform. Clearly in stone tool use and tool making, grips by both hands must be maintained against large external forces tending to rotate the enclosed objects.

Developed Oldowan tools. Finely controlled hard-hammer reduction in the production of these later tools provided insight into the relation of tool size, raw material, and manuf~tcturing techniques to grips and hand movements. The use of one or two fingers to control the angle of the striking platform was frequent during the production of the "unifacial pick/handaxe" (# 10) and even more so in the manufacture of retouched flakes. The experiments involved attempts to create steeper edge angles on flakes through marginal retouch. For both basalt and rhyolite flakes, relatively greater control was necessary to avoid crushing platforms, produce regular cutting edges, and avoid breaking the flake platform during production.

Flakes were held variously by the digitopalmar, four- or five-jaw chuck, and occasionally the buttressed pad-to-side grips. For the larger flake preforms held by the five-jaw chuck grips, movements of the fifth finger were frequently used to control the platform striking angle. The hard-hammer retouching of margins required the greatest skill of all the techniques used in our replication of Oldowan tools. The very short arm swing and precision required to execute this task are more sophisticated than the movements involved in the production of'other tools. For the unifacial pick/handaxe (# 10), there were marked differences in flaking properties and hand postures used to control the angle of the struck margin (i.e., the platform angle). A much lighter grip could be used to control the core, possibly due in part to the material (rhyolite) that flakes quite easily. Thin, large flakes were frequently removed, and unlike the ignimbrite, did not shatter as readily. The arm-swings of the right hand (with the hammerstone) were short, and obvious care was taken to accurately place the percussor on the manufactured platform. This tool required the longest manufacture time (about 3'5 minutes), during which it was constantly necessary to predict tile flake size removed, the position (angle) of the striking plattbrm, the force of the blows, and the point of each successive percussion. The fifth finger constantly adjusted the platform angle.

Frequently the index finger was rotated to lie along the outer margin of" the preform, where its palmar surface could achieve maximum contact with the surface of the stone (Figure 4). In this position the extrinsic flexor muscles of the index finger are effectively exploited in securing the grip. The index finger of the right hand was consistently positioned on the longitudinal axis of the hammerstone, with its palmar concavity directly opposite the point which contacted the preform. In this position it had maximal palmar contact with the percussor and was well separated from both the thumb and third finger, allowing for secure clamping of the stone. It also was possible to adjust the orientation of the striking end precisely through rotation of the index finger at the metacarpophalangeal .joint.

Late Acheulean Handaxe. The videotapes of Bordes and Crabtree manufacturing obsidian late Acheulean handaxes showed, in the initial stages, grips similar to those used in our manufacture of Oldowan core tools. A hammerstone was controlled with a three-jaw chuck thumb/finger grip to remove a flake from the preform. It was wielded during light

Figure 4. Position of index finger for precise positioning of tools.

HOMINID H A N D U S E 449

hammering more like a pestle than a hammer, however, with the striking surface directed slightly proximally. The flake was held in the left hand by the cradle grip. The position of the left index finger constantly changed as the flake was oriented for reduction. I,ater an antler billet (3-4 cm in diameter) was used for hammering. I t was held by the squeeze grip in close alignment with the long axis of the fbrearm and hand, overhanging the radial side of the palm at the second metacarpophalangeal joint, thus adding leverage to the hand. The thumb and its metacarpal area buttressed the tool against the fingers. Thc effectiveness of this tool depends on the ability to maintain its alignment with the long axis of the forearm. This is accomplished by strongly flexing the fourth and fifth fingers and drawing the fifth metacarpal toward the first, cupping the palm and squeezing the cylinder. The grip was frequently modified by extension of the index finger along the axis of the tool, where it lent precision to the billet's striking angle as short blows were carefully applied to the edge of the flake.

Blade Manufacture. Production of a blade by Tixier began with the preparation of a striking platform by a hammerstone and steel file (in lieu of a stone abrader), followed by preparation of the core. Large and small hammerstones were used like a pestle, with various adaptations of the three-jaw chuck thumb/finger grip. The first heavy blows with the hammerstone required the swing of the whole arm. When preparation became more delicate, index finger and wrist action were used more in controlling the hammerstone. The core was then placed between the feet to steady it while a blade was removed with an antler punch. The punch was held in the left hand with a modified digitopalrnar grip, in which the thumb was wrapped around the fingers, and was struck by a larger antler billet held by a squeeze grip in the right hand.

450 M . W . M A R Z K E A N D M . S . S H A C K L E Y

Experimental Toot Use

Digging. An Oldowan "chopper" was held in a digitopalmar grip between the four flexed fingers and the palm, as the retouched side was applied with considerable force to the ground. The thumb occasionally buttressed the side of the tool. The third metacarpal head was directly above the contact point of the tool, where it was exposed to reaction forces similar to those encountered in hammering.

Pounding. Nut-cracking was performed effectively with hammerstones similar to those used in the manufacture of Oldowan tools, and involved the same gripping postures and stresses at the center of the palm. In some cases, fbr nuts with hard shells, larger pounding stones were supported against the first and second metacarpals in an extended three-jaw chuck grip, with the fourth and fifth fingers buttressing the third finger or steadying the elongated oval stone. The nuts were positioned by a pad-to-pad pinch of the left thumb and index finger. Hammering movements involved short swings of the arm.

Chopping, Cutting and Scraping. A "chopper", held by the extended three-iaw chuck grip, was found effective fbr cutting vegetation and for chopping through the skin, muscle and sinews of the chicken. The tool lay along the second metacarpal of the palm and was manipulated primarily by the thumb, index and third fingers. The index finger was extended along the thin upper edge of the tool, where it controlled the position of the tip. The fourth and fifth fingertips steadied the tool, but the area of the palm over the fifth metacarpal did not appear to squeeze it against the area over the first metacarpal.

This grip was used alternately with the buttressed pad-to-side grip fbr cutting and chopping with large flakes. (The latter grip is also illustrated by Jones, 1980, 1981, for manipulation of a flake in removing skin from a goat.) A pad-to-side grip of smaller flakes was used to remove the meat from the bone.

For scraping and sharpening digging sticks, a flake was held in the right hand between the thumb and third finger, with the index finger curled around the edge of the flake controlling its position.

Grips and Hand Movements in Pongid Manipulative Behavior. Dcscripuons and illustrations ot chimpanzee and orangutan grips and movements in the manipulation of stones, sticks, and food objects indicate a preponderance of the hook, digitopalmar, and pad-to-side grips (Lawick-Goodall, 1968, 1971, 1973; McGrew, 1974, 1977; McGrew et al., 1979; Napier, 1960; Nishida, 1972; Sabater Pi, 1974; Tuttle, 1969; Waal, 1982, p. 51; and Wright, 1978). Kortlandt (1986) describes and illustrates a "relaxed power grip", in which cylindrical objects are held obliquely across the palm of the chimpanzee hand with the same orientation as tools in a squeeze grip. He considers this to be comparable to the squeeze grip. His term is contradictory, however, because the squeeze grip is not relaxed; it involves active squeezing by the palm as well as the fingers, with immobilization of the tool for use as a forceful extension of the hand. The grip he describes would fall in the category of finger/passive palm grips introduced here, and appears from his illustrations (and those available to us to which he refers) to cradle sticks and branches when they are carried or waved during displays. It is hoped that in the future he and others will report details of hand configurations and movements in the manipulation of tools by nonhuman primates. Experiments also should be designed to investigate whether the manipulation of a greater

HOMINID HAND USE 4 5 1

variety of hominid tools might elicit a greater range of stable and dynamic grips in captive primates than indicated by previous experiments. The disproportionate lengths of the thumb and fingers in the orangutan and chimpanzee would seem to preclude the effectiveness of many grips requiring buttressing and control of objects by the thumb against the palm and fingertips (Napier, 1960). The present evidence does indicate that there are differences between humans and great apes in the range of grips that are used effectively in the manipulation of tools. It is this evidence which directs our interest to the possible contribution of derived features in the hand to the human ability to both hold tools of many shapes and sizes by a large variety of grips and wield them with force and precision

Distinctive Pattern of the Human Hand

Long Thumb Relative to Hand Length. The thumb is long relative to the fingers in humans, with an average thumb/third finger index of 67 (Schultz, 1956). Several Old World and New World monkey species approach this index, and like humans are able to manipulate objects with control between the thumb and fingers. The human thumb pad on the distal phalanx is able to balance pressure of all the fingers as they secure and rotate an object together.

Stability at the Center of the Palm. In the region of the second and third metacarpals, there arc several distinctive features which together stabilize the palm against external and internal forces encountered in tool use and tool making. This is a region shown in our films to be targeted by external impulsive forces in hammering, pounding and chopping, and the third metacarpal also endures the largest internal axial compressive forces of all metacarpals during grasp (Chao et al., 1976). The capitate is robust, lacking a marked excavation on its radial side for the interosseous ligament with the trapezoid, and presenting a bulbous area on the palmar aspect between the proximal head and the distal facet for the third metacarpal (McIII) . The facets for the second and third metacarpals are therefore supported proximally fully by bone. The second metacarpal is the most robust of the metacarpals in humans, and the bases of the second and third proximal phalanges are stout (Susman, 1979). Stability of the third metacarpal base is enhanced by a styloid process on the dorsal radial aspect, which abuts against the capitate, and by the pisometacarpal ligament which runs across a groove between the hamate hook and carpometacarpaljoints of McIV-V in humans and attaches to the palmar base of the metacarpal (Marzke, 1986; Marzke & Marzke, in press). Finally, humans are distinguished by a well developed deep palmar fat pad, which is superficial to the deep branch of the ulnar nerve in the region of the third metacarpal (Spinner, 1984). It is thus in a position to protect the nerve from external forces.

Rotation of the Fingers. Of the seven palmar interosseous muscles characteristic of primates, four are incorporated into the adjacent dorsal interossei in humans. This arrangement appears to be particularly favorable to the functions of abduction, adduction and rotation of the fingers, although they also contract together in flexion of the proximal phalanges (Marzke, 1971). These movements play a significant role in aligning the finger with the tool for optimal gripping position (Landsmeer, 1955; Long, 1981), and facilitate rotation of objects held by the thumb and fingers (Long, 1981). During our filming, the fingers in all gripping postures were continually being rotated as they maneuvered the preforms and hammerstones into position for the removal of flakes. Asymmetries of the metacarpal heads

452 M. W . M A R Z K E A N D M. S. S H A C K L E Y

guide the proximal phalanx of the index finger into pronation and the fifth finger into supination as they are flexed by the interossei, allowing the palmar aspects of the fingers to conform to the shape of the tool (Lewis, 1977). Facility for rotational movements at the second and fifth metacarpophalangeal joints is complemented by similar facility at the second and fifth carpometacarpal joints (Lewis, 1977; Marzke, 1983). Pronation of the index finger may enhance the grip and maneuverability of tools held by pad-to-side and three-jaw chuck grips (Marzke, 1983), and supination of the fifth metacarpal and proximal phalanx contributes to the clamping of cylindrical tools in the squeeze grip (Lewis, 1977). It is possible that the provision for slight "give" at the second carpometacarpal joint protects the joint from stress when the phalanges are rotated into pronation by pressure of the thumb in pad-to-side grips.

Functional Unit of Digits l-IIl. There is a clear diti~rentiation in humans between the relatively long and robust M c I I - M c I I I and the shorter, less robust McIV-V (Susman, 1979). Much of the manipulative activity of the hand involves some combination of the thumb, second and third fingers. Several features of the joints enhance the effectiveness of grasp and movements of objects by this unit. First, the head of McIII as well as of McII is oriented toward the thumb metacarpal (Susman, 1979), a position which allows the full palmar aspect of'the third finger to oppose the palmar aspect of the thumb when it is drawn into flexion, adduction, and supination in the three-jaw chuck grips. Second, facility at the joints of the index finger for pronation (described above) further contributes to alignment of the palmar surface with oval or spherical objects, and of the radial surface with tools held in pad-to-side grips. Third, an independent deep flexor muscle to the distal phalanx of the thumb stabilizes the pad against pressure by the front and sides of the fingers. Fourth, a strong anterior oblique ligament stabilizes the anterior aspect of the first carpometacarpal joint when objects are pinched by the thumb and index finger" (Eaton & Dray, 1982). In chimpanzees the ligament is not so well developed (Lewis, 1977), and a more extensive palmar lip of bone runs proximally, in the position of the ligament, limiting excursion of the metacarpal in opposition to the ulnar digits. The configuration in humans thus permits wide excursion of the thumb in opposition, while at the same time allowing for stabilization of pinch grips. Also contributing to stability at the first carpometacarpal joint is an interosseous ligament between the first and second metacarpals, which is not present in apes (Bojsen-Moller, 1978).

Secure Grasp by the Distal Phalanges. Human hands are characterized by broad fibrofatty pads on the palmar aspect of the distal phalanges. The pads are divided into proximal and distal compartments (Shewsbury & Johnson, 1983), and are supported by broad, spatulate bones. The proximal deformable area of the pads is able to stay in contact with the surface of objects as they are maneuvered by the fingers, lending security to all grips employing the pads. Broad pads may not be essential to security of power grips exploiting the full palmar surface of the fingers and metacarpals, but they do seem to enhance maneuverability of the tools by the thumb and fingertips, and to resist displacement of objects from the hand when they are struck by other stones.

Morpho!ogy and Potential Grips by Fossil Hominid Han&

Hadar. Our present findings confirm and extend those reported by Marzke (1983) for Australopithecus afarensis. There are three derived human features in these hands: (1)

HOMINID H A N D U S E 453

orientation of the trapeziometacarpal II joint away from the sagittal plane, (2) radiodistal orientation of the joint between the capitate and McII, and (3) a transverse groove between the carpometacarpal joint surfaces and hook of the hamate, which in modern humans accommodates a pisometacarpal ligament to McII I (Lewis 1977; Marzke & Marzke, in press). The orientation and configuration of the McII joints indicate that the metacarpal may have had some "give", with slight potential for abduction and pronation. The distal orientation of the capitate surface for McII must have contributed resistance to axial compressive forces along McII. If there was a pisometacarpal I I I ligament (the hamate groove suggests that there was), it would have stabilized the base of the metacarpal against external forces tending to hyperextend it.

These features are incorporated into a pattern clearly emphasizing strong flexion of the fingers, and this pattern may have been exploited in climbing (Marzke, 1983; Stern & Susman, 1983; Susman et al., 1984). However, the thumb is considerably longer relative to the fingers than it is in apes, although not as long as in humans (Marzke, 1983). The thumb/finger proportions and McII morphology should have enhanced the force and control with which stones could be held by the pad-to-side grip, a staple of the chimpanzee manipulative repertoire, and by buttressed pad-to-side and extended three-jaw chuck grips. These same features should have also facilitated use of the three-jaw chuck thumb/finger grip in the manipulation of oval or spherical stones. Although there is not a clear indication of a well developed flexor pollicis longus, adductor pollicis may have stabilized the distal phalanx of these early hominid hands against pressure of the fingers, through an extension of its insertion tendon to the distal phalanx. This extension is found in great apes (Tuttle, 1969, 1970). A palmar lip at the base of the first metacarpal (recalling the structure in Pan) would have stabilized the base of the thumb during pinch. Large stones could have been steadied between the thumb, palm and fingers in a cradle grip, although maintenance of the grip and precise maneuvering of the stones might have been restricted by relatively narrow distal phalanges, limited excursion of the thumb toward the ulnar digits, and the lack of a well developed deep flexor muscle to the thumb. Well developed extrinsic flexor muscles (indicated by marked insertion areas on the middle phalanges) should have contributed to the firmness of these grips. The hand proportions would have permitted holding of cylindrical objects diagonally across the palm in a cradle grip, but the absence of modern human features at the fifth carpometacarpal joint may have precluded effective clamping of tools in this position with a squeeze grip.

Swartkrans. Assignment of the three hominid metacarpals to a taxon is uncertain, because they were not found in association with cranial or dental remains, or with one another. They come from a level dated to about 1'7-1"9 mya (Howell, 1978), at which stone tools also have been found (Brain, 1970). The bones are particularly notable for their strong intrinsic muscle attachments and robusticity (Napier, 1959). These features are compatible with two requirements for stone tool manufacture. One is maintainance of the grip of preforms by movements at the metacarpophalangeal joints generated by intrinsic musculature. The second is the ability of the bones to sustain the internal forces created by contraction of the well developed muscles. Pongid features of the first metacarpal base and a unique configuration at the metacarpal head indicate some difference from modern humans in the range of thumb movement (Napier, 1959; Lewis, 1977). There is some disagreement as to the range of McV mobility suggested by its proximal joint surface and

454 M. W . M A R Z K E A N D M. S. S H A C K L E Y

muscle markings (Day & Scheuer, 1973; Lewis, 1977; Marzke, 1983), which probably cannot be resolved without recovery o fa hamate. Potential for an effective squeeze grip is thus uncertain.

Olduvai. The hand bones found in Bed I, at the same level as other hominid remains and stone tools, have been attributed to Homo habilis (Leakey et al., 1964). Functional analysis of the bones by Napier (1962b) led him to conclude that they were capable of forming a human type of"power" grip involving the palm, but perhaps not a "precision" grip by the thumb and fingertip pads. Lewis (1977) questions both conclusions because of the lack of direct evidence in the hand bones for either. Susman & Creel (1979) infer from morphology of thumb and finger distal phalanges potential for "precision manipulation" superior to that of apes.

The Olduvai bones have the modern human broad, spatulate apex on the distal phalanges, which provides support to broad fi'iction pads. The ability for strong flexion of the pollical distal phalanx is indicated by a marked depression for at tachment ofm. flexor pollicis longus. From the shallow saddle surface on the trapezium it appears that the metacarpal could have drawn the thumb into a position where its tip might oppose all the fingertips including the fourth and fifth, providing the thumb was long enough. Unfortunately there is no direct evidence, or substantial indirect evidence of relative thumb length (see Napier, 1962a,b; Susman & Creel, 1979). Marked insertion areas on the middle phalanges reflect a well developed m. flexor digitorurn superficialis. This may have been exploited in climbing (Susman & Creel, 1979), and also should have contributed to a strong, secure grip of tools by the palmar surface of the fingers.

The total morphological pattern of the available hand bones is distinctively human in features which together enhance the ability to maneuver objects by the thumb and fingertips in varied orientations and to also maintain the grip of these objects against external forces. It would be surprising (but of course, not inconceivable) to find a thumb relatively shorter than the thumb ofA. afarensis in a hominid hand with these features.

Neandertal hands. All the skeletal features found here to be distinctive of modern humans are present in the hands of Neandertals, indicating potential for all grips used effectively in our experiments. The hands are notable for their apparent muscularity, large joint surface areas, and broad distal phalanges (see Musgrave, 1973; Trinkaus, 1983a,b, 1984; and Stoner & Trinkaus, 1981). Particularly well developed are the at tachment areas for m. opponens pollicis and the first dorsal interosseous muscle (Musgrave, 1971), muscles which secure the pad-to-side grips of objects (Long, 1981). Large insertion areas for the intrinsic musculature are consistent with the muscle force needed to secure grips and to resist rotation of cores out of the hand when they are submitted to impulsive forces. The large muscles imply considerable constraint forces on the finger joints, which would have been distributed over the large joint surface areas. It is interesting to note that the magnitude of constraint forces at the finger joints is greater for pinch grips than for power grasp (Chao et al., 1976; Moran et al., 1985).

D i s c u s s i o n

Tables 3 and 4 summarize our findings of functional and morphological correlates to habitual, varied, and effective use of tools by hominids. The gripping postures observed in

HOMINID HAND USE 4 5 5

the tool manipulation experiments emphasize use of the radial side of the hand, including the thumb, second and third fingers, in the manipulation of Paleolithic tools. Frequently the grips incorporate the palm as a passive buttress, particularly in the region of the second and third metacarpals. "Precision handling" of objects by the thumb and fingertips (tip/tip and pad/pad grips) and "power grasp" (squeeze and spherical grips), on which previous analyses of human hand function have concentrated, were rare in the varied and complex repertoire of grips and hand movements, which in many cases incorporated elements of both precision and power. This repertoire was so unlike those discussed in connection with modern tools that it was necessary to modify previous classifications of grips considerably in order to describe the manipulation of Paleolithic tools.

Positioning of the index finger with its thll palmar surface on the tool, opposite its point of impact, is important in the control of stone tools, and finger rotation is constant during the maneuvering of tools for precise applications. Control of the metacarpophalangeal joints by intrinsic muscles is essential to these movements and also to maintenance of the grip when stones are submitted to external forces that tend to displace them from the hand. Broad, flexible distal phalangeal pads also contribute to the ability to maneuver tools and to stabilize them against displacement with impulsive blows. All but the squeeze grip can probably be used quite effectively without benefit of modern human configurations and movements at the carpometacarpal joint of the fifth finger.

These findings throw interesting light on the hand ofA. aJarensis. The absence of modern human features in the region of the fifth finger probably precludes only the squeeze grip ti~om a potential repertoire of effective postures for the manipulation of natural objects as tools. The grips most likely to have exploited the pattern of human features found in the Hadar hand are the three-jaw chuck thumb/finger, buttressed pad-to-side, and digitopalmar grips, which proved to be preferred grips in activities involving the use of unmodified stones, including pounding, cutting, slicing, chopping, and digging. The capacity for an effective three-jaw chuck thumb/finger grip, coupled with apparent stabilizing features at the center of the palm, may represent facilitative and buttressing elements of an effective mechanism for pounding, chopping and digging with stones.

Our experiments suggest a link between the morphological pattern of the Olduvai hand and the associated developed Oldowan tools. Manufacture of these tools requires considerable control in the positioning of hammerstones and preforms for reduction. The mobile thumb and broad distal pads should have facilitated maneuvering of stones during orientation of the striking platform and hammerstone. At the same time, strong flexion and friction of the distal finger pads would have insured stabilization of" the tool against strong impulsive blows of the hammerstone. Wide excursion of the thumb would have permitted its positioning for stabilization of the tool against pressure of all four fingers in finger and finger/passive palm grips. In other words, these features should have contributed both a dynamic and enhanced stabilizing capacity to the grasp of tools.

From the prehistoric record of the Early and Middle Pleistocene, and from our experiments, it is clear that stones were held by the hands and habitually manipulated in ways that directed large forces toward the central region of the hand. Since large repetitive stresses may lead to joint degeneration (Radin et al., 1972), it is likely that stabilizing and protective features not already present at the stage ofA. afarensis were established early in the evolution of tool making. As tool manufacture and use demanded increasingly precise control and positioning of preforms and hammerstones, modern human thumb/finger

456 M. W. MARZKE AND M. S. SHACKLEY

Table 3 Preferred grips in the experimental manipulation of selected Paleolithic tools reported here

Antler Large Small Large/medium Small Hammerstone hammer flake flake preform preform

Finger grips 2-jaw p/s 3-jaw t/f 4/5-jaw p/p 4/5:iaw t/f

+

Finger~passive palm grips Butt. p/s + + Ext. 3-jaw t/f + + Cradle + Digito-palmar +

Finger~active palm grip Squeeze

Table 4 Morphological features in human hands that facilitate effective grips and associated use of stone tools

Features of the 2-jaw 3-jaw 4/5jaw Butt. Ext. modernhand p/s t/f t/f pad/side 3-j t/f Crad. Dig.-palm Squeeze

Long I/hand + + + + + + + length

Robust capitate, + + + + + + + + M c I I - I I I

McIII styloid + + +

Pisometacarpal I I I + + + ligament

Carpometacarpal II + + + + pronation

Carpometacarpal V + supination

PPII pronation + + + +

PP V supination + + +

McIII head radial + + + + + orientation

Palmar elements in + + + + + + + dorsal interossei

Broad distal + + + + + + phalanges

HOMINID HAND USE 4 5 7

proportions, and configuration of the interossei and metacarpophalangeal joints facilitating alignment of the palmar surfaces of the fingers with the tools, would have been at a premium. There are unfortunately no fossil hands from this period to document the sequence of skeletal changes in the hand. By the Late Pleistocene all the modern human skeletal features seem to be present in fossil hominid hands. One would expect to find robust, muscular hands with large joints surfaces and broad distal phalanges for support of large friction pads like those of Neandertals throughout the period when tools were held directly by the hands. Hands from the Upper Paleolithic are more gracile on the average than earlier hominid hands (Trinkaus, 1983a), perhaps reflecting techniques for tool manipulation that directed a lower magnitude of forces on the hand skeleton.

The pad-to-side grip of small flakes in our experiments was rare. We found, like Jones (1980), that it was tiring to maintain a tight pinch of the flake for cutting, and that a larger flake requiring a buttressed pad-to-side or extended three-jaw chuck grip was more effective during butchering. Grips and activities requiring fine manipulation of objects between the thumb and fingertip pads exclusively also occurred very rarely in our experiments. Manipulation of modern tools, and probably of Upper Paleolithic tools as well, elicits more of these precisely controlled movements of the thumb and fingertips.

Conclusions

Effective manipulation of unmodified stones and Paleolithic tools requires a variety of grips to accommodate the thumb, fingers and palm to the varied sizes and irregular shapes of the stones. During use, these tools are maneuvered primarily by the thumb and either the side of the index finger or the palmar surfaces of the second and third fingers. Frequently they are stabilized against the palm, particularly in the region of the second metacarpal. The hand skeleton is exposed to large internal forces generated by contraction of muscles that perform these functions, and by large external forces associated with wielding of the tools.

The derived features of the modern human hand form two patterns consistent with the twin demand for varied grip and resistance to forces. The first is the set of mechanisms for rotation of the fingers and maintenance of grasp by the distal finger pads, which together facilitate the continually changing blend of stabilizing and maneuvering actions that were found to be essential to the controlled manipulation of hand-held stone tools. The second is the set of stabilizing structures and robusticity at the center of the palm, where the internal and external forces are focused in tool use.

We do not see in the fossil record evidence to support Napier 's (1965) claim that "power" grips preceded precision function in hominid evolution. The present evidence suggests that the evolution of the unique morphological pattern of the hominid hand proceeded from the region of the second and third metacarpals, building upon the pad-to-side gripping capabilities common to other Hominoidea, and involved both buttressing and maneuvering elements from the start.

Acknowledgements

We thank M. Sauther and L. Christenson for filming our experiments, Dr R. Rupp~ for making the videotapes available, Dr C. Peters fbr supplying the mungongo nuts, and C. M. Hoffman and Dr R. F. Marzke for valuable discussions. Access to skeletal collections was

458 M. w. MARZKE AND M. S. SHACKLEY

kindly p r o v i d e d by D r R. T h o r i n g t o n (U.S. N a t i o n a l M u s e u m ) , D r D. J o h a n s o n

(C leve l and M u s e u m of N a t u r a l H i s to ry ) , Dr G. M u s s e r ( A m e r i c a n M u s e u m of N a t u r a l

His to ry ) , D r J . W. W. K i r s ch , M. R u t z m o s e r and P. Ba t che r ( M u s e u m of C o m p a r a t i v e

Zoology , H a r v a r d Un ive r s i t y ) , D r G. E. Er i ckson (Brown Un ive r s i t y ) , and D r A. Z i h l m a n

( U n i v e r s i t y of Ca l i fo rn ia , S a n t a Cruz ) . T h e inves t iga t ion of h a n d m o r p h o l o g y was

s u p p o r t e d by the L. S. B. L e a k e y F o u n d a t i o n and the D e p a r t m e n t o f A n t h r o p o l o g y ,

A r i z o n a Sta te Un ive r s i t y .

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