Functional anatomy of the hand of Australopithecus africanus

D.

Functional anatomy of the hand of Australopithecus africanus*

E. Ricklan

Department of Anatomy, Medical School. University of the Witmatersrand, 7 York Road, Parktown, Johannesburg, 2193, South Africa

The Sterkfontein hand bones, attributed to Australopithecus ajicanus, were analysed to determine potential hand function of the power grip type of this species. The metacarpus is as stable as that of modern humans, as indicated by the depth ofthe groove on the base of metacarpal 2, the styloid process of metacarpal 3, the base articular surface areas, and the ligament markings on the bases of the metacarpals. The flexion and rotation of metacarpal 5 might have been less than that of modern humans, due to a more marked ventral articular lip on the base. The metacarpus acts as a lever, acting in various planes. The extensor carpi ulnaris and extensor carpi radialis longus muscles were probably better developed than in modern humans. The extensor carpi radialis brevis and flexor carpi radialis muscles would probably have been as well developed as in modern humans. None of the long tendons have a mechanical disadvantage as compared to modern humans. The metacarpals have a high robusticity index. The proximal phalanges show some midshaft swelling, slightly greater curvature than in modern humans, and some side to side bowing: pongid features. The librous flexor sheath markings are well developed, but resemble those of modern humans rather than those of the pongids. A single middle phalanx resembles that of modern humans, and has well developed ridges for insertion of the flexor digitorum superficialis muscle. The distal phalanx of the thumb has a well developed region for insertion of the flexor pollicis longus muscle, and has a mechanical advantage over modern humans for action of this muscle at the interphalangeal joint. The features indicate that the hand of A. africanus was well adapted to powerful hand use, as in hammering, striking, chopping, scraping, and gouging actions, as well as for throwing and climbing activities.

Received 8 November 1986 Revision received 15 September 1987 and accepted 4 December 1987 Publication

date May 1988

Keywords: Australopithecus, hand, wrist, anatomy, biomechanics.

Journal of Human Evolution (i987) l&643-664

Introduction The relatively

large sample

offossil

hominid

postcrania

now available

(Ricklan,

1986a) has

made possible several studies of the functional capabilities of the hand of these early hominids. Thus, the Olduvai hand, OH 7, attributed to Homo habilis (Leakey et al., 1964; Day, The

1976), has been specimens

1982), Susman

have

been

(1983),

phalanx have Sonek (1986). recently

from

studied

by Susman

Hadar,

assigned

analysed Marzke

been The

by McHenry

from (1983)

& Creel

a functional

point

and

& Shackley

Marzke

published by Shrewsbury Sterkfontein capitate TM (1983).

(1979) and by Susman

to Australopithecus

Short

comments

of view

& Johnson 1526 has have

been

& Stern

ufurensis (Johanson by Tuttle (1987).

made

(1981),

Studies

(1983) and been studied

(1982).

et al., 1978, Stern

&

of the distal

by Shrewsbury & in this way most

regarding

the functional

morphology of the Swartkrans specimens (Napier, 1959; Robinson, 1972; Day & Scheuer, 1973), while Rightmire (1982) has considered the thumb metacarpal SK 84 in more detail. The remaining 17 Sterkfontein hand bones are as yet unpublished, apart from brief communications (Tobias, 1978; Ricklan, 1978, 19866). The purpose of this paper is to analyse some aspects of the functional capabilities hand of Austrulopithecus africanus, based on the morphology of 16 hand specimens * Paper presented at the symposium “The Longest Record: CA, in April 1986 in honor of Professor J. Desmond Clark. 0047-2484/87/07/80643

+ 22 $03.00/O

The Human

Career

in Africa”,

0

1987 Academic

of the from

held at Berkeley,

Press Limited

644

1). E.

Stcrkfontcin metacarpus,

Mc%mher hut some

1526 has heen Scheepers,

RI(:KI..~\S

4. ‘I’hr analysis is based prinripall) on the, morphology of the aspects of the phalanges also \vill hc discussed. ‘l‘hc capitatc ‘l’hl

the subject

1946; Clark,

of detailed

morphological

1947. 1967: 1, cwis.

and functional

1973. 1977; Kobinaon.

analyses

(Broom

1972: hlcHcnr),,

&

1983).

and will he dealt with only in passilig hcrr. By use of both uni\rariatc. and multi\.ariatc* allal) scs of th(, Stcrkf&nWin mctacarpals (Ricktan, 1988), it has been shown that the Stcrktontrin spccimcns arc most similar to corresponding

hones

of modern

This

when

linear

applies

(representing

humans.

size and shape

or

indicators,

.4. qfiicanushand

the Sterkfontcin

and quite

mcasurcmcnts

dissimilar

\vhrn

rcspcctivel>.),

bones

from

indicts

01‘

arc used.

arc compared

those

of the pongids.

linear

mc~asurcmrnts

111the following

to those of modern

analysis.

humans

in order

to determine ho\v closcl!~ a human functional moclcl might appl! to the fossil species. The movements of the hand ha\rc been classiticd into t\vo principlr t) pcs ol‘grip: a powc~~ grip,

and a precision

qrip

(Napier.

hccn expanded and rciincd 1982: Shrcwshuq, & Sonck, Analysis and

of the precision

size relationships

africanus thumb anatysed,

recognizing

is limited

The power the power

grip is dependant of the components

is a\,ailahtc.

Only

to aspects

of muscle

a distal

action with

to dcpcnd

opcratirq

the exception

movement

I,ittlc

according

of active. ma)

the po\vrr

stahlc.

about

Their

For

but, rxamplc.

on the stabilit)

the .l. \ct

txTn

1987). LVhilc function. this

of the hand

as a lvholr.

boncks. ‘t‘hc roh~laticit~

l
and on

~~t‘ttic bonr\

\vill br c.onsidcrcd.

of the metacarpus ttlr Sth carpom~tararpal,joi~~ts

movements

cvhcn

as

grip.

of the halld.

to the terminology

movement, occur.

ha\.c,

of thr morpholo#!

information

of thr ray (St\* 29-1-)has

phalanx

of the 1st and to a lcsscr dcgrrc,

arc relatively

the first type, capable

alld catrgorics

on ;I consideration

on the, rclcvant

The stability

these joints

largrl)

of the thumb.

of the polvrr

grip bvould appear

may be associated

With

the trrminolog>.

1st metacarpal having been only rcc.c.ntl) rrco\.crcd (‘l‘obias, the lital importance of the precision ,qrip in the’ stud) of hand

a

analysis

1956). :\lthough

(Landsmcrr, 1962: \\‘illiams & \\‘ar\vick. 1980: Kapandji. 1986). the principles of‘thc t\vo grip typrs remain unassailed.

arc’ classified

of Salter rrsistancc

an objc(‘t

as accessory

( 1955). ‘I‘his mrans to active.

gasped

that,joints

movcmc’nt

firmI\

of

arc not

is cncountercd.

in the hand

displacement at the CMJ. The reasons for this stability arc threefold. Firstly, the surfaces of the bones at thcJoint arc congruent. rithcr

(C:Al,J).

mo\.cmrnts

planar

may

cau5c

(as bc.t\vccn

metacarpal 3 and the capitatc), or complcs (as lJct\vccn metacarpal 2 and the capitatc. trapezoid and trapezium). The incoli,quity nccc.ssar! for rotation of the bollcs at thr.joirlt relative to one another is ahsrnt, and major rotational excursion:, are not possible. ‘I’ll{, hones arc in cfrct always in the ctosc-packed position dcscribcd by hlacConail1 (1941) in the description of more mohilc .joints. Only limited translational t> pc movcmcnts can occur. not only bouncl to one another 2-r ) nrr ’ ligaments, but similar ac>ts of ligaments and hamatc) (trapezium, trapezoid. capitatc

Secondly, mctacarpals patmar and intrrossrous

at their basc,s 1,) dorsal. occur brt\vrrn the distal and thcsc mrtacarpals.

row of carpals Furthermore. deep transvcrsr mrtacarpal ligamrnts hilld thrir hrada togrthrr. 1)uboussc.t (1981) suggests that movrmcnts of the mrtacarpals arc not limited by thcsc lattrr ligaments, although thry arr important in maintaining the rc,gularit! of the mrtacarpal

AUSTRALOPITHECINE

arch. Presumably, would

were movement

be more

important

interosseous another.

muscles

Thirdly,

no muscles

place preferentially radiocarpal

and

respectively).

at the CMJs

in limiting

act solely or directly

ulnocarpal,

and

5th

The

1st and

articular

dorsal

metacarpals

to one

Thus movements

will take

to and distal to the CMJs and

are completely

interphalangeal

stabilized,

CMJs

are

exceptions.

Each

(the joints

as in the close

of these joints

muscles respectively.

ofthe otherjoints,

has

as in hollowing

of the palm. The movement

and very little movement

occurs at CMJ3

Movement

of the 1st

and CMJ2

result in movements is greatest

(Kapandji,

at CMJ5,

at all less at

1982). The degree

offlexion-extension at thesejoints is stated by Dubousset (1981) to be 10” for CMJ4,25” the CMJ5, a few degrees for CMJ2 and even less for CMJ3. The wide range of shapes of articular humans

(Singh,

1959; El-Bacha,

limited available articular

movement

surfaces

interpretation facilitatory

of the fossil

to gripping

surfaces at the bases of the metacarpals

1981) and pongids

at these joints,

of hand function.

aspects of the stability

a

but the linkage between the 5th and

and that between the 4th and 3rd metacarpals,

of these joints,

ligaments

bipennate

that forces acting on the hand will cause movement

namely the thenar and hypothenar

4th metacarpals,

The

surface, as well as muscles which can actively move the metacarpal

does not affect the movements

CMJ4,

proximal

metacarpophalangeal

at the

at the joint,

movement.

over these joints.

at the more mobile joints

(1941),

CMJs.

more free, the deep transverse

metacarpal

It is only when the wrist joints

saddle-shaped

645

FUNCTION

also may serve to bind the shafts of adjacent

packed position of MacConaill

CMJ

HAND

hominids

seems to indicate

1979),

at the CMJs

of the hand rather

of the carpomctacarpal

joints

of modern

together

with the

that the shapes of the base

are only of very limited

The movements

actions

(Susman,

for

significance

in the

2-5 might best be viewed as

than essential

to these grips. Several

of A. africanus will be discussed

here.

Depth of the groove on the base of metacarpal 2 The base of metacarpal

2 has a dorsoventrally

directed

groove, into which the trapezoid

slots. The depth of this groove may be related to the stability of the 2nd CMJ: the greater the depth, the greater may be the stability. This depth can be measured as the difference between the maximum

length of the bone and the length from the most distal point of the

groove to the distal end of the head. In modern humans, the depth is 2.9 + 0.57 mm (males) and 2.5 + 0.51 mm (females) (X f SD). In A. af rzcanus the depth is 2.6 mm. Relative to the length, the depth is 4.2% 382) it is 3.9%. humans.

Ifr 0.82 (males)

Both absolute

The stability

and 3.9%

+ 0.84 (females).

In A. africanus (Stw

and relative depth in A. africanus is similar to that of modern

of CMJ2

due to this factor is thus similar in modern humans and in

the fossil. The length of the st_yloid process of metacarpal 3 Marzke (1983) noted that large forces are directed towards the distal end and shaft of metacarpal 3 in cylindrical and spherical power grips, as in the tight gripping of a solid object held in the palm. She noted that these forces rotate the metacarpal and displace the base in a palmar direction,

and that the styloid process stabilizes the metacarpal.

Although

she did not discuss this in further detail, it is assumed here that a first class lever system is being considered (Le Veau, 1977). Such a system is shown schematically in Figure 1. The fulcrum

(base of the metacarpal,

the head of the metacarpal,

A) 1ies between the effort Fl (a dorsally directed force on

M) and the resistance,

F2 (exerted

by the capitate

on the

646

Il. E. RICKLAN

I

I

Figure 1. bchcmatic reprrscntation ofiurccs acting on metacarpal 3 iside viw. ventral tu thr lvft). ‘l‘hr metacarpal z/i with styloid process SP articulates with the Capiratr C. A tool S ,qripprd in the palm exerts forces Fl. Fa, Fh and Fc un the metacarpal. The dorsal rotation around A is countered b) forcr FL? exerted by the capitatr. The length of SP is S. and that 4 the nun-styloid part of thr mctararpal is I,.

Table

1

Styloid process length in modern humans and Australopithecus africanus (mm)

.\laximum Length 01 metacarpal 3 Metacarpal 3 length excluding styloid Styloid length’ Relative styloid lrngth Styloid mechanical advantage’

(

’ The modern human sample comprises left and right hud skrlctons from thr Ra\ mend ILwt Collrction 111 Human Skeletons at the Universit! of thr \Vitwatcrsrand. ,Johanncshur~. 2 Styloid length = maximum length of mrtacarpal ‘i--mrr;icarpal 3 Itngth cuclc~linq St\-IcGd 4Relative styloid length = styloid length X lOO%/maximum Icnath ormrtacarpal ‘3. hStyloid mrchanical advantagr = Mrtacarpal 3 length rxcludinq styloid/qtyloid It-nqh. It IS the ratio r~t’l,i!, 111 Fiquw I, and represents thr relative Irn;th of thr mumcnt ums fur rotation aruund thr third (_:Ll,J, undrr tht, influence of a forcr rxtrted on th? distal end of thr metacarpal.

styloid

process

SP, which

is forced

vcntrally

to come

into contact

\tith

the dorsum

of the

capitate, C). The styloid process, even in modern humans, where it rcachrs its maximum development, is relatively short. The relative lengths of the styloid process (S) and tht non-styloid part of the metacarpal (L) in modrrn humans and in ;I. ~/GDZUJ arc ,gi\,en in Table 1. The styloid process in A. ajricanus (Stw 64) is absolutely short when compared with that of modern

humans.

This difkrcncc

is significant

(1’ < 0.05). Howe\.rr.

the length

AUSTRALOPITHECINE

relative to the metacarpal (P > 0.05), although

HAND

length is not significantly

a trend towards shortness

FUNCTION

647

different from that in modern humans

is noted. The ratio of the lever arm dorsally

rotating the metacarpal, to that ventrally rotating it is the “mechanical advantage” in Table 1. Its value is very high in the specimen of A. uf~icunus: (28.5), compared to mean values of 16.9 and 18.4 in modern

human

males and females respectively.

Consider Figure 1. Force F 1 exerted on the distal end of the metacarpal will tend to rotate the metacarpal dorsally, around the axis A at the CMJ. The force required to resist it, F2, is exerted by the capitate advantage”

is approximately

be some 18 times greater greater to counteract

the metacarpal

minimal

rotation

does

over-simplification. the styloid

than Fl in modern

head. However, in fact

and in A. africanus some 29 times

The

previously,

concept

of a rotation

the CMJ

resist the effort force.

These

and the capitate, and the ligaments

is stable,

In Figure

1, the object

others onto the metacarpal.

at this joint

is an

and other factors besides

would include

the surface

contacts and

X is grasped

in the hand and squeezed forces Fa . . ., Fb

parts of the object

(Marzke,

than the proximal

directed force is not Fl .L, to be counter-balanced

1983). The distal end will

end. The moment ofthe dorsally

by the moment of the ventrally

force, F2.S. Rather, forces Fa to Fc and others as discussed will contribute directed force. The effective force might produce a dorsal displacement rather

The presumed

by the

., Fc and

It is likely that the distal parts of the object are in fact more

than the proximal

thus tend to be more dorsally displaced

than a ventral mechanical

in counteracting

directed

to the dorsally of the styloid

displacement. disadvantage

possibly not of major functional importance

in toto

is planar,

and between the third and second, and soft tissues of the region.

lingers towards the palm. It exerts forces Fl and successively

process,

joint

the force in a gripping type action is in fact not totally directed through the

head.

firmly gripped

(axis A);

and (c) the effort force is directed

since the third carpometacarpal

occur.

As discussed

do indeed

Furthermore,

humans,

only by the styloid;

between the third metacarpal third and fourth metacarpals; metacarpal

of the “mechanical

Fl. This model assumes that (a) rotation occurs at the CMJ

(b) the force Fl is resisted through

on the styloid process. The magnitude

18 in modern humans and 29 in A. africanus. F2 would need to

significance.

forces Fl

in A. africanus of a short styloid process is thus Indeed,

the styloid process may be of minor

to Fc. The class one lever system must then be

modified. While some rotation involving such a system is known to occur, flexion-extension rotation

at the CMJ3

(Dubousset,

is “even more limited”

than the “few degrees” possible at the CMJ2

1981, p. 202).

What then is the function

of the styloid during gripping actions? Dorsal ligaments

of the

carpometacarpal joint pass between the capitate and this styloid (see for example diagrams in Fahrer, 1981; and in Williams & Warwick, 1980). The extensor carpi radialis brevis muscle also inserts into the distal region of the styloid process. The additional surface area provided by this process allows for greater attachment areas for these ligaments and tendons. These ligaments prevent not only ventral but also dorsal excursion of the metacarpal on the capitate. The styloid process of Stw 64 (and probably of Stw 68) is shorter than the mean for modern humans adults although the differences are not significant. The range for modern humans is 0.7 mm to 6.2 mm, which embraces the length of 1.9 mm in Stw 64. As regards the length, and the functional implications of the length, the styloid process of A. africanus would probably have been functionally similar to that of modern humans.

The stability

modern humans

and A. africanus.

of the CM,J3 due to this factor would have been similar in

D.

648 Table 2

Base areas (mm1 of metacarDals

E.

RIGIiLAS

2, 3 and 4 in modern humans and Australopithecus Modern

Metacarpal 2 Base R-U diameter’ Base D-V diameter’ Area ’ Relative area’

196 194 194 193

16.6 16.9 282.0 -1-00.9

Metacarpal Base R-U

3 diameter’

192

Base SV diameter’ .4rca 1 Relative area+ Metacarpal 1 Base R-U diameter” Base D-V diameter’ Arra’J Relative area+

africanus

humana’

Stvi 382 l-08 l-li :33+6 -13.41

I i8 Ii 7 I iti I is

Pli.6 332~0

“I.“:1 3.54i

13.7

Cl.9 I

187 186 181

16.7 ‘29.8 330.0

O-97 25.83 31-77

1it, Ii I I33

12% Ii.1 1Wh

W82 IJ% xt.37

I52

“93. I

“8+

192 188 187 178

1 I.3 13.4 153.0 257.6

O-88

151-i

I-00

1x

I’.’

15” 150

125.2 “21.1

21-51 3 l-70

I -I4

I .I18

15.1

IPI7

I@2

13.3 Ii3 ‘38.1 :1.%+.5 5tw 6-1.68 12.6 1&+I

186.5 333.13

LP76

Stw 65, 330 10.2

w97

12.5 127.5

Ih.91 ‘4.77

j 256.5) y

1 The modern human sample comprises left and right hand skeletons from the Raymond Dart Collection (,I Human Skeletons at the University of the Witwatersrand, ,Johanncsburg. 2 Base radio-ulnar and dorsoventral diameters. ‘3Base radio-ulnar diameter X Base dorsoventral diamctu. 1Area X 100%/maximum length. 5 This cannot be calculated as no specimrns are available Irom which mnx~mum crln be obtdincd. If lent@ IS estimated to be intermediate in lenqth betwren metacarpals 3 and 5, an estimatr of49.7 iusing Stw 64 and Stw 63 for lengths ofmetacarpals 3 and 5) ‘is obtained for metacarpal 1 length. Rrlativc area is thus cstimatcd to br 2.56.5.

Sire of the bases oj-metacarpals

Z-4

The larger

surface

contact

the base articular

or potential

proportional

contact

to its overall

product

of the dorsoventral

articular

surface

at

radio-ulnar

but also non-articular

could be argued that, since these areas larger non-articular areas arc associated with greater

stability.

CMJ?

which An approximation

mobility. and

area of the metacarpals,

the

The metacarpal

areas,

diameters. which

the greater

will be the area of’

in a planar joint will be inverseI> of this area can be obtained by the The

product

includes

arc not part of the joint.

not only

However,

it

are associated with the attachments of‘ligaments, with greater ligamcntous development, and hcncc base arcas

for modern

humans

and A. ajiicanus arc

given in Table 2. Larger specimens will in general have proportionally larger bases than smaller specimens, without this increased base area necessarily having the functional sianificance suggested here. To correct for this factor, the bast areas are expressed as a percentage of the length of the metacarpal, the latter measurement being considered a good reflection of the overall size of the metacarpal. There is a poor correlation between metrical length and robusticity (as defined below). Pearson’s coefficient of correlation between these two variables is -0.08 for metacarpal 1, -0.17 for metacarpal 2. -0.18 for metacarpal 3, -0.20 for metacarpal 4 and -0.12 for metacarpal 5, using the sample listed in Tables 2 and 6. Base area and metacarpal robusticity may therefore represent different uncorrelated functional entities. The length of metacarpal 4 in A. africanus cannot hc measured as the specimens are incomplete. Thus a relative area cannot be obtained in

AUSTRALOPITHECINE

metacarpal which

4 ofA. africanus. An estimate

is the average

Table 2. The mean

values

human

than

males

of metacarpal

HAND

has been made

on the basis of a metacarpal

3 and metacarpal

for the area

and

relative

649

FUNCTION

5 lengths.

areas

This

estimate

are significantly

length

is given

greater

in

in modern

in females

values

(P < 0.01). Th is may be related to metacarpal stability. The for A. africanus are not significantly different from the male or female modern human

mean

values

(P > 0.05).

The

stability

The shape of the base of metacarpal The

of CMJ2,

in A. africanus and in modern

similar

CMJ5

described

and

CMJl

variations

of a transversely

are

concave

flat or convex,

has pointed

out that since

to ventroradial, ventrally, but

convex

and a dorsoventrally in the hamate

or conjunct

by Dubousset

(1981)

rotation)

more accurately

and

and refers

that, in the Hadar the facet

type

rotation

Marzke

and

for the hamate

extends

onto

facet.

with a (1982)

5 runs from dorso-ulnar

will move the metacarpal not only could be considered a type of

(internal

as automatic

rotation).

longitudinal

aspect,

is considered

of flexion

and

unlike

supination

Kapandji

rotation.

and in the Swartkrans

the palmar

has

is that

Kapandji

1953). This movement

pronation

(1981)

facet articulating

hamate

to be an association

5 (AL 333-14)

El-Bacha

The basic pattern

metacarpal

concave

(MacConaill,

to this movement

metacarpal

surfaces.

the facet for metacarpal

(1983)

of extension

movement.

and hamate

flexion of the metacarpal at the joint also radially, a movement which

pseudo-opposition, (external

of rotation

the metacarpal

and dorsoventrally

transversely

is likely to be

5

capable

of both

3 and 4 due to this factor

humans.

(1982)

Marzke

metacarpal

notes

(SKW

the condition

27),

in modern

humans. The facet would abut on the hook of the hamate, limiting flexion, and hence limit automatic rotation or automatic opposition. In Stw 63, the ventral continuation of the hamate

facet

modern

humans.

is less marked Some

than

human

that

of SKW

individuals

27, but is slightly

have

a structure

more

similar

developed

to that

than

in

of Stw 63. A

comparison of casts of the Hadar 5th metacarpals shows that like Stw 63, the ventral lip is well developed in AL 333-14 and AL 333-19, but is poorly developed in AL 333-26 and AL 333-35. similar than

Extrapolating flexion

that

in modern

concurrently articular

with surface

interpretation

from

and hence

Marzke’s

automatic

humans. flexion.

would

analysis,

It is however

If this were be minimized.

of Stw 63 rests

Stw 63 would

thus

have

of the CMJ5

as A. afarensis,

possible

dorsal

rotation

to occur, Further

also on the nature

that

the limiting studies

capable

but slightly

translation

effect

are required

of the hamate:

been

could

of the ventral in this regard.

no specimen

of which

of less

occur lip of The has

yet been recovered from Sterkfontein. On available evidence then, it would seem that the fifth metacarpal of A. ufricunus was slightly, but possibly not significantly, less capable of flexion

and

lateral

rotation

than

that

of modern

humans.

It is also suggested

1983) that squeeze and spherical power grips would not have been within of A. ufarensis because of the low rotatory capacity of the fifth metacarpal.

(Marzke,

the capabilities Hazelton et al.

(1975) have shown that the total force exerted at the middle phalanges during powerful flexion of these lingers can be divided as follows between the four non-pollicial rays: 25.4% to the index, 33.9% to the middle, 25.2% to the ring, and only 15.2% to the little finger (mean values). Similar relative forces were recorded at the distal phalanx, the corresponding percentages being 25.7%, 33.0%, 23.6% and 17.2%. Thus only a small proportion of the grip strength is provided by the ulnar rays. The ulnar rays provide a dorsal buttress against which an object can be compressed by the vertically placed fingers,

650

D.

E.

RICKLAX

Fb

ECU Figure 2. The hand q-asps a tool in a power prip. Hammrrlng tgx li~rws E‘a and choppinq type fkxs Fb cause abduction at the wrist. These forces are countrrrd hv thr extensor carpi ulnaris muscle [ECU) hrrr as no cvidrncr relating 10 its and the flexor carpi ulnaris muscle (FCL’). FCL’ 1s ._ not rrprrsrnted development is available in thr fossil rrcord.

especially

those

of rays

Austrulopithecus, would might not significantly is the

ulnar

associated

margin

2 and

of Stw

with the origin

the specimen

3. A relatively

unmoving

be suitable to such a buttressing limit the power of the various grips.

precludes

63. A roughened

of the opponens an account

region

digiti minimi

of the degree

CMJ5,

function. Offurther occurs

muscle.

of devrelopment

as would

occur

in

while concurrently it interest in this regard

here,

and

was

Unfortunately,

probably damage

to

to be similar

in

of this region.

Ligamentous markings on the metacarpal5 These

markings

development constraints

on both

and position on movement

bases

and

heads

of the

in -4. africanus and modrrn in both

metacarpals humans.

appear This

may indicate

similar

spccics.

From the above considerations, it would seem that the CMJs of A. u&cnnus joints, probably similar in this regard to those of modern humans.

The metacarpus

were stable

as a lever

Because of the stability and relative immobile nature of the CM.Js, distal row of carpal bones function as a single unit in most actions.

the metacarpus and This unit acts as a

spacer-bar between the carpus and phalanges, on which objects are supported while being affixed by the thumb and phalanges, and on which the body rests in palmigrade postures. Forces from an object may be transmitted through the wrist into the forearm. Consider a tool grasped in a power grip (Napier, 1956, 1961; Landsmecr, 1962), or in full palmar prehension (Kapandji, 1982) (F’g i< ure 2). The tool is supported on the palm, and held obliquely in the hand, passing from proximo-ulnar to distoradial. It is supported by the palm at the position of the middle and distal palmar creases, corresponding to the skeletal

AUSTRALOPITHECINE

positions

of the

respectively would

ulnar

be transmitted

radiocarpal The

metacarpophalangeal

(Basmajian,

of the metacarpal

(i.e., along

row of carpal

a longitudinal

of the applied

force

ulnar

of abduction.

were applied

obliquely

The movements experimentally completely occur rotation

entirely more

midcarpal

joint.

deviation

(Volz,

was found

Additional the ulnar

also occur

In the abducted wrist, these,

radial

recently

(opening

scaphoid

of

positions

midcarpal

joint

1976; Youm

movements,

position,

Fb is exerted

on the

will tend

to be rotated

into a

such

and

was

& Sunderland,

1980; Fisk,

thus

et al., 1979).

The

capitate:

of the hamate

and

joints

concluded

to

1953). This has been

pole ofthe

as movement

the wrist is stabilized

but it has been shown carpometacarpal

Abduction

joint),

if these forces

respectively.

the proximal

& Warwick,

2); and (b) the use of the

et al, 1978; Volz

to lie within

of the

in this way

a proximoradially

could be induced

joint

(Bradley

1981).

the bones

force

are complex,

(abduction).

1980; Malek,

in which

directed

the hand

side of the midcarpal

(Williams

actions,

abduction

the

at the midcarpal

for abduction

lunate

during

and then to the

uses of a tool gripped

or flexion

and dorsal

the wrist

via the shafts

joint,

in which

side of the tool (Figure case,

joints

end of the tool

and then,

see Kauer,

the direction

of extension

ankylosis

eliminates

confirmed

degrees

from ventral

within that

almost

of bones;

a distoradially

2). In either

Slight

on either

to the midcarpal

as in striking

in which

metacarpophalangeal

exerted

of the skeleton,

bones, chain

on the radial

or scraper,

side of the tool (Figure

position

second

The two most likely and most common

force Fa is exerted

651

FUNCTION

Forces

will determine

are (a) the use of a tool as a hammer, tool as a chopper

1982).

from the tool to these regions

limb tend to move. directed

and

1975; Kapandji,

and the distal

joint

direction

HAND

rotation

axis of

i.e. near the

away

from the

of the capitate

and

1981).

by the long flexors

on the ulnar

side of the

namely the flexor carpi ulnaris (FCU) and extensor carpi ulnaris (ECU) muscles. the extensor carpi ulnaris muscle is more important (Tubiana, 1981). While

Of the

flexor carpi ulnaris muscle is more powerful than the extensor carpi ulnaris muscle, having a capacity of 2.0 Mkg as compared to 1.1 Mkg (Volz et al., 1979), since the wrist is automatically flexor

carpi

When

extended ulnaris

the hand

of force act about

in a power

muscle grasping

3. The

rectangle above,

small

represents into a single

important

as generally

lies in the head

rectangle the distal functional

represents row ofcarpal unit.

ulnaris

muscle

described

the proximal bones

rather

of the wrist

a pair of equal

of the capitate.

Separating

The radial side is on the left, and the ulnar action on the object held at the approximate

carpi

in the stabilization

the tool is in equilibrium,

the axis of rotation,

1977). The axis of rotation Figure

grip the extensor

is more

but opposite

for lever systems This

is schematically

row of carpal

and the metacarpus, the two rectangles

bones. joined,

than

the

(Figure

2).

moments (Le Veau, shown The

in

large

as discussed

is the midcarpal

joint.

side is on the right. The force Fl is produced by level of the metacarpal heads (the distal end of

the large rectangle). The force F2 is produced by the extensor carpi ulnaris muscle inserting into the ulnar side of the base of the fifth metacarpal. Lengths L and W are the perpendicular distances of forces Fl and F2, respectively, from the axis of rotation. The axis is represented in the diagram by a black dot, and lies in the head of the capitate near its proximal margin. Approximations of these dimensions most closely approximates the length of the metacarpus

can be made on the skeleton. L plus the length of the capitate.

Since the metacarpus

in length

is not of uniform

length,

but varies

fifth metacarpals, it is necessary for an average to be taken. compromise value for several reasons. Firstly, its length radial

and smaller

ulnar

metacarpals.

Secondly,

between

the second

and

The third metacarpal is a useful lies between those of the large

it lies in the same

plane

as the capitate,

652

D.

and articulates

with the major

3 is well represented complete

while

of this metacarpal The functional includes

Of second

the length

RI(:IiL:ZN

distal

in the Stcrkfontcin

specimens.

recovered,

part ofthe

E.

surfLceot‘thc

fossil collection,

and

fifth

metacarpals

of metacarpal

4 cannot

have been recovcrcd. length of metacarpal

3 is used,

the styloid

head

length

The

styloid

‘I‘hirdly.

metacarpal

1~ measured. r,tther

than

as no complete the maximum

specimens length

which

does not contrihutr to the length of the the capitatc dorsally. The metacarpo-carpal unit as depicted in Figure 3, as it o\.crlaps functional length is mcasurcd from the midpoint of‘thc base articular surface to the most distal point on the head. Functional length can be measured in both Stw 64 and Stw 68. The capitate

process.

capitatr.

StM. 6-l and Stw 68 being rrlativcl) ha\,e hecn only single specimens

is the perpendicular

and the line between

the ventral

process

distance

t,ct\~ccn

and dorsal

parts

the most proximal of the ulnar

margins

point

on the

of the distal

end. This approximates the anatomical proximodistal length. The width W of the hand, between the point of‘insertion of the extensor carpi ulnaris muscle and the centre of the head of thr capitatc. is approximated as follo\vs. FV = BRLT5 + BRU4

+ 0.5 BRUS.

metacarpal,

BRU4

Here. the

BRU5

radio-ulnar

is the radio-ulnar diameter

diameter

of the fburth

of the

metacarpal.

base and

of the fifth BRU3

the

radio-ulnar diameter of the third metacarpal. In the Sterkfontein fossil sample, Stw 63 is the sole tifth metacarpal, kchile the base 01’ metacarpal 4 is represented by two specimens (StM. 65 and Stw 330) and the base ot’ metacarpal 3 is represented by two specimens (Stw 6-t and St\v 68). 12:hcre more than one specimen is available, the average mcasurrmcnt of the spccimrns is used. The force F2 required to sustain the wrist against Fl depends on thts ratio brt\vrc.n \t

AUSTRALOPITHECINE

Table 3

HAND

653

FUNCTION

Length of lever arms1 in control of adduction of the hands of modem Australopithecus aficanus by extensor carpi ulnaris

humans

and

Modern humans2 males

Capitate length Metacarpal 3 length 0.5 Metacarpal 3 base breadth Metacarpal 4 base breadth Metacarpal 5 base breadth L’ W4 W/L (%)

females

x

SD

1;

SD

22.9 65.4 6.8 11.3 13.9 88.3 32.1 36.5

1.35 3.95 0.45 0.88 1.14 4.66 2.04 2.01

20.7 61.4 6.3 10.2 12.4 81.9 28.9 35.3

1.22 3.64 0.4 1 0.76 0.98 4.39 1.75 1.91

A. ajicanus~ 17.6 53.6 6.3 10.2 11.5 71.2 28.0 39.3

(TM 1526) (Stw 64, Stw 68) (Stw 64. Stw 68) (Stw65, Stw 330) (Stw63)

1 See Figure 3. 2 The sample size ofW/L is 147 (males) and 123 (females). The modern human sample comprises left and right hand skeletons from the Ravmond Dart Collection of Human Skeletons at the Universitv of the Witwatersrand. Johannesburg. s Average and composite measurements are given using available specimens (in parentheses). * L = Capitate length + Metacarpal 3 length. W = 0.5 Metacarpal 3 base width + Metacarpal 4 base width + Metacarpal 5 base width.

and L. The higher

the ratio,

the greater

force Fl which can effectively be balanced 3 gives the values of L and W for modern

the mechanical

advantage,

i.e., the greater

can be

by F2 (the extensor carpi ulnaris muscle). Table humans and for A. africanus. The ratio of W/L in

A. africanus is greater than the means of this ratio in both male and female sample of modern humans by some 9%. This difference is statistically significant in the female sample (P < 0.05) but not in the male sample (P > 0.05). Only 4 of 123 female (3.3%) and 12 of 147 male (8.2%) hands of modern humans (5.9% of270 hands) have ratios greater or equal to that of

A. africanus. The hand degree

of mechanical

Several object

potential

is transmitted

proximally, modern

such

Member

over the hands

4 hominid of modern

limitations

to the model

to the hand

not at the level of the heads

as if the object

were

broad.

exist.

amount

in both

species.

Thus

W/L

seems

to have enjoyed

However, would

that

to account

for angulation

of the carpus

relative

the force

of the metacarpals, the same

L would change

would

of an

but further apply

be altered

both

in

proportionally

by the same

amount in both species, and the inferences drawn would not be affected. be argued that width W is an approximation, and should for example, factor

some

humans.

It can be argued

and in A. africanus, and thus the length

humans,

the same

of the Sterkfontein advantage

proportional

Similarly, it could be corrected by a

to the metacarpus.

Once

again,

it

may be assumed that W/L in modern humans and A. africanus would be affected to the same degree. The proposed model attempts to reproduce the true situation as accurately as possible,

given

the constraints

of limited

fossil material.

Although the extensor carpi ulnaris muscle has a mechanical advantage the point of insertion of this muscle in Stw 63 is remarkable well developed associated according

crest is relatively larger than that of modern to the stresses placed upon them, a well marked

humans. tubercle

in A. africanus, (Figure 4). The

Since bones could indicate

develop large or

persistent forces exerted on the bone at that point by a muscle or tendon. It has been shown that muscle pull on a bone may be associated with cortical recession as well as outward bone

deposition

(Hoyte

& Enlow,

1964). The absence

of tubercle

development

thus does

D.

654

Figure 4. Stw 63. a fifth metacarpal centimeters. Note the well drveloped [arrowed).

not necessarily tuber& carpi

imply

ulnaris

muscle

the mechanical been

an absence

is an indication

large,

adapted

of muscle action

was powerfully

frequently

in this

from Strrkfontein .\l?mhrr 4. in dorsal nnv. ‘l‘he scale is m tubercle for thr insertion of the extensor carpi ulnaris muscle

ofmuscle

advantage

applied

pull in that region.

on a bony surface.

The

the prrsrncr

of a

seem that the extensor ‘I‘his, coupled

with

that the force Fl in .4. q/Ccanus could have

implies

or both.

for climbing

Howrvcr,

It would

used by A. ajicanus.

or persistently

of the muscle,

regard

RICKLAN

E.

hand

or for using

of this species tools

would

in actions

such

been

\vcll

as hammering

have

or

chopping. In equilibrium, important

Fl .L = F2. L2’, and thus Fl/F2

in the using

hyperabducting,

damaging

for good mechanical tend to rebound However, The Without would case,

of the action.

of the wrist carpi

of the this

abduct

The

or bonr

surfaces

For example,

after striking, in position

Firstly,

it prevents

of the joint. in a striking

decreasing

the energy

at the time ofstrikin,g,

force E’l is

the wrist

Srcondly, action,

imparted

as occurs

from

it allows

the tool will to the object.

in such actions.

rebound.

extensor

movement

efficiency

= i\,‘/L. The force F2 opposing

two reasons.

thr soft tissues

from the object

locking

will prevent

of tools for at least

extensor the force

ulnaris

thumb

action,

the

the wrist carpi

muscle

into

extensor

before ulnaris

F2 would

is important

positions pollicis

acting

brrvis

extension

abductor

the wrist

during

(Kapandji,

pollicis

longus

1982). muscles

of the thumb.

can also bc an acti\c

be the active

in stabilizing

and

and

at the joints

muscle

also

of abduction

force exerted

adductor

by the muscle.

of’the Fl \vould

wrist.

In this

then

be the

resistance force offered to the ob.ject held in the hand. For example, in gouging or scraping actions, the tool would be applied to the substrate with force Fl. In acti\c scraping with an instrument, the hand would bc brought from radial to ulnar deviation. as might also be required

in climbing

well developed

or in throwing

region

A. africanus was capable The

flexor

carpi

an object.

for the extensor of performing

ulnaris

muscle

Again

carpi ulnaris such also

actions

would

the mechanical

tendon.

advantage,

indicate

and the

that the hand

of

powerfully.

have

although when the wrist is in an extended position, have been less important. However, in propulsive

would

bcrn

rcsponsiblc

for such

actions.

as in grasping a tool. this muscle actilitics, such as in climbing,

would whcrc

adduction of the wrist is required. flexor carpi ulnaris muscle would ha\c excrtcd a force greater than that of the extensor carpi ulnaris muscle (\‘olz et al.. 1979). The tlexor inserts into the pisiform, which is not represented in the Stcrkfontein sample (Ricklan, 19864). Only

one early

hominid

pisiform

is known.

namely

AL 333-91

from

Hadar

[Bush

ef al..

1982). It is described as elongate, or rod-shaped. This has been noted by Tuttle ( 1981) and by Stern & Susman (1983) to resemble that of pongids but not that of modern humans, Badoux (1974) shows that an elongated pisilorm. acting as a lever and increasing the moment about the axis of carpal flexion, aids in supporting a body suspended from a

AUSTRALOPITHECINE

branch

by the hand.

such support. fifth

metacarpal

which

have

length).

Both hand

(329%

very

compensates is potentiated

length

pisiform that

for shorter

they

while

lingers

the

activities

flexors,

as might

in securing

nature

of this

with

may Many

specimens,

not necessarily

the insertion

that

way. The functional

in an arboreal

(coronal)

position

electromyographically in ulnar deviation, is flexed. brevis

indicate

details

and that

(based

on a

of 51.3 mm, the mean lower than that of

setting”

A. afarensis

that

ofwhich

(p. 282).

The

this view. As a locomotion

of the Hadar

shared

hand

carpi

ulnaris

certain

skeleton

will be published

of the flexor

plane,

of the wrist

The long extensors and

the power carpi

the lever system (40”-45”

differ

elsewhere.

muscle

It is

would

metacarpal,

longus, shows

and

according

have

in the same

15” of ulnar

to Tubiana

deviation

[1981])

is the

et al. (1975) have confirmed

Hazelton

in the support

the

a well marked

of insertion

than

carpi carpi

is damaged,

object,

ulnaris

the functional

brevis

impression

carpi radialis brevis as in modern humans.

(ECRL)

shows

of the

that

finger

position

in the coronal

allows

the

the extensor

Stw

in the region

64, a third of the styloid

muscle [Figure 5(a)]. In the third metacarpal

of the region

ulnar flexor

plane.

above),

muscles.

role in

discussion

in maximizing

(as discussed

radialis

but sufficient

longus

an important

et al. (1975)

the forearm

crest and adjacent

radialis

play

in view of the above

extension

of a heavy

extensor

carpi thus

of Hazelton

into line with

by the extensor and

muscles

It is of interest, the study

for insertion of the extensor process, impression is at least as well developed 68, the region

may be considered

of extension,

the extensor

ulnaris

important

to be brought

is extended

radialis

that

is more

Furthermore,

of the object

carpi

flexion.

muscle,

wrist

of the wrist,

extensor

of linger

ulnaris

of the

The wrist carpi

5

in the

that the force exerted by the linger flexors is greatest with the wrist intermediate with the wrist in extension, while the force is least if the

(ECRB),

increasing

efficiency.

supports

features

according to Kapandji [ 19821; or 20” of extension position of maximum efficiency of the finger flexors.

centre

and

in A. africanus and A. afarensis.

similar

In the flexion-extension

deviation

5 length

gibbon

of metacarpal

in the gorilla

a safe grip by the hand,

specimens

pisiform

the pongids.

implicit

(20%

of the

and

In A. afarensis the ratio is 28.8%

be useful

from those of the Sterkfontein

extensor

shorter

of the. carpus.

(Bush et al., 1982). This value is somewhat

5 ratio for the Hadar

locomotor

and

in the orang-utan

is relatively

flexion

in maintaining

to the length

and is higher than that of the gibbon and orang-utan. While did not quantify the relationship between metacarpal and large and mechanically that “. . . A. afarensis possessed

concluded wrist

adaptation,

wrist

is long relative

of 14.8 mm for AL 333-91 and a metacarpal

pisiform/metacarpal

been

of the wrist are important

respectively),

the

655

FUNCTION

the pisiform

by a long pisiform.

the gorilla and chimpanzee, Stern & Susman (1983) advantageous

34.4%

concludes

333-14 and AL 333-89)

pisiform,

and flexion

and gorilla

hands,

(1974)

this flexion pisiform

and

long

Badoux

chimpanzee

ofAL

length

In the chimpanzee

HAND

is represented

This Stw

to show

that the development of the point of insertion of the tendon is as well developed as in Stw 64. In Stw 382, a second metacarpal, the base shows a marked depression for the insertion of the extensor

carpi

radialis

longus

muscle

[Figure

5(b)].

This

region

is rather

better

developed than in modern humans. Apart from the stabilizing role of the extensor carpi radialis longus brevis muscles, and their role in active extension of the wrist to allow powerful flexion of the fingers, as discussed previously, the extensor carpi radialis longus muscle has another important function. It is an abductor of the wrist. While using a power grip to manipulate a tool, before a striking action is performed, the wrist is cocked in an abducted position. This

656

D. E. K~CKL.AI\‘

Figure 5. (a) Stw 64. a third metacarpal from Sterkfbntein ,Ilcmber 4. Thr radial side is shown. The scale is in centimeters. Note the well developed region for the insertion of the extensor carpi radialis brevis tendon (arrow) and the styloid process S. (b) Stw 382. a second metacarpal from Sterkfontein Member 4. The radial side is shown. The scale is in centimeters. Note the well developed region for the insertion of the extensor carpi radialis longus muscle (arrow).

TO,

.:

,

I

Figure 6. A schematic diagram of the forearm F and hand H, seen from the ventral or from the dorsal aspect. T is a hammer-type tool held by the hand. To strike X, the hand is adducted through angle Al (inset) from HI to H2. Ifthe wrist is cocked (abducted) from HI to HO. to strike X rhr hand is adducted through angle A2 from HO to H2.

AUSTRALOPITHECINE

HAND

657

FUNCTION

FI

F2

Figure 7. A schematic diagram of the left hand, as seen from the ventral aspect. The radial side is on the left. M + DC is the metacarpal + the capitate. PC is the proximal row of carpal bones. W2 is the width from the radial side of the base ofmetacarpal 2 to the proximodistal line through the head of the capitate which is the axis of rotation. Fl is an adduction force and F2 is an abduction force. See text for details.

cocking

allows

the wrist.

the head of the tool to swing

The extensor

pull it into the cocked gravity

cannot

Figure or dorsal hand

is cocked

in an arc by passive

longus

and to support

muscle

and/or

acts against

it there,

when

active

the weight

adduction

the wrist is in a position

where

6 is a schematic diagram of the forearm (F) and hand (H) viewed from the ventral aspect. T is a hammer or axe-type tool, and X is the object to be struck by T. If the the forearm,

to strike

angle Al. The tool is similarly in an abducted

position

(i.e.,

flexed),

X with

rotated

T the hand

is adducted

from Tl to T2 through

HO, and then adducted

from

to H2, through

as would

occur

in many

powerful

actions,

For this analysis

we may ignore

to H2

angle A2, the tool

angle A2. A2 is larger than Al, and thus the momentum is greater if the wrist has been cocked. When the forearm the

swing

of the too is

is further

& Sunderland, discussed, the 7. F2 is a force

produced by the flexor carpi radialis muscle (FCR), on the radial side of the wrist. force opposing F2, which may be the weight of the tool as the hand is cocked position.

Hl

angle Al. If the wrist

increased. Since abduction occurs about an axis in the head of the capitate (Bradley 1953; Volz, 1976; Youm et al., 1978; Volz et al., 1979), as previously mechanical situation is similar to that of Figure 3, and is shown in Figure

abducted

of

of the tool, to

this function.

is similarly moved through head of the tool at impact cocked

radialis

position,

perform

is in line with

through

carpi

Fl is the

the lever arm due to the length

into

the

of the

tool itself, and consider the force to act directly on the metacarpus. In equilibrium, F1.L = F2.W2. Thus W2/L = Fl/F2. The larger the ratio ofW2/L, the larger is the force Fl which can be moved by F2; or, the greater is the mechanical advantage of F2. Table 4 gives the values of measurements L and W2 in modern humans, and in A. gfricanus. Two different methods were used to calculate W2/L: W2lLl and

658 Table 4

D. E.

RICKLXS

Length of the lever arm1 in abduction by extensor carpi radialis longus, in the hands of modern humans and Australopithecus

ajkicanus

Capitate length Metacarpal 2 length Metacarpal 3 length 0.5 Metacarpal 3 base brradth ,Metacarpal2 base breadth Ll’ L2-’ \v2 ’ W2/LI (%)’ W2IL’ (%)” ’ See Figure 7. L ‘l‘h e modern human sample cornpI-iscs left and ri,+t hand skrlctwa of the M’itwatrrsrand, ,Johatltlc\bu~g. Human Skeletons at the University i Avrrage and composite measurements arr given. bawd on avaitablr kId1= &pirate length + Metacarpal 2 length; L2 = Capitate length + Metacarpal 3 Icngth; W2 = 0.5 Metacarpal 3 base w-idth + Metacarpal 2 hasr xvidth. i Sample sizes of W2/Ll are 177 (mates] and 143 (femalcs). ‘) Sample sizes ofW2/1,2 arc 173 imales) and I41 (fematcsl.

W2/L2,

where

+ third

metacarpal

representing

length

to calculate larger

length

length.

a large

However, possibly

L 1 = capitate

than

male

individual,

W2,

this

expected

measurement

+ second

The reason

metacarpal

L. Table

and

may

not

must

measurement

4 shows

thr

sprrimrns

length.

is remarkable,

The flexor carpi 68, there

indicating in modern

that

radialis

is no specific

a somewhat humans

muscle

inserts

facet or tubercle

ZoIInxiun(~1

and L2 = capitate

be entirely

he included.

there

is no significant ofwhich modern

to counteract is included dilfcrcnce

the in the

between

ofthe two methods is used for humans have a mechanical

more powerful

[Figure

Icngth possibly

of A4. ujCanus.

typical Thus,

advantage as regards this lever system. Since the lever arms of.4. humans are similar, the large facet for the insertion of the extensor in A. africanus than

(

iin p
of Stw 382, its length

modern man and il. africanus (P > 0.05), trrespective calculating the ratio. Neither .4. africanus nor

muscle

Kay mow1 I)ut

is that Stw 382 is a very large metacarpal,

metacarpal

base

firm

qjicanus and modern carpi radialis longus

or habitual

use of this muscle

5(b) 1.

into the base ofmetacarpal for this tendon.

similar

3. In Stw 6-l and Stw

to the condition

in modern

humans. However, the ventral prominence of these bases may suggest a powerful muscle. To summarize the preceding discussion of the lon,g wrist tendons, the following deductions have been made regarding these muscles in ‘4. africanus: (1) The (2) The

ECU was probably ECRL was probably

better developed better developed

than in modern humans. than in modern humans.

(3) The ECRB was developed at least as well as in modern humans. (4) The FCU cannot be assessed, as no pisiform has been recovrered from Sterkfontein Makapansgat. (5) The FCR was probably

developed

to a similar

extent

as in modern

humans.

or

AUSTRALOPITHECINE

HAND

659

FUNCTION

FI

Figure 8. A mechanical model ofthe action ofthe flexor pollicis longus muscle. The distal phalanx D and proximal phalanx P of the thumb are shown in schematic side view. The ventral surface is to the right. L is the length, and 2W the dorsoventral thickness of the distal phalanx. Fl is produced by the flexor pollicis longus muscle, and F2 is the resultant force exerted on the phalanx by an object grasped by the thumb. A is the axis of rotation.

(6) The ECU

in A. africanus had a mechanical

advantage

for adduction

of modern humans. (7) TheECRLinA. af’rzcunus may have had a mechanical modern humans for abduction, and certainly large as that of modern humans. The

hand

powerful objects

in A. af~icunus was,

flexion

of the fingers,

using a power

the wrist

joint;

hammering, production scraping, abduction climbing. position: Kapandji

grip, where

(b) the resisting striking,

chopping,

of strong hammering,

in these

respects,

the supporting the hand

of abduction gouging,

adduction of the hand, striking and chopping,

of the hand,

as required

for actions

advantage

greater

well

adapted or large

than that of

for the following: objects,

of the wrist, with

as would a tool,

(a)

position

be exerted or

as

the holding

have been held in an extended

scraping

than that

had a lever arm for this movement

of heavy

would

greater

of at

during

in climbing;

(c)

as required for actions such as gouging, using a tool; and (d) production of strong such as hammering,

chopping,

throwing

or

In each of these actions the tool is considered to be held in the human tool using in the palm of the hand, following the oblique arch of the hand as described by side to the distoradial part of the hand. (1982), p assing from the proximoulnar

The distal phalanx of the thumb as a lever The major

Sterkfontein

first distal

features

of its morphology.

phalanx, Several

Stw 294, resembles important

functional

that

of modern aspects

humans

of power

in all

grip action

are deduced from the morphology. In Figure 8 the distal phalanx D is represented in side view, articulating with the proximal phalanx P. Fl is the force produced by the flexor pollicis longus muscle inserted into the ventral surface of the base of the distal phalanx. The application of force Fl will cause flexion about the axis A, which lies at the dorsoventral midpoint of the trochlear

D.E.

660 Table 5

Lever arms in flexion

Austrdopithecus

of the distal interphalangeal

joint of the thumb of modern human and

africanus

I,

13 I 131 170

x.7 1.4 L’i.3

LV/I. I%)

131

184

2L2’-’

\I;

RICKLA1N

o..u CJ.27

ICI8 lrlx

148 I I5

1% I(Ir

1‘The modern human sample comprises left and right hand skclvt<,ns Human Skeletons at the University of thp Witwatersrand. ,Juhantwsburq. 2 2\V = dorsovcntral diamercr ofbasr. ( Approximate measurement. as thr distal end of Stw 294 is d,m~a~ctl.

II.,ii

7.7 3.0

“2.’ I i..i from

iC8

Il.28

3.i

1.4& 1.28

Itj.L’i 21.0

thr Ra\-m~lntl

Dart

(:A=ctiun

01

Figure 9. Stw 294, a 1st distal phalanx from Sterkfontcm Member -1. ‘l‘hc ventral aspect is ahown. The scale is in centimeters. Note (a) the region for the insertion 1Lthr flexor pollicis longus muscle 1F). and (b) the apical tuft (‘l‘).

joint,

at distance

W from

the ventral

surface.

W is half the dorsovcntral

base of the distal phalanx. An object grasped by thr thumb will exert force F2 on the distal end of the phalanx. Thus position

where

an object

is gripped

by the distal

phalanx,

diameter

of the

due to the flcxion of thejoint in equilibrium, such as in a F1.W

= F2.L,

and

W/L

=

F2/Fl. The values of W, L and W/L in modern humans, and in Stw 294 are given in Table 5. The ratio W/L of Stw 294 is significantly larger than both male and female sample means ofmodern humans (P< O-05). Since W/L is greater in il. aj?icanus than in modern humans, by some 14-17%, F2/Fl will consequently be greater than in modern humans. This mechanical advantage in A. africanus implies that for any given force Fl, the resultant F2 exerted by an object on the thumb will be higher in .i. africanus than in modern humans by 14-I 7%. Examination of the region of insertion of the flexor pollicis longus muscle into the base of Stw 294 shows it to be at least as well developed as in modern humans, implying that the muscle would have been as well developed in A. africanus as in modern humans (Figure

9). A. africanus would

thus have been capable

of powerful

gripping

with the thumb.

AUSTRALOPITHECINE

Table 6

Indexes

of robusticity

of metacarpals

Modern

HAND

in modern

humans and

Australopithecus africanus

humans’

male Metacarpal

661

FUNCTION

female

A. ajicanus”

n

x

SD

n

x

SD

2

198

12.7

0.90

161

3’3 4

189 182

13.0 12.6

0.89 0.99

156 156

12.0 12.5’ 11.8

0.8 1 0.87 0.96

13.1 14.5 _i

(Stw 382) (Stw 64,683 394)

5

170

14.6

1.34

144

13.5

1.28

15.0

(Stw 63)

1 The modern human sample comprises left and right hand skeletons Rom the Raymond Human Skeletons at the University of the Witwatersrand, Johannesburg. 2 Composite indexes are calculated using available specimens. 3Excluding the styloid process. 4 Significantly different from A. africanus. 5 Unable to he calculated as no full fourth metacarpal of A. africanusis known.

Dart Collection

of

Robusticity Robusticity can be expressed as a percentage ratio: 0.5 (SRU + SDV) SRU is the radio-ulnar diameter of the specimen at midshaft, SDV dorsoventral

diameter,

and L the length

ofA. africanus and modern humans the value of the index of robusticity each metacarpal,

the differences

0.05). In each case robusticity in males strength

or potential

applicable

grip

can be shown

are not significant,

strength using

their locomotion

to that of modern an adaptation

humans,

to greater

of the male

between

that

curved

midshaft

the greater

specimens. specimens attachment

of modern

humans of modern

This

of robusticity

in the female

metacarpal

3 (P <

of the female

although

their hand

than modern

hand

This

would

be

the pongids

grip strength

IfA. africanus used his hand

robusticity

relative

hand.

way. Thus

must be

in a similar

way

man may be interpreted

as

strength.

and middle phalanges

features of the few Sterkfontein specimens are considered here. of the Sterkfontein proximal phalanges is somewhat intermediate

than

those

radio-ulnar

than

of robusticity,

The proximal

more

except

in the same or in a similar

capabilities.

potential

The relevant non-metrical The overall morphology

of the index

robust than are females (P < 0.01). The greater may be interpreted as a reflection of greater grip

the hand

to have a low index

large to support

The values

(male and female samples) are given in Table 6. While ofA. africanus is higher than that ofmodern humans in

males are more than in females

only to groups

of the bone.

X 100%/L, where the corresponding

expansion

is a pongid

and

those humans,

in most feature.

There

of the pongids. but (Stw

Thus,

less so than 28,

is slight

293,

400),

radio-ulnar

the shafts

are slightly

in the pongids. but

not

bowing

all

There (Stw

of one

(Stw 28), a pongid feature. The ridges on the sides of the phalanges of the fibrous flexor sheaths of the long flexor tendons of the fingers

is

122) of the for the (flexor

digitorum superficialis and flexor digitorum profundus muscles) show greater development in the Sterkfontein hominids than in modern humans. The single middle phalanx is similar to that of modern humans, in all features. Well marked depressions for the insertion of the flexor digitorum superficialis muscle are

D. E. RICKLAN

662 present,

complementing

powerful

linger

longitudinal moments degree

curvature imposed

to indicate

that

Stern hand

& Susman to indicate

(1983) regard an adaptation

grasping more

features

which

they

would

of the lingers,

from

muscle

the OH-7

actions.

than

is a rcmodclling

phalanges

superficialis

hand

rather

of the proximal

Susman

that

& Creel

individual

had

OH-7

be consistent

or the Hadar

with lingers

to climbing

the high

for insertion

capabilities.

of

OH-7

Similarly,

of the Hadar &4. qfurensis activities which require those of modern

specimens.

well adapted

the

hending

of the subadult

grasping

of A. ujGmzu.s resemble

The phalanges

region

phalanges

strong

to strong

(1979) consider

and the well developed

on the middle

suggesting

in the pongids

response

the markedly curvrcd phalanges for suspensory and climbing

resemble

tool using

phalanges,

(1979) suggests

of the lingers.

of the proximal

digitorum

powerful

on the sides

of the ray segments

by lengthening

of curvature

the flexor

the ridges

in .4. africanus. Susman

flexors

humans

but nevertheless

to powerful

show

gripping

actions

activities.

Conclusion The hand both

of.4. qfricanus have features

bones

a powerful

signs

grip

and

powerful

hand

which and

of stability similar to that of modern modern humans have similar

ulnaris

and extensor

radialis

carpi

in modern

muscles,

were probably

radialis

longus

humans;

and

the long flexors similarly

muscles

Marzke modern have

& Shackley

hominids, humans

been

including

(1987),

conclude

able

to exploit

of natural

muscles humans.

developed

in il.

and flexor

carpi

appear

pollicis carpi

to have

longus

ulnaris,

been

Only

of hand

about

The Hadar

to he the preferred in the

muscle extensor

had a mechanical

to the problem have

as tools.

digging.

of L4. carpi

better

would

found

and

for

shows

brevis

approach

objects

of grips

chopping

suitable

metacarpus

radialis

The extensor

the ,4. ufurensis hand

a variety

cutting,

The

as well as the flexor

in a multi-cultural

in manipulation

pounding,

ufurensis considered

that

adaptations

probably

carpi

in the two species.

carpi radialis longus and flexor pollicis longus advantage in A. africanus compared to modern early

wcrc

the extensor

of the lingers,

developed

indicate

movements.

humans. Similarly. the mctacarpals degrees of robusticity. The extensor

africanus and

africanus than

would

wrist

use in

as capable hominid

grips

squeeze

as

would

in activities grip

was

.4.

hecause of the absence of certain features in the region of the fifth finger in modern humans. In this regard, similar conclusions were reached by Marzke ( 1983). The hand of il. af manus was at least as similar to modern humans as that of A. ufarensis, and thus on this basis should have been capable of similar hand

actions.

to be defective, which are found

While

a discussion

of the possible

uses to which

the hand ofA. africanus might

have been put is beyond the scope of this paper, the morphology of the hand bones species suggests that A. africanus would have been well suited for powerful manual and

that

comprising

indeed

such

tasks

might

the suite of fossil hand

have bones

been

performed

hy some

of the

of this tasks,

individuals

from Sterkfontein.

Acknowledgements I wish to thank Professor P. V. Tohias for useful related work, as well as for his continued support and University of the Witwatersrand and to the sponsors Longest Record: The Human Career in Africa”, held in honour

of J. Desmond

Clark,

for their financial

comments and criticisms of this and encouragement. I am indebted to the and organizers of the conference “The in Berkeley, California, in April 1986

assistance

which

enabled

me to attend

AUSTRALOPITHECINE

the conference, Anthropology, the Hadar appreciated.

and to prepare University

material

this paper.

of California,

on two occasions;

Miss A. D. Williamson

J. W. K. Harris,

E. Delson

HAND

Professor

Berkeley,

F. C. Howell of the Department

kindly allowed me to examine

this assistance

and warm

has provided invaluable

and the anonymous

663

FUNCTION

secretarial

hospitality assistance.

of

casts of is much I thank

JHE reviewers for helpful comments

on

previous versions of this paper.

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