The Femur in Early Human Evolution HENRY M. MCHENRY AND ROBERT S. CORRUCCINT Department of Anthropology, L'niuersity of California, Davis, California 9561 6 and Department of Anthropology, Southern Illinois L'niuersity, Carbondale, Illinois 62901

-

KEY WORDS Femur Australopithecus . Homo sp. indet. Plio-Pleistocene hominid . Multivariate analysis

.

ABSTRACT Uni- and multivariate analyses of 5 fossil and 215 extant hominoid femora show that two morphological patterns of hominid femora existed about two million years ago. Femora classified as Homo sp. indet. (KNMER 1472 and 1481) are more like Homo sapiens although not identical. Those classified as Australopithecus robustus (SK 82 and 97) and A. boisei (KNM-ER 1503) are similar to one another but uniquely different from any living hominoid. The strong mophological contrasts imply biomechanical and possible locomotor differences, although these are as yet unknown. Two million years ago significant changes occurred in human evolution. The brain began to increase rapidly in size from that time until the birth of Homo sapiens (Holloway, '74; Jerison, '75; Leutenegger, '73; McHenry, '75d,e, '76; Pilbeam and Gould, '74; Stephan, '72; Tobias, "71). Stone tools first appear (Howell. '76; Isaac, '76a,b; M. Leakey, '71, '76; McHenry, '75d). The tools are associated with living floors, chopped bones of large mammals, and primitive shelters, all implying that by two million years ago humans were eating meat, organizing activities around a home base, and probably sharing food (Isaac, '76a,b; M. Leakey, '71, '76). It is a t about two million years t h a t the genus Homo first appears and is contemporary with Australopithecus boisei in East Africa and A . robustus in South Africa (Howell, '76; R. Leakey, '71, '73a,b, '74, '76a,b; Leakey and Walker, '76; Clarke et al., '70; Clarke and Howell, '72). Recent evidence suggests there was a dramatic climatic change a t that time as well, leading to dessication in East Africa (Ceding et al., '77). In East Africa about two million years ago, at least two species of hominids appear which can be distinguished by cranial capacity and masticatory apparatus. Australopithecus boisei is distinct from Homo sp. indet. in having a cranial capacity not much above 500 cc, compared with about 700 cc for Homo sp. indet. (R. Leakey, '74; Day e t al., '75; McHenry, '76). Posterior teeth ofA. boisei are relAM. J. PHYS. ANTHROP.(1978) 49: 473-488.

atively very large; the cheek teeth of Homo sp. indet. are intermediate in size relative to body weight compared to other hominoids (Pilbeam and Gould, '74; Wolpoff, '73). The sagittal crest, flaring zygomatic arches, deep temporal fossae, thick alveolar bone, and numerous other characteristics are associated with the large posterior teeth ofA. boisei (Tobias, '67). What is still poorly explored is the possibility of postcranial differences between the two species. Few specimens come with postcranial material associated with teeth or crania. Thus postcranial taxonomic assessments must be done independently of the teeth and crania. Five femora have been given taxonomic designations despite the fact that they were not associated with dentitions or crania: two from Swartkrans (SK 82 and 97) classified as A. robustus (Lovejoy and Heiple, '72; McHenry, '72; McHenry and Corruccini, '76b; Napier, '64; Robinson, '72); one from Area 123 a t Koobi Fora (KNM-ER 1503) classified as Australopithecus sp. indet., and two from Area 131 at Koobi Fora (KNM-ER 1472 and 1481) classified as Homo sp. indet. (Day et al., '75, '76; R. Leakey, '73a,b; McHenry and Corruccini, '76b). These five specimens provide the opportunity to examine postcranial differences between the two or more taxa of hominids recognized a t two million years ago. The purpose of this report is to test the hypotheses that (1)there are no significant mor-

473

474

HENRY M MCHENRY AND ROBERT S CORRIJCCINI

phological differences in postcrania reflecting locomotor c o n t r a s t s between t h e s e fossil hominids and Homo sapiens; (2) there are no significant morphological differences in postcrania of locomotor significance among t h e fossil hominids themselves at two million years. These are t h e simplest hypotheses and ones t h a t gain support from t h e work of several authors. Lovejoy, ('73, '751, and Lovejoy e t al. ('73) show that modern human hips are not demonstrably different from Australopithe('us in features related to bipedal walking. Wolpoff and Lovejoy ('75: p. 276) give t h e opinion t h a t t h e postcranial skeleton of Ausidentical to t h a t of Homo tralopithecus is " in form and proportion" except in traits related to parturition and muscularity. From these two hypotheses one predicts t h a t equivalent body parts such as femora from different fossil hominids at two million years should not differ morphologically among themselves nor from Homo sapiens more than would be expected within a modern hominid species. We propose to test these predictions morphometrically since species variability is most easily and accurately studied by this method (Blackith and Reyment, '71; Sneath and Sokal, '73). MATERIALS AND METHODS

The two Swartkrans femora (SK 82 and 97) were discovered in 1949 by Broom and Robinson ('491 in t h e pink breccia but were not described until 1964 by Napier ('64). They have subsequently been described and analyzed by numerous authors (Day, '69; Lovejoy and Heiple. '72; McHenry. '72; Robinson, '72; Lovejoy. '73, '75; Walker, '73; Wood, '76; Lovejoy e t al., '73; McHenry and Corruccini, '76b). There is general agreement t h a t they belong to the most abundant species a t Swartkrans (Paranthropus or Australopithecus robustus). Although absolute dating of t h e Swartkrans cave deposit is as yet uncertain (Rrock et al., '77; MacDougall and Price, '74; Partridge, '731, biostratigraphic correlations with dated sites yield dates slightly younger t h a n two million years ago (Cooke, '70; White and Harris, '77; Vrba, '74, '75). The KNM-ER 1503 specimen comes from Koobi Fora area 123 at Lake Turkana where it was found by M. Muluiba in 1972 (R. Leakey, '73a). Area 123 has not as yet been definitely tied into t h e stratigraphic sequence at Koobi Fora (White and Harris, '77). Descriptions can be found in R. Leakey ('73a), Wood ('76) and

Day et a]. ('76). R. Leakey ('73a) refers t h e specimen to Australopithecus sp. indet. Since the majority of specimens from Area 123 a t this time a r e A . boisei and since t h e specimen resembles t h e robust australopithecine femora from South Africa, i t is likely that KNM-ER 1503 should be classified as A . boisei. Two of the Lake Turkana femora (KNM-ER 1472 and 1481) are classified a s Homo sp. indet. (R. Leakey, '73b). They both derived from Area 131 a t Koobi Fora and well below t h e KBS Tuff. The discovery came in 1972 from beds which also yielded KNM-ER 1470, the nearly complete Homo cranium, although from many meters away and at different levels. Early dating placed these beds at older than 2.6 million years ago (Fitch and Miller, '70, '76; Fitch et al., '741, although the fauna indicated a date of 2.0 million years ago in the Mesochoerus limnctes zone correlated with t h e Shungura member F of Omo (Maglio, '72). Redating of the KBS Tuff with radiometric and fission track reveals two sets of dates, 2.4 (Fitch et al., '76; Hurford et al., '76) and 1.8 (Curtis et al., '75). Descriptions of these femora can be found in R. Leakey ('73b1, Wood ('76), Day et al. ('75), and McHenry and Corruccini ('76b). We compare these fossil femora to a large series of extant hominoids including 57 Homo sapiens, 42 Pan troglodytes, 16 Pan paniscus, 66 Gorilla gorilla, and 34 Pongo pygmaeus. The source and sex of this comparative sample is given elsewhere (McHenry, '72). All measurements were taken by one of us (H.M.M.) t o eliminate inter-observer error. There are two analyses, one based on t h e proximal femur only (10 measurements) and one based on t h e complete femur (20 measurements). The measurements are shown in figure 1 and are described as follows: 1. Vertical diameter of t h e head (Head diamj: t h e maximum diameter of t h e head taken vertically. 2. Vertical diameter of t h e neck (Neck vert diam) : the minimum vertical diameter of t h e neck. In t h e analyses this measurement was modified by subtracting measurement No. 3 from i t to avoid redundancy and to reflect t h e cross-sectional neck shape. 3. Anteroposterior diameter of t h e neck (Neck a-p diam): t h e diameter of t h e neck taken perpendicular t o t h e previous measurement at t h e same point as t h e vertical diameter.

475

EARLY HOMINID FEMORA 6

14

11

----I

16

17

r

11 Fig. 1

Diagram of measurements taken on the femur. Numbers refer to descriptions in the text

476

HENRY M. Mt'HENRY AND ROBERT S. CORRUCCINI

4. Transverse diameter of the proximal shaft (Shaft tv diam): the transverse diarneter of the shaft taken just below the lesser trochanter. 5 . Anteroposterior diameter of the proximal shaft (Shaft a-p diam): the a-p diameter of the shaft taken a t the same position as the transverse diameter. In the analyses this measurement was modified by subtracting it from measurement No. 4. 6. Proximal width (Proximal wd): the projected distance between the most medial point on the head and lateral point on the greater trochanter taken perpendicular to the long axis of the shaft. In the analyses this measurement was modified by subtracting one-half of measurement No. 1 to approximate biomechanical neck length as defined by Lovejoy ('75). 7. Neck length (Neck 1): the distance between the head-neck border and the intertrochanteric crest taken along the neck axis on the posterior side. 8. Lesser trochanter to head (Less trochhead) : the maximum distance between the inferior border of the lesser trochanter and the medial surface of the head. In the analyses this measurement is modified by subtracting one-half of the head diameter to approximate the distance between the lesser trochanter and the center of the head. 9. Lesser trochanter to neck (Less trochneck): the minimum distance between the inferior border of the lesser trochanter and the superior border of the neck. 10. Greater trochanter projection (Gt troch projection): the projected distance between the superior most point on the greater trochanter and the superior border of the neck derived from subtracting measurement No. 9 from the distance between the inferior border of the lesser trochanter and the center uf the superior border of the greater trochanter. 11. Maximum length (Length): the maximum distance between the proximal and distal ends of the femur taken parallel to the shaft axis. 12. Bicondylar width (Bicon wd): the maximum transverse diameter of the distal end taken perpendicular to the shaft axis. 13. Anteroposterior diameter of the distal shaft (Dist a-p diam) : the maximum a-p diameter of the shaft a t the distal end excluding the distal epiphysis. 14. Transverse diameter of midshaft (Mid

tv diam): the transverse diameter of the shaft taken a t the midpoint of the shaft. 15. Anteroposterior diameter of midshaft (Mid a-p diam) : the a-p diameter of the shaft taken a t the midpoint. In the analysis this measurement was modified by subtracting i t from measurement No. 14. 16. Anteroposterior diameter of the lateral condyle (Lat con a-p): the projected distance between the most posterior point on the lateral condyle and the lateral lip of the patellar surface taken perpendicular to the axis of the shaft. 17. Anteroposterior diameter of the medial condyle (Med con a-p): the projected distance between the most posterior point on the medial condyle and the medial lip of the patellar surface taken perpendicular to the axis of the shaft. 18. Transverse diameter of the lateral condyle (Lat con tv!: the transverse diameter of the lateral condyle taken on its posterior aspect. 19. Transverse diameter of the medial condyle (Med con tv): the transverse diameter of the medial condyle taken on its posterior aspect. 20. Condylar notch width (Inter con wd): the distance between the lateral edge of the medial condyle and the medial edge of the lateral condyle taken a t the middle of the posterior aspect. Each measurement for each individual was checked for error by calculating multiple regression predictions of each variable from two other variables within each species and finding those which had deviations greater than three standard deviations. Means, standard deviations and coefficients of skewness and kurtosis were also calculated to check for errors and deviations from normality. Statistical procedures follow those reported in McIIenry and Corruccini ('75a,b, '76a,b) and McHenry et al. ('76). We derived allometrically corrected shape variables by first normalizing the data to insure equal weight in each variable, calculating a standard size vector (Mosimann, '701, adjusting each variable for average within-species allometric influences (Corruccini, '721, dividing each allometrically adjusted variable by the size vector, and standardizing by the grand mean and standard deviation. The new shape variables are thus relatively free of size distortions SO that comparisons between specimens will not

477

EARLY HOMINID FEMORA

be biased by the effects of size. We then apply standard statistical tests including principal components and canonical variates analysis. It may be expected that the 10-variableanalysis will yield statistically more reliable results, since the ratio of sample size to variables is larger and there is a larger sample of fossils (van Vark, ’76). This consideration, however, primarily concerns the confidence limits of the groupings rather than the multivariate distances themselves. RESULTS

Proximal f e m u r The means, standard deviations, and shape variables for the ten proximal measurements appear in table 1. Since the shape variables are expressed in standard deviation units from the grand mean, i t is easy to discern patterns of variation among the subjects. These units should not be taken as comparable to standard deviation units within a group, but rather as standard deviations from the grand mean. So, for example, the femoral head diameter of SK 82 is 3.0 standard deviation units from the Homo supiens mean based on the shape variables, but since the within-H. sapiens variation

is much less than the total variation, the actual distance between fossil and modern human femora is much greater if measured by conventional within-group standard deviation units. Table 1 shows that femoral head size in the three fossils classified as Austrulopithecus boiseihobustus (SK 8 2 , W and KNM-ER 1503) are significantly small, averaging 2.13 standard deviation units below the grand mean and 2.71 units below the Homo sapiens mean. The two fossils classified as Homo sp. indet. (KNM-ER 1472 and 14811, on the other hand, are very near the grand mean, with an average distance of 0.62 units below H. supiens. In the vertical diameter of the neck, KNM-ER 1503 is near the top of the range of variation of H. supiens whereas the other fossils fall within the observed human range. In the adjusted anteroposterior neck diameter which reflects neck flatness or roundness, all of the Australopithecus boiseilrobustus fossils are near the “flat” end of the human range of variation or outside of i t (as in the case of KNMER 1503), a point made clear by Wood (’76). This dimension is in strong contrast to the 2 Homo sp. indet. fossils which have more

TABLE 1

Means, standurd deviations, and mean shape uuriahles

of

the ten proximal femoral measurements

Measurements

Homo s.

Pan t.

n

1

2

3

4

5

6

7

R

9

10

57

40.8 (3.8) 0.58 33.4 (2.2) -0.66 30.7 (2.0) -0.98 44.6 (5.8) 0.45 33.5 (3.9) - 0.37 34.0 - 2.42 36.8 -1.78 40.0 -0.12 43.4 0.04 35.1 -2.19

4.9 (1.7) -0.52 3.7 (1.3) -0.32 3.5 (1.2) -0.04 7.6 (1.7) 0.29 5.0 (1.2) 0.68 7.6 0.98 7.0 0.34 4.7 -0.85 6.3 -0.48 9.1 1.74

23.1 (2.8) 0.87 19.1 (1.6) 0.38 17.2 (1.5) 0.12 22.8 (3.4) -0.36 15.3 (2.41 -1.16 18.8 --1.18 19.3 - 1.30 20.6 -0.47 23.4 -0.10 18.3 -4.54

28.4 (2.8) -0.08 26.8

5.7 (2.1) -0.49 4.7 (1.3) 0.23 2.2 (0,9) 0.23 6.3 (2.2) 0.97 5.4 (1.9) -1.32 5.4 0.09 8.3 -0.42 9.6 -1.05 10.3 -1.92 8.4 --0.95

61.3

33.6 (4.5) 0.77 23.5 (2.7) -0.46 22.5 (2.2) -0.25 30.5 (5.7) -0.74 27.2 (3.5) 0.52 40.5 1.98 $6.7 2.88 42.5 2.32 45.0 2.14 86.2 1.22

61.8 (5.9) -0.41 51.5 (3.3) 0.32 48.1 (3.6) 0.79 72.2 (9.1) -0.68 51.7 (5.0) 1.17 69.8 0.73 71.2 0.02 68.0 0.73 75.3 0.78 61.7 0.61

58.7 (6.0) 0.26 48.1 (3.2) -0.64 44.2 (2.4) -0.88 67.9 (9.6) -0.88 45.7 (4.6) -0.96 61.9

10.1 (2.7) -0.83 12.4

42

Pan p

16

Oorr Elu

66

Pongo

34

SK 82 SK 97

ER 1472 ER 1481 ER 1503

(1.6)

0.42 23.0 (1.9) -0.39 36.1 (4.7) -0.65 22.8 (3.3) 0.93 30.4 -0.40 32.6 0.70 31.4 1.56 31.3 1.20 30.7 0.91

(7.7) 0.70 49.2 (3.6) -0.54 46.0 (4.2) -0.65 66.6 (11.6) 0.56 44.3 (6.81 -1.43 63.5 0.88 65.2 0.86 61.0 0.33 67.3 0.73 68.7 1.72

0.65

65.3 0.95 60.0 0.22 65.3 0.42 53.9 -1.09

(2.31 0.65 11.6 (2.11 0.92 17.8 (4.6) 0.01 11.2 (4.5) 0.51 6.1 -2.60 3.0 -2.84 1.8 -2.91 4.8 -2.80 7.0 -1.81

478

IIENRY M MCHENRY AND ROBERT S. CORRUCCINI

*SK97 *SK 82

8

KNM ER 15.03

KNGER

CANONICAL V A R I A T E I

CANONICAL V A R I A T E I

a

SK

97

IZ

D

SK

82

**KNMER

1503

H

0

0 J

a a

0

I

I

I

P R I N C I P A L COMPONENT

II

I

I

1

I

I

PRINCIPAL COMPONENT

IT

Fig. 2 Results of the multivariate analyses of the proximal femur. Circles encompass approximately 95% of the individuals. A Plot of canonical variates I and 11. B Plot of canonical variates I and IV. C Plot of principal components I and 11. D Plot of principal components 11 and I11

rounded necks. Relative to t h e rest of t h e proximal end, t h e transverse shaft diameters of theHomo sp. indet. femora a r e large, almost above t h e observed human range of variation. The modified anteroposterior shaft diameter reflects t h e flatness or roundness of t h e shaft just below t h e lesser trochanter and shows t h a t t h e Homo sp. indet. fossils a r e relatively small (meaning rounded), especially KNM-ER 1481 which lies outside the observed human range of variation. None of the fossils significantly deviates from Homo supiens in t h e rela-

tive size of t h e proximal end (measurement No. 61, but all of t h e fossils have significantly long necks (measurement No. 7 ) relative to humans. The long neck of all early hominid fossils has been noted by numerous investigators (e.g., Napier, '64;Lovejoy et al., '73; McHenry, '72). The two measurements relating t h e position of t h e lesser trochanter (No. 8 and No. 9) show that none of t h e fossils significantly deviates from t h e range of variation in modern humans. The projection of t h e greater trochanter, however, is below t h e observed

EARLY HOMINID FEMORA

479

range in Homo sapiens in all of t h e fossils ex- variate 1 is like principal component 2, varicept KNM-ER 1503 which is close to t h e bot- a t e 2 is like component 1,and variate 4 is like tom of t h e range. component 3. In this analysis we entered t h e Figures 2c and 2d display the results of fossils as a single group of five to avoid creatthe principal components analysis of t h e ten ing any samples of less t h a n three for which proximal measurements. In this analysis we within-group dispersion cannot be estimated. entered all specimens separately without a The plot shown here reveals more information priori groupings. Only t h e first three compo- about t h e dispersion of t h e groups than was renents have significant eigenvalues (over 1.0). ported earlier (McHenry and Corruccini, '7Gb). The first principal Component accounts for The significant facts to emerge from this anal42.9%of t h e total dispersion. It acts primarily ysis are t h a t in the total dispersion, all of t h e to separate Pongo femora from Gorilla and fossils are different from Homo sapiens, and Homo. The fossils cluster and fall slightly t h e Australupithews hoisei/robustus speciabove t h e two standard deviation range of mens are much more different than those clasHomo. Measurements with high correlation sified as Homo sp. indet. In fact, the Australowith this component are those in which Pongo pithecus fossils fall outside t h e two standard deviates most strongly from the others, illus- deviation range of H. sapiens on the second trating t h e uniqueness of the orangutan canonical variate whereas KNM-ER 1472 and femur with its high neck shaft angle reflected 1481 are within t,he human range. The uniquein a high value in measurement No. 8 allesser ness of t h e fossils is most strikingly illustrochanter to head), a low value in measure- trated by canonical variate 4 which is strongly ment No. G (proximal width), and other traits. correlated with the suite of measurements Principal component 2 (20.2%) is t h e axis of t h a t makes the early hominid femora unique: human uniqueness with t h e fossils falling small heads, large necks, and low greater within t h e human range. Measurements with trochanters. high correlations with this component are Whole f e m u r neck l e n g t h (high negative correlation), greater trochanter projection (high positive Table 2 presents t h e means, standard deviacorrelation), and t h e modified anteroposterior tions, and shape variables for measurements shaft diameter reflecting t h e roundness of t h e of the shaft and distal end (No. 11 to No. 20). shaft (high positive correlation!. Humans The comparative sample differs slightly since tend to have longer necks, less projecting these measurements were taken four years greater trochanters, and rounder shafts (ex- after t h e proximal measurements were taken, cept for PongoJ than t h e other hominoids in and i t was often impossible to relocate t h e t h e sample. The same configuration holds in same individual specimens. The most outgeneral for t h e fossil femora, especially in standing feature of this set of traits is t h e neck length and greater trochanter projec- contrast between Homo sapiens and t h e pontion. The third principal component (13.31%;) gids. Humans tend t.o have relatively longer maximizes t h e projection of the fossils, espe- femora than apes, relatively narrow and more rounded shafts, high lateral margins of t h e cially those classified as A ustralopithecus. Traits with highest correlation are those de- patellar surface (reflected in t h e high value of scribing t h e shape of t h e neck: vertical neck t h e anteroposterior diameter of the lateral diameter (high negative correlation) a n d condyle, measurement No. 161, and relatively modified anteroposterior neck diameter (high small medial condyles (No. 19) in comparison positive correlation). In vertical neck diam- with apes. In most respects t h e two fossil femora (KNM-ER 1472 and 1481) fall within e t e r t h e A u s t r a l o p i t h e c u s boisei/robustus specimens a r e relatively large and the Homo the observed human range of variation in specimens a r e relatively small. In t h e antero- shape variables although KNM-ER 1481 is posterior directions, all of t h e fossil hominids relatively short (No. 11) and both fossils have have somewhat flattened necks but t h e Aus- relatively very small medial condyles (No. 19). tralopithecus specimens much more so t h a n The medial condyle of KNM-ER 1481 is worn and the measurement may be too small, althose classified as Homo. Results of t h e canonical variates analysis though it was carefully reconstructed on t h e are shown in figures 2a and 2b. The results a r e original with plasticine when measured. The entire suite of 20 measurements makes similar to those of the principal components analysis except for t h e fact t h a t canonical the base for t h e principal components analysis

480

HENRY M. MCHENRY AND ROBERT S. CORRUCCINI

Fig. 3 Stereogram of t h e first three principal components resulting from the analysis of the whole femur TABLE 2

Means, standard deviations, and mean shape uariables o f the ten shaft and distal femoral measurements Mearurements ,L

11

12

Homos.

51

Pan t

35

401.4 (30.1) 1.29 293.2 (14.41 - 0.42 340.1 (34.7) -0.72 266.0 (24.4) - 0.84 401.0 1.36 396.0 0.78

72.5 (5.8) -0.03 63.0 (3.9) -0.05 84.9 (11.4) 0.73 56.3 (6.9) -1.23 69.3 -0.73 72.6 -1.30

Gorilla

54

Pongo

28

ER 1472 ER 1481

13

14

15

16

17

18

19

20

34.1 (3.3) 0.95 27.1 (1.6) -0.44 35.8 (4.7) -0.17 24.8 (3.2) -1.01 32.3 0.24 33.6 -0.78

23.9 4.1 -1.06 25.2

-1.6 (7.0) -1.07 2.8 (1.7) 0.35 8.8 (2.8) 0.54 4.3 (2.01 0.93 1.3 -0.58 3.2 -0.46

57.3 (7.3) 1.28 38.8 (2.7) -0.58 49.8 (5.7) -0.68 36.2 (4.51 -0.34 52.5 0.73 60.5 1.41

54.6 (4.8) 0.49 43.1 (2.8) -0.80 62.6 (8.9) 0.70 39.6 (4.3) - 1.22 50.2 -0.54 55.0 -0.28

22.2 (10.8) 0.25 17.5 (1.4) 0.33 23.0 (3.7) -0.46 16.2 (2.81 0.07 17.6 -1.52 22.0 0.00

23.5 (2.9) -0.76 22.2 (1.7) 0.48 31.1 (4.4) 0.80 19.1 (3.1) -0.43 19.0 -2.48 19.3 -3.22

20.5 (5.8) -0.43 17.9 i2.21 0.15 33.3 (3.1) 0.21 17.1 (3.0) 0.35 20.2 -0.23 23.0 0.03

(2.1) -0.37 36.7 (4.7) 1.07 21.9 (3.5) - 0.22 26.4 -0.41 25.6 - 1.29

shown in figure 3. When the whole femur is analyzed in this way, the dominant feature is the hominid distinctness which appears as the first principal component explaining 51.5%of the total variance. Homo sapiens, and the two Homo sp. indet. fossils are widely separated from the pongids on this axis, the fossils projecting even further from the apes than does Homo sapiens. Traits with high correlation

with this axis include just those measurements which are most unique in Homo: (in order of magnitude) ; anteroposterior diameter of the lateral condyle (No. 16, reflecting the large lateral border of the patellar surface), total length (No. ll), projection of the greater trochanter (No. lo), transverse diameter of the medial condyle (No. 191, anteroposterior diameter of the midshaft (No. 15, reflecting

EARLY HOMINID FEMORA

the rounded shape of the midshaft with the prominent linea aspera), neck length (No. 7), and several others. The second principal component (28.3%) separates the Pongo femur from the rest with the fossils in a n intermediate position. Traits with high correlations include (in order of magnitude): bicondylar width (No. 11, relatively small in Pongo and the fossils), anteroposterior width of the medial condyle (No. 17, relatively very small in Pongo but average in the fossils), transverse shaft diameter (No. 4, relatively large in Pongo and very large in the fossils), lesser trochanter to head (No. 8, very large in Pongo reflecting the high neck-shaft angle but not exceptionally large in the fossils), and the anteroposterior diameter of the distal shaft (No. 13, small in Pongo, medium in fossils). The third principal component (11.0% of total variance) acts primarily to separate Gorilla with the fossils falling intermediate but closer to Gorilla than to Homo. In order of magnitude the traits with the highest correlation with this axis include the transverse diameter of the lateral condyle (No. 181,anteroposterior diameter of the neck adjusted to reflect relative flattening (No. 3), and lesser trochanter to neck (No. 9). The values for these traits are not simply interpretable in a univariate fashion but reflect the multivariate complexity.

481

million-year-old femora are not morphologi cally alike: of those analyzed here, there are two clusters, one containing the two Homo sp. indet. femora (KNM-ER 1472 and 1481) and the other containing the three Australopithecus boiseilrobustus specimens (SK 82, SK 97, and KNM-ER 1503). Wood ('76) reports similar results. A multivariate complex of traits differentiates these two clusters, but relative femoral head size is certainly one important feature. Lovejoy ('751, Wolpoff ('761, Wood ('761, McHenry and Corruccini ('76b,c) and Robinson ('721, have recently reviewed the problems of assessing femoral head size in early hominids. Compared to shaft diameters or to a size vector derived from a suite of femoral measurements, the femur head in Australopithecus robustus (SK 82, SK 97) and A . hoisei (KNM-ER 1503) is relatively very small indeed. Lovejoy ('75) and Wolpoff ('76) state that head size should be normalized by femoral length but this can only be done for one Australopithecus specimen (A. africanus, Sts 14). It appears that Sts 14 is within the modern human range, although both the head and the distal third of the femur are missing in Sts 14. As yet no complete robust australopithecine femur is known so that normalization of femoral head size by femoral length is impossible. There are difficulties in using femoral length alone as the size standardizer, anyway. First, the index of femoral head diDISCUSSION ameter divided by femoral length shows that The results clearly do not support our first modern humans have relatively smaller hypothesis: there are significant morphologi- femoral heads than Pan, Gorilla or Pongo. cal differences between the femora of the ear- Presumably this is because the femur has inly hominids and those of Homo sapiens. The creased in relative length in human evolution. most conspicuous single trait which differen- Biegert and Maurer ('72) show that relative to tiates all hominid femora of two million years trunk length the human femur is much longer ago from those of the present is the long than that of any other catarrhine primate exfemoral neck (although there are numerous cept the lesser apes. Biegert and Maurer ('72) other traits which taken in multivariate com- also provide an estimate of the trunk length in bination also separate the femora of extinct Sts 14 (390 mm) which can be combined with from extant hominids). Both Australopithecus Lovejoy and Heiple's ('70i estimate of femoral boiseihobustus and Homo sp. indet. femora length (280 mm). The resulting index shows are outside the modern human range of varia- that the Sts 14 femur is 11% smaller than tion. The results of the principal components Homo sapiens, although well above Pan (by analyses of proximal and whole femur re- 21%).Yet another problem with standardizing ported here confirm the earlier findings by femoral head size with femoral length is McHenry ('75d) and McHenry and Corruccini allometry: there is an obvious relative growth ('76b): the Homo sp. indet. specimens are mor- differential between t h e two dimensions phologically different from Homo sapiens al- which can have a significant effect on a ratio though not as different as those fossils clas- in an animal as small as Sts 14. It is for this reason that in this study we have exponentisified as Australopithecus boiseilrobustus. The other significant finding is that all 2- ated dimensions by their average within-

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group allometric coefficient with t h e size vector before forming a n index. Femoral head size can be related to t h e forces which pass through t h e hip joint, particularly body weight and t h e pull of t h e hip abductor muscles (Lovejoy et al., '73). Using Sts 14, Lovejoy e t al. ('731 show t h a t t h e forces in A . africanus a r e similar to H. sapiens relative to body weight but femoral head pressure is 50% less in t h e fossil form. They assume that femoral length and head size a r e allometrically related by t h e exponent 312 to derive this pressure estimate. This exponent is too high and should be closer to 514 based on our calculations using their own d a t a (Corruccini and McHenry, '78). The pressure becomes much more similar between extinct and ext a n t hominids when empirically derived allometric adjustments a r e made. Femoral neck length, t h e most conspicuous trait in separating all fossil hominids from Homo sapiens in this study, is relatively long in all of t h e fossil hominids, a fact pointed out by numerous investigators (Campbell, '66; Day, '69; Clark, '67; Lovejoy, '73, '75; Lovejoy et al., '73; Napier, '64; Robinson, '72; Zihlman, '67). Its measurement can be made in numerous ways. Napier ('64) and Wood ('76) use the distance between t h e intertrochanteric line and t h e neck-head border. Using the ratio of neck length defined in this way to transverse shaft diameter, Wood ('76) reports a value of approximately 118 and 120 for KNM-ER 1481 and 1472, respectively, which is well below t h e australopithecines and within the human range. IJsing t h e measurements defined above in the same index, we find t h a t t h e two Homo sp. indet. femora have values closer to 144 and 135 which is well above Homo sapiens and similar to t h e other early hominids. This finding is confirmed by t h e shape coefficients for measurement No. 7 in table 1 which show all early hominids with much longer femoral necks t h a n all other hominoids. Femoral neck length can be related functionally to the power arm of the abductor muscles of t h e hip (Lovejoy et al., '73). The shape of t h e femoral neck has received some attention from Wood ('76) who shows t h a t specimens classified as Australopithecus have flatter necks t h a n those referred to as Homo sp. indet. Using t h e measurements defined in this paper, we confirm Wood's assertions, but t h e differences between specimens are much less. The two Swartkrans specimens, for example, a r e only 5% smaller t h a n KNM-

ER 1481 in the index vertical neck diameter/ anteroposterior neck diameter. Our index of neck shape given in table 1 (No. 2, vertical d i a m e t e r m i n u s anteroposterior d i a m e t e r ) shows considerable variation within H. sapiens which encompasses all of t h e early hominid values. The morphology of t h e distal end of the femur in the two Homo sp. indet. femora resembles Homo sapiens and t h e two A. africanus specimens from Sterkfontien, South Africa (TM 1513 and Sts 34, Heiple and Lovejoy, '71; Robinson, '72). They also resemble t h e 3-million-year-old Hadar femora (A. L. 128, 129) as well (Johansen and Coppens, '76; Johanson e t al., '76). These specimens have the suite of distinctive bipedal traits defined by Heiple and Lovejoy ('71) including a high bicondylar angle. a deep patellar groove, a relatively large lateral lip on t h e patellar surface, a relatively small medial condyle, and a flat contour of the distal articular surface. The shape vector describing biepicondylar width (No. 12) shows KNM-ER 1472 and especially 1481 to be relatively small (although not outside t h e observed human range of variation), a point made by Heiple and Lovejoy ('71) about TM 1513. It is unfortunate t h a t no distal femora a r e as yet known for A. robustuslboisei except possibly KNM-El3 993. Judging from t h e numerous hominid proximal tibia from E. Turkana (KNM-ER 741, 803B and G, 813B, 1471, 1476B and C, 1481B and C, 1500 A, C, H, and J, 1810) few obvious differences emerge and yet some of these tibia must be A . boisei. This implies t h a t t h e knee joint of Australopithecus and Homo may have been similar. The results of this study support t h e taxonomic assessments of R. Leakey ('73a,b),Day ('76a,b), Wood ('761, Day e t al. ('75, '76): KNM-ER 1481 and 1472 are much more similar to Homo sapiens than are other early hominid femora. Their placement in t h e genus Homo is justified along with t h e known crania assigned to Homo a t about two million years ago (e.g., KNM-ER 1470, 1590; R. Leakey, '73b). The clustering of these two fossil femora with Homo sapiens is not fortuitous in these analyses since they were entered into t h e proximal and whole femur principal components analyses as separate specimens. What is even more convincing is t h e fact t h a t in t h e canonical variate analysis the two fossils were entered into t h e analysis grouped with the

EARLY IIOMINID FEMORA

Australopithecus specimens. Since t h e function of canonical variates analysis is to minimize within group dispersion and maximize between group differences, it is remarkable t h a t t h e two early Homo femora would assert their position between t h e Australopithecus group and H. sapiens. In fact, t h e total Mahalanobis D2distance between Homo sp. indet. and Homo sapiens in t h e proximal femur analysis is less than half t h a t between Australopithecus and H. sapiens and about equal to t h e distance between Homo sp. indet. and Australopithecus. Given t h e morphological contrast between the robust australopithecine femora and those classified as Homo sp. indet a t two million years ago, how does Australopithecus africanus fit into this picture? Is t h e hip architecture of t h e hominids from Sterkfontein, Makapansgat, and Hadar (classified as A. africanusj more like A. robustzdboisei, Homo or is i t unique? Robinson ('67, '72) and Napier ('64) have championed t h e view t h a t A. africanus resembled Homo sapiens in hip architecture and t h e robust australopithecines were unique. McHenry 1'72, '75a,b,c), Zihlman ('711, Lovejoy et al. ('73) and others have provided evidence t h a t trait by trait t h e hip architecture of A. africunus and A. robustus was similar although multivariate analysis of the ilium and acetabulum showed differences (McHenry and Corruccini, '75a). The question is difficult to resolve because there are so few equivalent skeletal parts known from t h e two australopithecine forms. Sterkfontein provides us with two well preserved distal femora of A . africanus and Swartkrans with two proximal femora of A. robustus. The one proximal femur t h a t is known of t h e South African A . africanus (Sts 14) is crushed and distorted beyond reliable reconstruction (Day, '73; Walker, '73). A solution to this problem may come when the Hadar specimens (especially AH 288) are completely described and analyzed (Johanson and Taieb, '76). Do the morphological differences in femoral anatomy imply locomotor differences? It is difficult to imagine more than one form of hominid bipedalism since there is only one form of it present today. Opinions vary widely on this issue. Early reports stressed the bipedal nature of the australopithecines without specifically referring to t h e form t h a t bipedalism took (Dart, '25, '49a,b, '57, '58; Broom, '38; Brooni e t a]., '50; Broom and Schepers, '46;

483

Broom and Robinson, '49, '50, '52; Clark, '47a,b, '48). Some investigators did not think t h e evidence supported the conclusion t h a t australopithecines were bipedal (Keith, '31; Kern and Straus, '49; Ashton and Zuckerman, '51, '56a,b; Zuckerman, '54, '70).After t h e discovery of t h e Sterkfontein (Sts 14). Swartkrans (SK 50) and Makapansgat (MLD 7, 8) pelvic remains, t h e dominant opinion became t h a t bipedalism was established in one o r more forms of Austrulopithecus, but t h a t it differed somewhat from t h e gait of modern Homo supiens (Washburn, '50, '60, '63, '68; Washburn and Moore, '74; Howell, '55, '67; Clark, '55, '59, '67; Mednick. '55; Chopra. '62; Straus, '62; Robinson, '63, '68, '72; Davis, '64; Day and Napier, '64; Napier, '64, '67; Campbell, '66; Genet-Varcin, '66, '69; Zihlman, '67, '71; Zihlman and Hunter, '72; Day and Wood, '68; Day, '67, '69, '74, '76a,b; Preuschoft, '71; R. Leakey, '71, '73a; Robinson et al., '72; Wood, '74a,b, '76; Sigmon, '71; Oxnard, '73a,b, '75a,b; Zuckerman e t al., '73; Jenkins, '72). Many of these authors speak of the "incompleteness" or "inefficiency" of australopithecine bipedalism. The work of Lovejoy and colleagues has led to different conclusions (Lovejoy and Heiple, '72; Heiple and Lovejoy, '71; Lovejoy et al., '73; Lovejoy, '73, '75) : morphological differences between early hominid hindlimbs and those of modern humans do not imply locomotor difference. The morphological contrasts in the hip a r e related to t h e increase in t h e birth canal size which increased in size during human evolution because of encephalization. They show t h a t t h e femoral neck of Austrulopithecus is relatively large providing a longer power a r m for the abductor muscles of t h e hip. They also show t h a t other morphological differences do not affect t h e performance of bipedal striding. Our findings support t h e view t h a t there a r e morphological contrasts between t h e femora of early hominids and modern hominids, but we also show t h a t t h e contrasts are m u c h g r e a t e r between Australopithecus boisei/ robustus and ]€orno supiens t h a n between Homo sp. indet. and Homo sapiens despite t h e fact t h a t t h e fossils occur at approximately t h e same time. Our morphometrical study does not! of course, discern t h e nature of t h e locomotor differences. The proximal ends of the three robust Australopithecus femora studied here are extraordinarily unique in shape. Although they follow Homo supiens on t h e axes which em-

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phasize human distinctness, t h e overall marphology of these femora is unique among living hominoids. In t.he canonical variates analysis t h e robust Australopithecus specimens align most closely with Homo sp. indet. although about eight times as far as the distance between t h e two species of Pan. Such strong morphological contrasts imply import a n t differences in t h e hip biomechanics of t h e robust australopithecines and other hominids. Perhaps changes in pelvic inlet related to parturition of smaller brained fetuses account for all of t h e morphological contrasts between A . robustudboisei and Homo sapiens (Lovejoy et al.. ’ 7 3 ) . Further material and biomechanical analyses are clearly needed to resolve this issue. CONCLUSIONS

By about two million years ago two patterns of hominid femoral morphulogy are present: one t h a t is more similar to but not identical withHomo sapiens !Homo sp. indet.), and one t h a t is unique (Australopithecus robustus/ h o i a d . The contrasts are documented by uniand multivariate analyses. The magnitude of t h e morphological differences imply biomechanical distinctions and possibly locomotor contrasts as well. The picture t h a t is emerging from t h e two million year old fossil beds of t h e African Plio/Pleistocene shows a t least two very differently adapted hominids with one (Homo sp. indet.) walking, tool making. meat eating, shelter building, perhaps food sharing and hunting more like Homo sapiens and t h e other (Australopithecus rohustudboisei) adapting in a way unknown in modern times. ACKNQWLEDGME NTS

We thank R. E. F. Leakey, C. K. Brain, and E. Vrba for generous permission to study t h e original fossils; L1. Brothwell and T. Molleson of the Division of Anthropology, British Museum (Natural History), 1,. Barton of t h e Powell Cotton Museum, H. Lawrence and C. Mack of t h e Museum of Comparative Zoology, Harvard University, R. Thorington of the Division of Mammalogy, Smithsonian Institution, W. W. Howells of t h e Peabody Museum, Harvard University, and M. Poll of t h e M u s k Royale de 1’Afrique Centrale for t h e use of t h e comparative material in their charge; L. J . McHenry for assistance, editing, and encouragement. Financial support was provided in part by t h e Wenner-Gren Foundation and t h e

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