Hominid fossil sample from Kanjera, Kenya: Description, provenance, and implications of new and...

AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 96:7-23 (1995)

Hominid Fossil Sample From Kanjera, Kenya: Description, Provenance, and Implications of New and Earlier Discoveries

THOMAS PLUMMER AND RICHARD PO?TS Department of Anthropology, University of California, Los Angeles, California 90024-1 553 (T.P.); Department of Anthropology, National Museum of Natural History, Washington, DC 20560 (R.P.); and National Museums of Kenya, P.O. Box 40658, Nairobi, Kenya (R.P.)

KEY WORDS Modern human origins, Provenance, Cranial thickening, Kanjera, Homa Peninsula

ABSTRACT Anatomically modern hominids were first collected from Kanjera, Kenya, by L.S.B. Leakey in the 1930s. Their apparent association with an archaic fauna was quickly challenged, throwing their age into doubt. Further unpublished hominid fragments were collected in 1974, 1975, 1981, and 1987. We review the context and morphology of the entire hominid sample. A minimum number of five individuals is represented by both cranial and postcranial elements. Several individuals have thickened cranial vaults, a characteristic originally thought to reflect their great antiquity. Vault thickening resulted from diploic expansion and may have been a response to acquired or inherited anemia. The entire hominid sample postdates the Kanjera Formation, deposited from the early into the middle Pleistocene. Most of the sample was derived from the black cotton soil capping the stratigraphic col- umn. The morphology and context of the Kanjera hominids is consistent with human skeletal remains from nearby Holocene sites. Hominid 3 was probably an intrusive burial into an early Pleistocene bed. o 1995 Wiley-Liss, Inc.

In 1932-33 and 1934-35, the Third and Fourth East African Archaeological Expedi- tions led by L.S.B. Leakey recovered the remains (mostly cranial) of a minimum number of three fossil hominids a t Kanjera (Leakey, 1935) (Table 1). Based on their apparent association with a middle Pleistocene fauna and their anatomically modern appearance, Leakey claimed the Kanjera remains provided clear evidence of the antiquity of Homo sapiens. Considerable controversy followed this claim. P.G.H. Boswell, a geologist accompanying Leakey on the Fourth East African Archaeological Expedition, suggested that the modern hominids and archaic fauna were actually brought together by sediment slumping (Boswell, 1935). Moreover, Leakey did not map the locality, and lack of excavation pho- tographs (due to camera failure) prevented any precise check on the location of the finds. P.E. Kent, Leakey’s geologist on the

Fourth Expedition, echoed Boswell’s con- cerns, stating “. . . in the absence of exact knowledge of the site of the in situ fragments the certain dating of Kanjera Man became a matter of great difficulty, and again the verdict of ‘not proven’ was regret- fully reached” (Kent, 1942, p. 128). The re- sulting controversy over the provenance of the hominids has led some researchers to accept Boswell’s (1935, p. 371) call to place them in a “suspense account,” while others have accepted the Kanjera remains as some of the earliest evidence of anatomically modern humans in East Africa (Brauer, 1984; Clark, 1981; Leakey, 1981).

Leakey returned to Kanjera in 1941 (Leakey, 1943), 1955 (Leakey, 1958), and

Received July 13,1993; accepted May 24,1994 Address reprint requests to Thomas Plummer, University of

California, Los Angeles, Department of Anthropology, 405 Hilgard Avenue, Los Angeles, CA 90024-1553.

0 1995 WILEY-LISS, INC.

8 T. PLUMMER AND R. POTTS

TABLE 1. Summarv of hominid finds from Kaniera’

Suecies No. Year EXD Context Part MNI

Hominid 1 “Hominid 2” Hominid 4 Hominids 1, 4? Hominids 1, 4? Hominid 3

Hominid 3? Hominid 5 Hominids 6, 7

Hominids 6, 7

Hominids 6, 7

Hominids 6, 7 MNI total

1932 1932 1932 1981 1987 1932,1935

1981 1932? 1974, 1975

1981

1987

1987

L L L K S L

K L Y

K

S

S

Surface In situ? Surface Surface In situ, BCS Surface + in situ

Surface Surface Surface

Surface

Surface

In situ, BCS

Cranial 1 Cranial, rib ? Cranial 1 Cranial Incisor Cranial, 1

Cranial Femur Cranial 1

Cranial, 1

Cranial, tibia,

temporal

femur, phalanx

innominate

innominate, tibiae

phalanx, ?radius

5

L = Leakey, Y = Yale Expedition, K = National Museums of Kenya Survey, S = Smithsonian Expedition. Note that the Hominid 2 fragments are not definitely human. MNI = Mimimum number of individuals.

apparently in 1958, judging from the years recorded on some Theropithecus specimens from the locality. He never relinquished his belief in the antiquity of the hominid remains, which in his view represented fully developed H. sapiens in the middle Pleis- tocene (Leakey, 1972,1974). Subsequent expeditions (Pilbeam, 1974, unpublished report; Pickford, 1984) have recovered further, unpublished hominid fossils (Table 1). The 1987-88 Smithsonian Expedition to Kan- jera recovered additional hominids in both surface collections and excavations, and provided a stratigraphic framework for inter- preting the context of the artifact and fossil samples from the locality. In the last 25 years revisions in chronology and an ex- panded hominid sample have demonstrated the great antiquity of anatomically modern H. sapiens in Africa (e.g., Brauer, 1984, 1989; Rightmire, 1984,1989; Stringer, 1989). A reassessment of the morphology and antiquity of the entire Kanjera hominid sample is thus timely, given their modern appearance and the great age originally accorded them.

Since the age and phylogenetic interpretation of the Kanjera hominids is dependent upon their true stratigraphic provenance, it is necessary first to summarize the geology of the locality.

GEOLOGIC CONTEXT OF THE KANJERA HOMlNlDS

The Kanjera exposures (Oo 20’ S, 34“ 30’ E) occur in the foothills of Homa Mountain

on the Homa Peninsula of Lake Victoria (Plummer and Potts, 1989) (Fig. 1). Fossilif- erous outcrops occur in two areas, designated the Northern and Southern Expo- sures (Behrensmeyer et al., in press). Most work has been concentrated in the Northern Exposures. There, the Kanjera Formation consists of five beds, from oldest to youngest, KN-1 through KN-5. KN-1 consists of gray, massive to poorly bedded tuffaceous silts and sands. KN-2 is subdivided into two facies: KN-2a and KN-2b. The former is gray to greenish well-bedded tuffaceous sands and gravels, and the latter gray lacustrine laminated silts and clays. KN-3 consists of yellow-orange tuffaceous silts and sands interbedded with green to gray clay and clayey silt. KN-4 consists of dark gray to gray brown tuffaceous silts and sands with gravel lenses interbedded with sandy and silty clay. KN-5 is a massive, dark green-gray cal- careous clay. The Kanjera Formation is un- conformably overlain by the Apoko Forma- tion (ApN), dark greenish-brown to gray- brown sandy clay with a sandy gravel channel facies. The stratigraphic section is capped by black cotton soil (BCS). Strati- graphic interpretation is complicated by nu- merous local faults, which have juxtaposed units of different ages. Vertebrate fossils were recovered in KN-2a through KN-5, ApN, and the BCS. Paleomagnetic stratigraphy and faunal analysis indicate Kanjera Formation deposition began in the early and continued into the middle Pleistocene (Beh- rensmeyer et al., in press). Thus, deposition

HOMINID FOSSIL SAMPLE FROM KANJERA. KENYA 9

began much earlier than originally believed (Leakey, 1935,1972). ApN deposition probably occurred during the middle to late Pleis- tocene, while the BCS is probably latest Pleistocene to Holocene in age.

A map sketched by Kent in 1934 and re- produced in Pickford (1984, p. 216) indicates that all of Leakey’s hominid finds were on the flats north of the KN-2a Island (Kanjera Flats; see Fig. 1). Fragments of Hominids 1, 2, and 4 were found in close proximity to each other. All of the Hominid 1 and 4 fragments were surface collected, while the Hominid 2 fossils were found several inches into the sediment. Approximately 60 yards to the southeast Leakey discovered fragments of a cranium, phalanx, and femur, designated Hominid 3. Excavation there recovered two in situ cranial fragments. Thus, parts of Hominids 2 and 3 were recovered in situ in a presumed Kanjera Bed. As the context of Hominid 3 seemed more secure, this specimen became the cornerstone of Leakey’s stratigraphic placement of all the hominids (Leakey, 1935).

Leakey’s (1935, p. 27) diagram of the Hominid 3 site section places the in situ hominid fragments at the base of grayish calcified stratified sands near their lower contact with greenish gray clays. The grayish calcified stratified sands were in turn overlain by sandy gravelly clays. Several lines of evidence suggest that Leakey’s lower two units correspond to the two KN-2 facies defined by the Smithsonian Expedi- tion (Fig. 1). The descriptions of his units match our observations of the KN-2 facies. Moreover, the grayish calcified stratified sands were said to have yielded Theropithe- cus fossils in other parts of the Northern Exposures (Leakey, 1935). Controlled surface collection by the Smithsonian Expedi- tion, as well as fossil trace element analyses (Plummer et al., 1994; Plummer, 19921, indicate that nearly all of the T. oswaldi fossils from Kanjera were derived from KN-2a. Leakey (1935, plate VIA) provides a photograph illustrating the horizon from which the Hominid 3 fragments were derived. The deposits in the photograph strongly resem-

10 T. PLUMMER AND R. PO’ITS

ble those defined as the KN-2a Island by the Smithsonian Expedition. Lastly, KN-2a deposits interfinger with and occasionally overlay KN-2b clays on the Kanjera Flats in the approximate location of the Hominid 3 site. In summary, we interpret Leakey’s basal greenish gray clays as corresponding to KN-2b, while his grayish calcified stratified sands correspond to KN-2a. Bed KN-2 underlies sediment samples having re- versed geomagnetic polarity, indicating that Hominid 3 was derived from a bed with an early Pleistocene age (Behrensmeyer et al., in press).

The overlying sandy gravelly clays in Leakey’s section are somewhat harder to de- fine. At the Apoko Site excavation of the Smithsonian Expedition, carried out in the approximate area of discovery of Hominid 3, a gritty, surficial mantle draping over the outcrops was found to be weathered and pe- dogenically altered KN-2b. The sandy gravelly clay noted by Leakey could also have been a weathered aspect of the underlying Kanjera Bed, or ApN channel sediments.

The comments of Kent (1942) complicate the above interpretation of the Hominid 3 site stratigraphy. His first criticism was lev- eled against Leakey’s site diagram. Kent noted that while drawn horizontally, the lower two beds are actually tilted. This ob- servation provides independent confirma- tion that Leakey (1935) was describing KN-2 deposits. Some, but not all, KN-2b outcrops are tilted on the Kanjera Flats. With- out knowing the precise location of the site, it is impossible to know what the dips of the beds are, and whether Kent’s criticism was warranted. Kent (1942) associated the hominids with his “middle beds,” equivalent to KN-3 and KN-4 of the current scheme. This assignment is at odds both with his own descriptions and sketch map (placing Leakey’s sites west of the KN-3 and KN-4 outcrops) and with Leakey’s bed descriptions, which linked Hominid 3 to the Theropithecus beds.

In summary, evidence suggests that the Hominid 3 discovery site, and most likely that of Hominids 1,2, and 4, was associated with KN-2a and KN-2b outcrops on the Kan- jera Flats. The extensive geologic trenching of the Smithsonian Expedition failed to doc- ument sediment slumping (Behrensmeyer

et al., in press), dispelling Boswell’s (1935) assertion that the hominids and fauna were brought together by this process. It thus seems likely that two fragments of Hominid 3 were associated with an early Pleistocene bed.

The remains of a minimum of two additional hominids (Hominids 6 and 7) were surface collected by the Yale Expedition in 1974 and 1975 (Pilbeam, 1974, unpublished report). Unlike Leakey’s finds, which were made on the Kanjera Flats, the Yale hominids were found southeast of the KN-2a Is- land, approximately 10 m N of the BCS Site trenches (BCS T1-3) of the Smithsonian Ex- pedition (Fig. 1). Pilbeam (1974, unpublished report) suspected that they were derived from the BCS.

A National Museums of Kenya project surveyed and mapped sites on the Homa Peninsula in 1981, collecting more hominid remains from Kanjera in the region of Leakey’s sites and from the Hominid 6 and 7 site (Pickford, 1984). The Smithsonian Ex- pedition’s excavations recovered in situ hominid fossils in the top 10 cm of the BCS at the Black Cotton Soil Island (BCSI) Site and in Black Cotton Soil Trench 3 (BCS T3), the approximate areas of discovery of Homi- nids 1, 2 and 4, and Hominids 6 and 7, respectively (Fig. 1). Additional hominid cranial and postcranial fragments were surface collected at the site of earlier Hominid 6 and 7 finds.

Our review of the discovery contexts of the Kanjera hominids suggests that Hominid 3 was recovered from an early Pleistocene bed (KN-2a1, while some or all of the other hominid fossils were derived from the BCS.

DESCRIPTION OF THE KANJERA HOMlNlDS

Hominids 1-5 As the specimens collected by Leakey

have already been described and figured (Leakey, 1935; Tobias, 1968; Brauer, 19841, we will only summarize their morphology. Table 2 presents descriptive measurements of the Kanjera cranial vault fragments. Hominid 1 consists of seven cranial fragments (Fig. 2). The frontal is approximately half complete, and includes glabella and na-

HOMINID FOSSIL SAMPLE FROM KANJERA, KENYA 11 TABLE 2. Length, breadth and thickness measurements (mm) for the Kanjera hominid cranial vault sample’

SDecies No. Exu Part

Hominid 1 Frag. 1 Leak Frontal

Thickness Region/ landmark L Br Min Max Landmark

116.0 95.0 squama 6.0 7.0 bregma coronal suture

Frag. 2 Leak Parietal 110.6 57.3 4.6 5.9 lambdoid suture 9.6 (near lambda) sagittal suture 8.7

3.7 Frags. 3-5 Leak Occipital

Parietal

Temporal

“Hominid 2” KH 9 Leak ? KH 10 Leak ?

KJ 12831 Leak Frontal

KH 58 Leak Frontal?

Hominid 4

Kanjera-Hominid 1 and/or 4? K J 7673 NMK Vault KJ 7674 NMK Vault KJ 7675 NMK Vault KJ 7677 NMK Vault

Frag. 1 Leak Occipital

Frag. 2 Leak Parietal

Frag. 3 Leak Parietal Frag. 4 Leak Frontal Frag. 6 Leak Parietal

KH 8 Leak Occipital?

Kanjera Hominid 3

Hominid indeterminate

Kaniera Hominids 6 and 7 a 12827

KJ 12828

KJ 12829 KJ 12829’ KJ 1282g2 KJ 22924

K J 12829 KJ 12829 KJ 7499

KJ 12829 KJ 7690 KJ 7501A KJ 7501B KJ 12829 KJ 12829 KJ 12829 KJ 12829 KJ 12530 K J 12530 KJ 22925 KH 50 KH 51

Yale

Yale

Yale Yale Yale Smith

Yale Yale NMK

Yale NMK NMK NMK Yale Yale Yale Yale Yale Yale Smith Smith Smith

Parietal

Occipital

FrontaYparietal Parietal Parietal Temporal

Parietal Parietal? Parietal?

Frontal? Vault Vault Vault Vault Vault Vault Vault Vault Vault Vault Vault Vault

lambda inion asterion

asterion

asterion

squama squama

lambda

lambda

50.2 28.9

69.4

41.8

26.3 23.1 21.8 21.1

69.1

70.3

59.6 62.9 61.9

58.2

97.8

64.8 bregma

inion ext. occ. prot.

68.8 49.2 46.9 52.6

41.3 34.1 50.9

24.3 39.4 44.0 35.7 45.3 41.9 31.7 19.2 32.3 31.4 47.1 48.2 50.9

asterion

asterion? (on suture)

45.4 18.7

57.3

37.0

19.6 12.6 18.4 13.2

43.9

60.1

53.8 33.2 41.3

51.4

60.1

55.7

37.9 44.1 42.1 49.4

24.9 22.3 41.1

23.5 22.8 28.4 25.7 31.7 32.4 28.1 15.6 27.3 23.5 31.6 32.8 30.4

6.4 4.3

3.5 4.0

5.1 4.7 5.1 3.1

5.6

5.8

5.1 6.4 8.6

6.3

7.3

4.7

4.1 6.5 6.4

10.0 4.6 4.3

_ - 7.0 7.0 7.4 9.5 4.5 5.5 9.2 6.0 6.5 6.8 7.4 5.9

9.8 6.4

3.9 5.7

6.1 6.3 5.7 5.4

16.3

12.8

9.8 10.1 10.2

11.4

11.8

11.9

10.6 9.1 9.7

11.0 8.9 6.7

_ - 9.1 9.0

10.4 10.0 5.4 8.5 9.2 9.2 9.1 9.2 9.0 8.5

(8.3) 9.6

10.1 (8.0) 7.5

8.8

8.5

11.7

11.7

(10.3)

10.9 11.9

5.7

7.8

‘Measurements in ( Smith : Smithsonian Expedition. ‘Piece conjoin.

) estimated due to breakage. Leak = Leakey, Yale = Yale Expedition, NMK = National Museums of Kenya Survey,

12 T. PLUMMER AND R. POlTS

Fig. 2. Cranial fragments from Hominid 1 (A-E) and Hominid 4 (F,G). A, B: Frontal bone, frontal and lateral aspects. C: Occipital fragment, posterior aspect. D: Conjoining left parietal and occipital fragment, lateral aspect. E: Zygomatic fragment, frontal aspect. F, G: Frontal fragment, frontal and lateral aspects. Scale = 4 cm.

HOMINID FOSSIL SAMPLE FROM KANJERA, KENYA 13

sion. The frontal squama on the left side extends to the coronal suture just lateral to bregma. Conjoining fragments of the left parietal form a single piece running along and preserving much of the sagittal suture. While it does not conjoin with the frontal fragment, this piece of parietal does articu- late a t the lambdoidal suture with a portion of the occipital. Three conjoining fragments preserve much of the occipital squama and nuchal region, as well as small portions of the left and right temporals. A facial fragment preserves a small portion of the left zygomatic and maxilla, including the lateral and inferior orbital margins.

Hominid 2 consists of three cranial fragments and a rib shaft fragment. These scrappy specimens are not clearly attributable to the Hominidae, and while included in Tables 1 and 2, might better be excluded from future discussion of the sample.

The cranium of Hominid 3 is more fragmentary than that of Hominid 1, and again consists largely of frontal, parietal, and occipital pieces. The two nonconjoining frontal fragments are both from the left side of the bone. The most important of these was recovered in situ by Leakey, and preserves a portion of the superior and lateral orbital margins and the zygomatic process of the frontal. The second fragment preserves a small portion of the anterior temporal line and conjoins with a small parietal fragment along the coronal suture. Posterior to these, a piece of the left parietal conjoins to a fragment of the left occipital squama along the lambdoidal suture. Pitting and minor in- flammation of the outer table of the former fragment suggests that this individual suf- fered from a scalp infection (D. Ortner, pers. comm.). Lastly, a small portion of the right parietal, superior to the temporal line but not reaching the sagittal suture, is preserved. This fragment was also found in situ.

Hominid 4 (Fig. 2) consists of two frontal fragments, one preserving nasion, glabella, and the medial portions of the left and right superciliary arches. The other fragment is less obviously hominid, but was thought to be part of the frontal squama.

Leakey’s (1935) vault reconstructions of Hominids 1 and 3 are long, narrow, and

strikingly parallel-sided. All of his finds are anatomically modern (Leakey, 1935; Brauer, 1984; this study). This assessment is best demonstrated by the supraorbital fragments of Hominids 1, 3, and 4. Archaic H. sapiens crania characteristically have a supraorbital torus: a continuous bar of bone extending across the superior margins of both orbits and incorporating glabella, the superciliary arches, and the supraorbital trigones. In contrast, the superciliary arches and supraorbital trigones of anatomically modern humans are generally gracile and easily distinguishable. The flat to slightly convex glabellar regions of hominids 1 and 4 and the gracile superciliary arches of all three hominids clearly align them with anatomically modern humans. The frontal squama of Hominids 1 and 4 are high, with the Hominid 1 lateral profile being nearly identical to that of a late Pleistocene anatomically modern human cranium from Naivasha, Kenya (Brauer, 1984). The Hom- inid 1 facial fragment is lightly built and preserves a trace of the canine fossa. Its occipital is strongly curved, but appears anatomically modern. It has an asymmetric nuchal swelling, better developed on the left side. This may be indicative of asymmetri- cally distributed muscle activity, perhaps through partial muscle or neural failure.

A left femoral diaphysis and a pes phalanx fragment were collected with the Hominid 3 cranium. Moderate compact bone thickness, relatively circular shaft shape, and the presence of a strong pilaster all align the former specimen with anatomically modern human femora (Kennedy, 1983; Ruff et al., 1993). A right proximal femur fragment in the Na- tional Museums of Kenya (KNM-KJ 12826) is not described by Leakey (1935). A cast of this specimen in the Natural History Mu- seum, London, is accompanied by a note stating, “Washed out by rain at skull site.” Oakley et al. (1977) refer two femoral fragments to Kanjera Hominid 5, one presum- ably being this specimen. It preserves the lesser trochanter, the femoral neck, and the anteromedial portion of the head. The diameter from the superior to the inferior margin of the neck is 32.4 mm, while the A-P neck diameter is 28.0 mm. The shaft A-P diameter just below the lesser trochanter is ap-

14 T. PLUMMER AND R. Poll's

Fig. 3. Hominid fossils recovered in situ in the BCS by the Smithsonian Expedition. A Right temporal fragment KNM-KJ 22924, lateral aspect. B: Right I' KNM-KJ 22927, lingual aspect. Scale = 1 cm.

proximately 29 mm. Again, the preserved portions are indistinguishable from those of anatomically modern humans.

The National Museums of Kenya survey recovered a few possible and probable hominid fossils in the areas of collection of Leakey's Hominids 1-4 (Pickford, 1984). KNM-KJ 7676 and KNM-KJ 7679 are possible hominid cranial and molar fragments, respectively, collected in the area of the Hominid 3 discovery. A proximal rib fragment attributed to Hominid 3 (KNM-KJ 7678) is probably bovid. The fossils most convincingly accorded hominid status are vault fragments collected in the area of the Hominid 1, 2, and 4 discoveries (KNM-KJ

The Smithsonian Expedition carried out two small excavations in the approximate areas of Leakey's finds. No fossils were recovered in the KN-2b deposits excavated at the Apoko Site, near where Leakey collected Hominid 3. Excavation at the BCSI Site, in the approximate area of discovery of Homi- nids 1, 2, and 4, recovered a hominid right upper medial incisor (KNM-KJ 22927) (Fig. 3). It was found in situ in the top few centi- meters of BCS capping a KN-2a outcrop (Fig. 1). It has a pronounced cingulum and is shovel shaped. This specimen is 20.5 mm high and the mesial-distal width of its oc- clusal surface is 8.5 mm.

7673-7675,7677).

While assigning the Kanjera remains to H. supiens, Leakey (1935) felt that their long, parallel-sided braincases, wide fron- tals, and thick vaults were primitive features consistent with their presumed antiquity. The crania are undoubtedly long, but their extreme parallel-sidedness is probably a reconstruction artifact (Plummer, pers. obs.; Brauer, 1984). One feature at least, the thick vault bones, could be interpreted as primitive. This feature is discussed in detail below, following the description of Hominids 6 and 7.

Hominids 6 and 7 Every project performing research at

Kanjera since the 1974 Yale Expedition has recovered human fossils southeast of the KN-2a Island. The tibia and parietal fossils (discussed below) indicate that a minimum number of two individuals have been sam- pled at the site. Following the informal numbering scheme established by Leakey (1935), these have been designated Homi- nids 6 and 7. In total, the Yale Expedition surface collected 15 cranial fragments and two pieces of a right innominate from this area. Four specimens in particular deserve mention.

KNM-KJ 12827 consists of two conjoining fragments preserving the anterosuperior portion of the left parietal (Fig. 4). Fifty-four


Fig. 4. Cranial fragments from Hominid 3 (A) and Hominids 6 and 7 (B-D). A Occipital break illustrating diploic expansion. B: Pitting of the outer table of left parietal KNM-KJ 12827. C: Break in KNM- KJ 12827 illustrating diploic expansion. D: Break in KNM-KJ 12829 illustrating diploic expansion. Scale = 1 cm.

point-six millimeters of the coronal suture is preserved along the anterior margin, but the medial edge around bregma is chipped off. Forty-three point-four millimeters of the sagittal suture is preserved posteriorly. The internal surface preserves a few indistinct mid-meningeal grooves. The parietal exhib-

its moderate curvature laterally and posteriorly, and must have been relatively long and flat when complete.

KNM-KJ 12828 is an irregularly shaped portion of the left half of the occipital. It includes much of the moderately developed external occipital protuberance and a por-

16 T. PLUMMER AND R. POTI'S

tion of the left half of the superior nuchal line. No sutures are preserved.

KNM-KJ 12829 is a collection of 10 smaller cranial pieces. Two parietal fragments were found to conjoin along a short stretch of sagittal suture. One of the pieces preserves a small portion of an intersecting suture, either the coronal or lambdoidal. If the former attribution is correct, these pieces would have to have been derived from a different individual than KNM-KJ 12827. Thickness differences between these specimens supports this interpretation.

KNM-KJ 12825A is a right ilium fragment preserving the superior half of the acetabulum but lacking the iliac crest and sac- ral articulation. Anteriorly it preserves the moderately developed anterior inferior iliac spine, and a portion of the margin between this landmark and the anterior superior iliac spine. Posteriorly the specimen preserves the apex of the greater sciatic notch. Its maximum length is 101.6 mm and its greatest height is 83.0 mm. It exhibits a moderately developed arcuate line and lacks a prominent iliac pillar.

KNM-KJ 12825B is a portion of the posterior ischium, including a very fragmentary ischial tuberosity. Its greatest length is 48.0 mm, greatest height is 54.2 mm, and greatest thickness across the ischial tuberosity is 23.7 mm. Much of the cortical bone on the posterior and lateral surfaces of the fragment has flaked off. While it does not conjoin to KJ 12825A, the two pieces are propor- tioned similarly, and may have come from the same innominate.

While surveying and mapping sites on the Homa Peninsula, Pickford (1984) collected more hominid remains attributable to Hom- inids 6 and 7. These included several small cranial vault fragments, left mandibular condyle (KNM-KJ 7687) and corpus (KNM-KJ 7497) fragments, a maxillary fragment (KNM-KJ 7691A), two conjoining left innominate fragments (KNM-KJ 74951, and two left tibia diaphysis fragments (KNM-KJ 7493, 7494A). A possible femoral condyle fragment, KNM-KJ 7683, as well as two rib fragments, KNM-KJ 7691C and D, were also recovered.

Mandibular condyle KNM-KJ 7687 has a maximum length and breadth of 41.4 mm

and 27.5 mm, respectively. Condylar A-P and M-L dimensions are 15.4 mm and 26.9, though the A-P measure is reduced due to chipping of the articular surface. KNM-KJ 7497 is a small fragment of the anterior portion of the left mandibular corpus. Its maximum length and breadth are 43.5 mm and 16.4 mm, respectively. The symphysis is missing and tooth sockets are indistinct. Corpus height at the mental foramen is approximately 39 mm. KNM-KJ 7496 is a badly preserved premolar fragment. KNM-KJ 7691A is a poorly preserved maxillary fragment, including 9 portion of the maxillary sinus.

KNM-KJ 7495 consists of two conjoining left innominate fragments preserving the ilium bordering the acetabulum to a point just above the base of the ischial tuberosity. The iliac and ischial contributions to the acetabulum are preserved, as is most of the posterior surface of the ischium. The posterior margin of the acetabulum and the spine of the ischium are both missing. The inferior margin of the sciatic notch is preserved up to the beginning of its apical curvature. The ischiopubic ramus and pubis are not preserved. The anterior margin of the ilium, including the anterior inferior iliac spine, is missing. It has a maximum height of 142.7 mm and breadth of 56.8 mm. Its estimated acetabular height is 60.2 mm, and the maximum breadth of the ischial tuberosity is 27.6 mm. These fragments could have come from the same individual as innominate fragments KNM-KJ 12825A and B.

Left tibia diaphysis KNM-KJ 7493 has a length of 276.9 mm and maximum A-P and M-L diameters of 38.9 and 27.9 mm, respectively (Fig. 5). Its approximate midshaft A-P and M-L diameters are 31.1 mm and 27.9 mm. A deep sulcus is preserved on the posterior surface of the proximal diaphysis, corresponding to the insertion of tibialis posterior. The tibia exhibits porcupine gnaw marks on both its proximal and distal ends. Like the rest of the bone, the gnawed patches had small carbonate nodules adher- ing to them, indicating that gnawing had occurred prior to final interment.

KNM-KJ 7494A is also a left tibia diaphysis (Fig. 5). It has a maximum length of 271.2 mm and maximum A-P and M-L diam-


Fig. 5. Hominids 6 and 7 remains surface collected by the National Museums of Kenya survey team and the Smithsonian Expedition. A Proximal manus phalanx KNM-KJ 22926, probably from digit 2. B: Right mandibular ascending ramus fragment KNM-KJ 22928. C: Left to right. Posterior aspect of left tibia1 diaphyses KNM-KJ 7493 and KNM-KJ 7494A. D Porcupine gnaw marks on distal break of KNM-KJ 7493. Scale = 3 cm.

eters of 43.0 mm and 29.6 mm, respectively. Estimated midshaft A-P and M-L diameters are 36.8 mm and 26.2 mm. Unlike KNM-KJ 7493, this specimen does not exhibit any un- usual sulci.

Smithsonian Expedition surface collection recovered two small vault fragments (KNM-KJ 22923, KNM-KJ 229251, a right mandibular fragment (KNM-KJ 229281, tibia diaphysis fragments which conjoined to KNM-KJ 7494A, proximal tibia fragments (KNM-KJ 22921, KNM-KJ 229291, a

first manus phalanx (KNM-KJ 229261, a possible radial tuberosity fragment (field number KH 54), and a possible terminal phalanx fragment (KNM-KJ 22922) at the site of earlier Hominid 6 and 7 finds (Fig. 5). The two vault fragments are again thick (Table 2). KNM-KJ 22928 is a massive right ascending ramus fragment. The coronoid process and gonial angle are missing, as is the mandibular corpus anterior to the empty third molar socket. The specimen has a well developed mylohyoid groove and mandibu-

18 T. PLUMMER AND R. POTTS

lar foramen. Its maximum length is 81.8 mm and its maximum height is 66.0 mm. Maximum corpus breadth (buccal-lingual) is 21.0 mm at the rear of the M3 socket, while corpus height in this area is 34.1 mm. The condyle’s A-P and M-L diameters are 17.6 mm and 25.8 mm, respectively, and this specimen seems a good match for the left condyle collected by Pickford (1984) (KNM-KJ 7687). In terms of general propor- tions, KJ 22928 is comparable in size to the very large, probable Holocene human man- dible from Loiengalani, Kenya (Schepartz, 1987).

KNM-KJ 22929 is the anterolateral quar- ter of a proximal left tibia articular surface. It includes the anterior portion of the lateral condyle and a portion of the intercondylar area. KNM-KJ 22921 may also be a proximal tibia fragment.

KNM-KJ 22926 is a complete proximal manus phalanx, probably from digit 2. It is 44.2 mm long, has proximal A-P and M-L measurements of 12.3 mm and 14.4 mm, respectively, and distal A-P and M-L measurements of 7.3 mm and 11.3 mm. Its midshaft A-P and M-L measurements are 7.5 mm and 10.7 mm. Lastly, KH 54 is a small diaphysis fragment with a large tuberosity; it is probably a fragment from a very robust proximal radius.

The Black Cotton Soil Site trenches (Fig. 1) were dug in order to determine whether parts of Hominids 6 and 7 were eroding from the BCS. KNM-KJ 22924, a right temporal fragment, was recovered from the top 10 cm of Trench 3 (Fig. 3). This specimen has an A-P length of 52.6 mm and a height of 49.4 mm. It preserves the mastoid portion of the temporal posterior to the external auditory meatus, as well as small sections of the ad- joining parietal and occipital. Both the mastoid process and the digastric fossa are moderately developed. The occipitomastoid crest is weakly developed medial to the well developed occipital groove. Asterion is preserved, though its exact position is difficult to determine due to sutural obliteration. Breaks medial, superior, and posterior to the mastoid process expose mastoid air cells. Like many of the surface collected hominid fossils from this area (and also the hominid

fossils collected by Leakey, the Yale Expedi- tion and the National Museums of Kenya survey) the temporal fragment was covered with calcium carbonate nodules.

While the Hominid 6 and 7 cranial remains are fragmentary and some of the vault pieces are thick, the preserved morphology is anatomically modern. Parietal KNM-KJ 12827 exhibits moderate curvature both laterally and posteriorly and must have come from a large skull with a high parietal boss. The weakly expressed superior nuchal line and external occipital protuberance and curvature of KNM-KJ 12828 aligns it with anatomically modern occipi- tals. Temporal fragment KNM-KJ 22924 has a moderately developed mastoid process and a weakly developed occipitomastoid crest, the latter feature strongly expressed in some archaic H. sapiens fossils (e.g., Omo 2 from Ethiopia; KNM-ES 11693 from Eliye Springs, Kenya) (Brauer, 1984; Brauer and Leakey, 1986).

The Hominid 6 and 7 postcranial fossils are also indistinguishable from those of anatomically modern humans. Innominate fragment KNM-KJ 12825A lacks an iliac pillar, a feature well developed in H. erectus in- nominates and the archaic H. sapiens innominate from Broken Hill, Zambia (Day, 1971; Stringer, 1986; Kennedy, 1992). The ischium of the KNM-KJ 7495 innominate fragment lacks the medial torsion of H. erectus and possibly African archaic H. sapiens ischia (Day, 1971; Kennedy, 1992). Tibia1 diaphyses KNM-KJ 7493 and KNM-KJ 7494A do not exhibit the compact bone thickening characteristic of H . erectus and archaic H. sapiens diaphyses (Kennedy, 1985; Ruff et al., 1993).

DISCUSSION Cranial thickening in the Kanjera sample

Leakey (1935) felt that cranial thickening within the Kanjera sample was a primitive feature consistent with their presumed antiquity. As Table 2 demonstrates, maximum parietal thicknesses for Hominids 3 and 6/7 (12.8 mm and 11.8 mm, respectively) com- pare favorably with a range of fossil hominids (mean thickness at parietal boss: Nean-


TABLE 3. Thickness measurements (mm) of the diploe and outer and inner tables as well as the ratio of diploic to tabular bone of selected Kaniera hominid cranial vault fraements'

Species No.

Hominid 1 Fragment 1 Fragment 2

Fragment 1

Fragment 2

Hominid 3

Hominids 6, 7 KJ 12827

K J 12829 K J 12829D K J 7501A KJ 7499 K J 22925

Part Diploe

Frontal Parietal

Occipital Occipital Parietal Parietal Parietal

Parietal Parietal Parietal Parietal Parietal Parietal Parietal? Parietal? Vault

2.3 1.7

10.3 10.1 8.7 8.3 9.3

7.4 9.1 7.7 8.9 6.8 8.1 5.1 4.0 5.4

TO",,,

2.1 2.2

1.4 2.5 1.3 1.3 1.3

0.5 0.6 1.3 0.3 1.0 1.5 1.9 1.5 1.2

2.4 1.5

0.8 1.1 0.6 1.0 1.0

0.51 0.46

4.68 2.81 3.58 2.61 3.04

0.2 10.57 0.1 13.00 0.6 4.05 0.5 11.13 0.3 5.23 0.7 3.68 1.3 1.59 1.0 1.60 0.3 3.60

'Multiple measurements were taken on Fragments 1 and 2 of Hominid 3 and KNM-KJ 12827.

derthals, 11 mm; Ngandong, 14 mm; Zhou- koudien, 16 mm; Kabwe, 10 mm; and Swanscombe, 11 mm) (Webb, 1990). How- ever, the maximum thicknesses of the Homi- nid 1 frontal and parietal fragments (9.6 mm for both) are restricted to small areas on the coronal and lambdoidal sutures, respectively. The squama of these bones are con- siderably thinner, as is the preserved portion of the Hominid 4 frontal squama (Table 2). Vault thickness varies within the sample, with its greatest expression in Homi- nids 3,6, and 7.

In order to roughly quantify the contributions of different vault components to cranial thickening within the Kanjera sample, measurements of total thickness and inner and outer table thicknesses were taken along breaks using a binocular microscope and needle-tipped calipers (Table 3). The average ratio of diploic to tabular bone in healthy modern human adults is 1.4 and does not exceed 2.3 (Reynolds, 1962). The contribution of tabular bone to total vault thickness seems to have been quite substan- tial in Asian H. erectus as well (Weidenreich, 1943; Webb, 1990). In contrast, the ratio of diploic to tabular bone in 14 individuals suffering from sickle cell anemia ranged from 2.3 to 7.3, with a mean of 4.4 (Reynolds, 1962). As in individuals suffering from anemia, thickening of the Hominid 3 and 617

vault fragments is largely due to diploic expansion (Fig. 4; Table 3). Moreover, thickening of parietal KNM-KJ 12827 was accompanied by outer table thinning (Table 3) and pitting (Fig. 4).

Angel (1966) introduced the term porotic hyperostosis to describe thickening of the diploe and pitting of the skull vault andor orbital roof. This condition has been convincingly linked to anemia, caused by defec- tive hemoglobin in red blood cells (Stuart- Macadam, 1987a,b; Ortner, 1993). In the hereditary anemias (e.g., sickle-cell anemia), an amino acid substitution results in the defect. In acquired anemia (e.g., iron- deficiency anemia), inadequate availability of dietary iron results in abnormal hemoglobin. In both types, red blood cells are defec- tive and have shortened life spans, leading to greater turnover and increased demand for hematopoietic tissue. During infancy and early childhood all marrow is already fully occupied with red hematopoietic tissue; in anemic individuals long bone red marrow volume is increased at the expense of cortical bone, while diploe may increase at the expense of the tabular bone in the skull (Or- tner, 1993). In adults, red marrow space can increase at the expense of fatty marrow, and anemia may not lead to bone remodelling.

Modern Lake Victoria is host to a variety of parasites, including malarial plasmodia

20 T. PLUMMER AND R. POlTS

and schistosomes, providing the potential for both forms of anemia among the circum- lake populations. Maintenance of a balanced polymorphism for sickle-cell anemia is an adaptation against malaria, with heterozy- gotes of the sickle-cell hemoglobin gene having increased resistance (Ortner and Put- schar, 1981). A high parasite load and/or an iron-poor diet can lead to iron deficiency anemia. Cranial thickening in the Kanjera sample may indicate that individuals in past populations were also exposed to heavy parasite loads, and thus subject to hereditary and/or acquired anemia. Similar argu- ments linking cranial thickening to anemia in lakeside skeletal samples in East Africa have been made by Angel et al. (1980) and Robertshaw (1987, and pers. comm.). The demand for hematopoietic tissue in the Hominid 3, 6, and 7 crania may have de- clined with adulthood, with remodelling bringing about an outer table of normal thickness, as noted in some fragments (Ta- ble 3).

Age of the Kanjera horninids The provenancing of the hominid sample

has always been problematic, because six of the seven stratigraphic units a t the locality are bone-bearing, and most of the hominids have been surface-collected. The National Museums of Kenya’s hominid sample, as well as several fragments from Hominids 1 and 3 in the Natural History Museum, Lon- don, were included in an energy dispersive x-ray fluorescence analysis provenancing study. The low elemental concentrations of the entire hominid sample indicates they postdate Kanjera Formation deposition (Plummer et al., 1994). However, the absolute age of the hominids cannot currently be determined. The recovery of in situ human fossils from BCS outcrops near the areas of surface collection of Hominids 1, 2 and 4, and Hominids 6 and 7 suggests a BCS origin for these individuals. Black cotton soil formation can be a relatively fast process, tak- ing between 1,000 and 10,000 years (G. Re- tallack, pers. comm.) Carbon-14 dates on organic material from Kanjera BCS samples have yielded estimates of 1690 f 70 BP and 1080 f 60 BP for the lower and upper halves, respectively, of one outcrop. How-

ever, the BCS at Kanjera includes allochtho- nous clasts (Behrensmeyer et al., in press), and during its formation could have incorpo- rated bones from older stratigraphic units, so the above dates represent minimum ages for the hominids.

A portion of hominid tibia KNM-KJ 7494A from the Hominid 6 and 7 collection was submitted to Beta Analytic, Inc., for 14C dating. Not enough collagen was preserved for a date on organic material. An accelerator mass spectrometry date of 9930 f 265 BP was obtained using bone apatite at the Eid- genossische Technische Hochschule Univer- sity in Zurich, Switzerland. Bone apatite dates are often unreliable, and the above date cannot currently be confirmed.

The above account does not resolve how Leakey (1935) recovered two fragments of the relatively young Hominid 3 in an early Pleistocene outcrop. A possible explanation is suggested by Leakey’s original work on the peninsula.

Origin of the hominid sample Leakey’s hominids were found within 60

ms of each other, while the Hominid 6 and 7 remains were found several hundred meters away. The features these collections share include 1) long cranial vaults, as exemplified by Hominids 1 and 3 and parietal fragments from the Hominid 6, 7 collection; 2) thick cranial vaults, as exemplified by Hom- inids 3 ,6 , and 7; 3) shallow burial. Only the Hominid 3 fragments were found at a depth of several feet; all other specimens recovered in excavation were less than 10 cm from the surface; 4) presence of both cranial and postcranial elements, suggesting the erosion of associated clusters of bones; and 5) presence of multiple individuals at sites.

Some fossils in Leakey’s and the more recent collections are suggestive of elevated levels of robusticity. The large fossa in tibia diaphysis KNM-KJ 7493, massive mandibular fragments KNM-KJ 7687 and 22928 and robust radial tuberosity KH 58 indicate that at least one of the Hominid 6 and 7 individuals was of massive build. While the postcranial remains collected by Leakey are scrappy, the swollen nuchal region of Homi- nid 1 is also suggestive of at least moderate robusticity.


During his expeditions to the Homa Pen- insula, Leakey became intrigued by the shell middens found around the Lake Victo- ria shoreline, and the prospect of associated burials. He dug a trench through one such mound at Kanjera (Leakey, 1935,1936; Rob- ertshaw et al., 19831, in which he discovered pottery and faunal remains. The search for associated human remains ended with D.G. MacInnes’ discovery of a skeleton in ancient lake deposits at Kanam East. This individual, like Olduvai Hominid I (Leakey, 1932) and Kanjera Hominid 3, was first thought to be eroding from an in situ context. It was soon recognized as a burial, and subsequent excavations recovered six skeletons (designated Homa shell-mound 1-6) from ex- tremely shallow graves (Leakey, 1935). This general context, clusters of burials near the lake margin in shallow graves, is similar to the Kanjera hominid finds. Moreover, like the Kanjera sample, the skulls recovered from the Kanam burials are long, the vaults of several specimens thick (an average of 9 mm is given for the vault of Homa shell- mound l), and the mandibles and postcrania are massive. Other burials associated with shell middens were excavated at Kanam by the 1974 Yale Expedition, and there also are of robustly built individuals (Plummer, pers. obs.). In summary, the morphology and the context of the Kanjera hominids is consistent with human skeletal remains from nearby Holocene sites. Contra Boswell (19351, sediment slumping played no role in the incorporation of the hominid fossils into the early Pleistocene beds. Shell middens excavated by Robertshaw et al. (1983) at Kanjera and Kanam recovered fauna (including a few human bones), artifacts (including rare obsidian pieces), and pottery attributed to the Oltome (or Kansyore) tra- dition. Though the middens have proven difficult to date, Robertshaw (1987) suggests that they were accumulated 4,000-8,OOO years ago. Like the burials Leakey investi- gated at Kanam, the Kanjera hominids may also represent Oltome burials.

CONCLUSIONS The anatomically modern Kanjera homi-

nids have been in a “suspense account” since Boswell’s (1935) scathing attack on Leakey.

Leakey’s (1935) stratigraphic section of the Hominid 3 excavation, Kent’s unpublished map and the excavations of the Smithsonian Expedition strongly suggest that Kanjera Hominid 3 was derived from a KN-2 outcrop on the flats north of the KN-2a Island. No sediment slumping was revealed anywhere at the locality, repudiating Boswell’s (1935) explanation for their association with an early Pleistocene fauna. The discovery of hominid incisor KNM-KJ 22927 in the BCS near where Leakey’s Hominids 1, 2, and 4 were collected might link them to this unit. Alternatively, they could have been derived from a Kanjera Bed outcrop that was pe- dogenically altered to BCS. Further anatomically modern human cranial and postcranial fossils were surface collected in 1974 and 1975, and 1981 and 1987. Designated Hominids 6 and 7, these were recovered southeast of the KN-2a Island and were derived from the BCS.

The great thickness of the Hominid 3 vault was thought by Leakey (1935) t o reflect its antiquity. Very thick cranial fragments are also present in the Hominids 6 and 7 sample. However, not all of the Kan- jera hominids have thickened vaults, and when present, thickening is the result of diploic expansion. Cranial thickening within the sample may have resulted from genetic or acquired anemia.

While there is no question that they postdate the Kanjera Formation, the absolute age of the Kanjera hominids is still in doubt. The explanation, which reconciles Leakey’s discovery of Hominid 3 in an early Pleis- tocene bed with its relatively recent age, is that the Kanjera hominids were intrusive burials into the outcrops of the locality. The Kanjera finds may be related to Holocene burials associated with shell middens and Oltome ceramics known elsewhere on the Homa Peninsula.

ACKNOWLEDGMENTS We thank the Office of the President, Re-

public of Kenya, and R.E. Leakey, M. Isa- hakia and M.G. Leakey of the National Mu- seums of Kenya for permission and support in conducting the field and laboratory studies. We also thank Peter Andrews, Theya Molleson, and the staff of the Natural His-

22 T. PLUMMER AND R. POTI’S

tory Museum, London, for permission and assistance in studying Kanjera Hominids 1 and 3. We are grateful to David Pilbeam for sharing unpublished results of his 1974 expedition. Geological research at Kanjera by A.K. Behrensmeyer is gratefully acknowl- edged. J. Clark assisted with illustrations and manuscript preparation. Kenya field work was supported by a Scholarly Studies Program Grant to R.P. Funding by the Wen- ner-Gren Foundation, Leakey Trust, L.S.B. Leakey Foundation, Boise Fund, Donner Foundation, J.F. Enders Fellowship Fund of Yale University, and the Smithsonian Fel- lowship Program is also gratefully acknowl- edged. This is a publication of the National Museum of Natural History’s Human Ori- gins Program.

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