Location

The PTK site was discovered in 2012 by The Olduvai Paleoanthropology and Paleoecology Project (TOPPP) at the junction of the main and secondary branches of Olduvai Gorge (Tanzania; Fig. 1 and Supplementary Fig. 1). The site is situated ∼500 m south of the well-known FLK 22 Zinjanthropus (FLK 22 Zinj) site and, to date, it is known to comprise three distinct archaeological levels (Supplementary Fig. 2). Two of these levels, corresponding to what has been defined as ‘upper Zinj’ and ‘lower Zinj’21, occur in the same clay stratum as the FLK 22 Zinj level, underlying volcanic Tuff IC, dated by 40Ar/39Ar to 1.832+0.003 Ma (ref. 22). PTK’s third archaeological level underlies the Zinj clay, within the tuffaceous layer known as the ‘Chapati Tuff’23, and corresponds stratigraphically to the top of the Olduvai Bed I archaeological level designated as FLK NN 2. TOPPP’s 2014 excavation of this third level at PTK yielded abundant Mode I stone artefacts and a large faunal assemblage, which includes the MHL hominin proximal phalanx OH 86, described here.

Figure 1: Geographic location of the ‘PTK’ site. The location of the new PTK site (from which the OH 86 proximal phalanx was excavated) compared with two other major and penecontemporaneous Middle Bed I (Olduvai Formation) sites of FLK 22 Zinjanthropus and FLK NN that also occur near the junction of the Main and Side Gorges. (a) Informal views of excavations at each site. (b) Relationship of the sites in aerial view; lower middle image=a panoramic view of the Gorge looking north, with PTK indicated. (c) Political map of Africa with Tanzania highlighted in black and the approximate location of Olduvai Gorge represented by white dot and a schematic plan view of sites near the junction of the Main and Side Gorges. Full size image

Specimen identification and anatomical description

OH 86 is a complete manual proximal phalanx that is nearly entirely encased in a very thin layer of the carbonated tuffaceous silt from which it derives (that is, the ‘Chapati Tuff’; Fig. 2). Although the areal spread of this concretion on OH 86 is encompassing, its submillimetre thinness guarantees little impact on the gross measurements that we derived on the specimen and analyse and discuss in this study. Basic osteometrics of OH 86 are listed in Supplementary Table 1. On the basis of several lines of morphometric evidence (see below), as well as qualitative features, we assign OH 86 to the fifth ray of the left hand.

Figure 2: OH 86 views. The OH 86 hominin manual proximal phalanx in (from left to right) dorsal, lateral, palmar (distal is top for each) and proximal views. Scale bar, 1 cm. Full size image

Applying published qualitative criteria36—including asymmetry of the specimen’s flexor ridges and distal trochlea, as well as the orientation of the latter, and its base’s palmar outline—OH 86 compares most favourably to a modern human manual proximal phalanx from ray V. Quantitative data—including head mediolateral width/base mediolateral width ratio=0.72, base mediolateral width/overall superoinferior length ratio=0.39—corroborate this qualitative diagnosis. Last, we tested this corresponding qualitative/quantitative ray assignment by conducting a discriminant function analysis of OH 86 and a comparative sample composed of modern human proximal phalanges from rays II and V (which tend to be more similar to each other because of asymmetries caused by muscle insertions, among others). This analysis confirms the results of the initial qualitative and quantitative tests, also indicating that OH 86 most probably derives from a fifth ray (with a probability six times more likely than ray II, using the seven shape variables, and with a probability 10 times more likely than ray II, using the seven raw dimensions; Supplementary Fig. 3).

Assuming that the assignment of OH 86 as a fifth proximal phalanx is correct, it must also derive from a left hand on the basis of the pattern of asymmetry of the distal condyles: in palmar and dorsal views, the presumed radial condyle projects more inferiorly than does the presumed ulnar condyle. Further, the putative ulnar basal tubercle (insertion for the hypothenar muscles) is larger and protrudes more ulnarly and proximally than does the radial basal tubercle.

Mosimann shape ratios. The overall size (as approximated by the geometric mean, GM) of OH 86 is within the range of modern humans and chimpanzees (Supplementary Fig. 4a), as it is the case of other hominins except Au. sediba (below the human range). In terms of relative length, OH 86 is in the midrange of humans and the upper range of gorillas, but below chimpanzees and monkeys (Supplementary Fig. 4b). OH 86 exhibits a MHL, dorsopalmarly short trochlea, although this value also overlaps with the lowermost range of African apes (Supplementary Fig. 4c). No trend is evident in mediolateral trochlear width, although it is worth noting that the trochlear proportions of OH 86 are virtually identical to those of the fossil Qafzeh 9 (H. sapiens; Supplementary Fig. 4d). With regard to midshaft dimensions, OH 86 is dorsopalmarly short (but still overlaps with the modern human outlier range; Supplementary Fig. 4e), and as a consequence it is also mediolaterally wider than are the midshafts of the proximal phalanges of other hominins (Supplementary Fig. 4f). Extant taxa exhibit similar values of relative basal dorsopalmar height as does OH 86 (Supplementary Fig. 4g), although Pan clearly stands out in its having relatively higher bases (but still overlapping with the remaining sample). In this respect, although all hominins fall within the modern human variation, a clear trend is evident: fossil Homo and OH 86 show very similar values, in the low interquartile range of H. sapiens, whereas all australopiths are in the upper interquartile range. Last, with regard to relative basal breadth (Supplementary Fig. 4h), modern humans (and cercopithecid monkeys) possess wider bases than do African apes. Pliocene australopiths fall in the African ape range, whereas the early Pleistocene Au. sediba, fossil Homo and OH 86 all exhibit values within the human range.

When all these dimensions of proximal phalanx form variation (that is, the seven Mosimann shape ratios and the associated GM) are summarized by means of a principal components (PCA; Fig. 3a) and a cluster (Fig. 3b) analyses, the closest form affinities of OH 86 are revealed to be with Homo. In fact, based solely on the two major axes of form variation, our PCA (Supplementary Fig. 4 and Supplementary Table 5) clearly separates H. sapiens from Pan, Gorilla and cercopithecoid monkeys (Fig. 3a). Further, although all fossils exhibit their closest form affinities to H. sapiens, OH 86 is the oldest hominin phalanx within the modern human form space (as represented by the two first axes, accounting for 90.1% of total form variation). In addition, when all dimensions of phalangeal form are summarized using an unweighted pair group method with arithmetic mean (UPGMA) dendrogram based on group centroids (Supplementary Fig. 4), it reveals a ‘Homo’ cluster nested within the ‘hominin’ group—with OH 86 being the oldest fossil in the sample placed within this ‘Homo’ cluster (Fig. 3b). In sum, modern human phalangeal form (Fig. 4 and Supplementary Table 5) is characterized by moderate relative total proximodistal length, midshaft mediolateral robusticity and overall size (that is, intermediate values of PC1, similar to Pan) in combination with a mediolaterally wide and dorsopalmarly short trochlea and base (which, together with their shorter lengths, differentiate human and Pan proximal phalanges). More specifically, the bases of Homo proximal phalanges are mediolaterally wider and dorsopalmarly shorter than are those of australopiths (Supplementary Fig. 4g,h). In addition, on the basis of trochlear shape, the intermediate phalanges of australopiths—as well as those of OH 7—are clearly distinct from those of extant and fossil Homo7. This previous finding regarding manual intermediate phalanges corresponds to our independent analyses of complete manual proximal phalanges, indicating together that modern human phalangeal morphology can be accurately characterized quantitatively. Importantly, OH 7 does not conform to the modern human characterization, even though it is penecontemporaneous with the MHL OH 86.

Figure 3: The form of the human proximal phalanx. (a) Plot showing the two major axes of proximal phalanx V form variation (that is, shape and size space). Major taxonomic groups can be distinguished (using convex hulls); OH 86 is the earliest fossil specimen within the human variation. (b) UPGMA cluster analysis summarizing eight dimensions of phalangeal form space: OH 86 is the oldest specimen within the Homo cluster. The cophenetic correlation coefficient is high (0.8681), indicating that the dendrogram is faithfully preserving the pairwise distances between the original dimensions. (These analyses exclude OH 7 because this hand skeleton does not preserve complete proximal phalanges32.) Full size image

Figure 4: Phalangeal curvature in extant and fossil hominoids. (a) Included angle values (in degrees) in a modern and fossil sample of fifth proximal phalanges. OH 86 is (exclusively) within the modern human variation (distinct from australopiths). Boxes represent 25th and 75th percentiles, centreline is the median, whiskers represent non-outlier range and the dot is an outlier. Samples for each boxplot are Homo sapiens (n=36), Pan paniscus (n=8), Pan troglodytes (n=16), Gorilla (n=22), Pongo (n=16) and Hylobatidae (n=22). (b) The fossil hominin specimens analysed in this study are compared in lateral view. All pictures were taken from the originals with the exception of AL333-62 (cast) and ATE9-2 (modified from the literature56; Supplementary Table 4). Scale bar, 1 cm. Full size image

Phalangeal curvature. From functional and evolutionary perspectives, it is highly relevant that all australopith fifth proximal phalanges exhibit higher values of phalangeal curvature than do any of the extant and fossil Homo specimens (Fig. 4), denoting a biological transition in hominins towards less (if any) commitment to arboreal locomotion (at least as it is revealed from manual proximal phalanx anatomy). As with overall phalangeal form (Fig. 3), OH 86 falls exclusively within the modern human range of variation of fifth proximal phalanx curvature (Fig. 4); pooling results for curvature of all non-pollical proximal phalanges place OH 86 once again within the modern human range and in the lowermost range of Gorilla (Supplementary Fig. 5). In addition, compared with the manual phalanges of O. tugenesis, Ar. ramidus, Au. afarensis, Au. sediba, the Swartkrans hominins and OH 7—whose powerfully built flexor insertions result in proximal phalanx diaphyseal morphology that includes distinctive, palmarly concave ‘outbowing’—the diaphysis of OH 86 lacks such pronounced flexor insertions and is thus much straighter in medial and lateral views (Fig. 2).