The hand of Homo naledi

A nearly complete right hand of an adult hominin was recovered from the Rising Star cave system, South Africa. Based on associated hominin material, the bones of this hand are attributed to Homo naledi. This hand reveals a long, robust thumb and derived wrist morphology that is shared with Neandertals and modern humans, and considered adaptive for intensified manual manipulation. However, the finger bones are longer and more curved than in most australopiths, indicating frequent use of the hand during life for strong grasping during locomotor climbing and suspension. These markedly curved digits in combination with an otherwise human-like wrist and palm indicate a significant degree of climbing, despite the derived nature of many aspects of the hand and other regions of the postcranial skeleton in H. naledi.

H. naledi finger length of ray two is similar to modern humans while the fingers of ray 3 and ray 4 are long compared with modern humans and most other hominins (apart from Ar. ramidus). Hand 1 also has a relatively long PP3 and digit 3 relative to interarticular length of the Mc3 compared to other hominin, but falls within the range of variation of modern humans. However, there is substantial variation in relative PP and digit length across different rays and across different taxa. Early H. sapiens Qafzeh 9 is unusual compared to other early and modern H. sapiens in having long PPs and digit length for all rays. H. naledi proximal phalanges are most similar in overall robusticity to other australopiths, apart from Au. afarensis, and modern and early H. sapiens. Ar. ramidus and Au. afarensis are generally less robust because their PP length is longer than later hominins. Au. africanus, Swartkrans specimens and particularly Neandertals tend to be slightly more robust than H. naledi and H. sapiens.  Table 4. Proximal and intermediate manual phalanges used in the phalangeal curvature analyses. The 1 st polynomial coefficient (A) from the polynomial function (y=Ax2+BX+C) for each specimen and sample means are provide. The first coefficient (A) expresses the nature and degree of the longitudinal curvature whereas the second (B) and third (C) reflect aspects of the orientation of that curve with respect to the rest of the element (i.e. element rotation, element position in 2-dimensional space). Only the first coefficient (A) was used in phalangeal curvature analyses. See Supplementary Text for further details on the high-resolution polynomial curve fitting methodology.

UW101-1726 right scaphoid
Preservation This specimen is complete and well-preserved, apart from erosion of the cortex at the tip of the tubercle. Morphology This scaphoid measures an estimated 19.1 mm in its maximum dimension (roughly ML), 12.7 mm in DP height and 8.4 mm in PD length. The radial facet is continuously convex in both the DP and ML dimensions and measures 11.5 in DP height and 13.6 ML breadth. The capitate facet is shallow and oval-shaped, measuring 10.3 mm in DP height 12.1 mm ML breadth. It is "closed" along the distal border due to a large trapezoid facet. The lunate facet is palmarly-positioned and oriented medially, but is not well-defined, measuring an estimated 6.1 mm in DP height and 6.8 mm PD length. The tubercle is short and palmarly-oriented, and its base robust. In dorsodistal view, the trapezium-trapezoid facet is 8.8 mm ML breadth and 15.6 mm in PD length. It is mildly convex ML and extends onto the scaphoid tubercle.

UW101-1732 right lunate
Preservation This specimen is complete and well-preserved, apart from cortex missing from the distal tip of the palmar beak of the capitate facet and non-articular palmar surface of lunate body. Morphology This specimen is the antimere to UW 101-418B left lunate. UW101-1732 measures 13.6 mm in PD length, 14.0 mm in DP height and 13.4 mm in ML breadth. The capitate facet measures 11.9 mm in DP height and is ML narrow (7.8 mm). There is no separate articulation for the hamate. The scaphoid facet is poorly defined and measures an estimated 7.6 mm in DP height and 2.7 mm in PD length. The scaphoid facet can be easily seen in distal view, oriented equally laterally and distally. The triquetrum facet measures 8.3 mm in DP height and 7.0 mm in PD length and is mildly convex in both dimensions. The radial facet is broad ML (13.5 mm) and DP (12.4 mm) and dominates the proximal surface.

UW 101-418B left lunate
Preservation This specimen is complete and well-preserved apart from a fragment of cortex missing from dorsal half of the lunate facet and the non-articular palmar surface of lunate body. There is a overall porosity of the cortex on the radial facet. Morphology The specimen is the antimere to UW 101-1732 right lunate and its overall morphology is identical to that described for UW 101-1732, apart from a slightly more DP convex triquetrum facet. This bone measures 14.4 mm in PD length, 13.7 mm in DP height and 11.6 mm in ML breadth. The capitate facet measures 12.0 mm in DP height and 7.6 mm in ML breadth. The scaphoid and triquetrum facets measure an estimated 9.0 mm and 7.9 mm in DP height, respectively, and 4.4 mm and 7.4 mm in PD length, respectively. The radial facet is 12.4 in DP height and 11.4 ML breadth.

UW 101-1727 right triquetrum
Preservation This specimen is complete and well-preserved, apart from cortex missing from the dorsolateral edge of the hamate facet. Morphology This triquetrum measures 13.6 mm in ML breadth, 9.9 mm in DP height and 7.2 mm in PD length. The lunate facet measures 7.4 mm in DP height and 6.2 mm in PD length and is mildly concave in both directions, matching the convexity of the lunate's triquetrum facet. . The hamate facet is generally triangular-shaped, measuring 12.4 mm in ML breadth and 7.7 mm in DP height. The lateral half of the hamate facet is flat or mildly concave, while the medial half is strongly convex and oriented equally distally and dorsally. This morphology matches the more dorsally-place triquetrum facet on the hamate. The hamate facet wraps around onto the distal side of the bone at its most medial extent. This portion articulates with a slightly proximally-facing portion of the distal end of the triquetrum facet on the hamate. In palmar view, the lateral half of the triquetrum is nonarticular, convex both ML and PD, and a deep sulcus is not present. The medial half is dominated by the pisiform facet, which is small, oval-shaped and well-defined. It measures 7.6 mm in ML breadth and 5.9 mm in PD length. The pisiform facet is oriented primarily palmarly, but also slightly medially, and is strongly concave ML and generally flat PD. The medial non-articular side is deeply excavated.

UW 101-1731 right trapezium
Preservation This specimen is complete, but much of the cortical surface is eroded, exposing trabecular bone. The palmar, distal and dorsal surfaces have exposed trabeculae, which obscures the extent of the Mc2 articulation. However, the remaining morphology is not  The triquetrum facet measures 6.8 mm in DP height and 12.9 mm in PD length. It is dorsally placed on the hamate, such that there is non-articular surface at the proximal end, palmar to the facet rather than the articular surface extending the entire DP height of proximal hamate. The triquetrum facet is proximally and dorsally oriented at the proximal half, and proximally-palmarly oriented at the distal half, creating the typical concavo-convex triquetrum facet morphology.

UW 101-1321 right first metacarpal
Preservation This specimen is complete and generally well-preserved. There is cortex missing from the lateral and medial edges of the palmar articular surface of the head and from palmar and lateral surface of proximal shaft, just distal to the trapezium facet. Morphology This bone measures 37.4 mm and 35.8 mm in total and interarticular PD length, respectively. The Mc1 base and proximal shaft are remarkably small compared to a much more robust midshaft and distal half of the bone. The base and proximal shaft measure 9.6 mm and 6.6. mm in ML breadth and 10.2 mm and 8.6 mm in DP height, respectively. The trapezium facet is also small, measuring 8.3 mm in ML breadth and 6.8 mm in DP height. In comparison, the midshaft and proximal shaft measure 9.5 mm and 11.4 mm in ML breadth, and 8.1 mm and 9.7 mm in DP height. The head measures 11.1 in maximum ML breadth and 12.6 in DP height. In palmar view, the shaft appears "pinched", created by a crest running along the sagittal midline with strongly sloping medial and lateral walls that end in flaring crests for the intrinsic thenar muscles. On the lateral side of the shaft, a well-developed flange for the opponens pollicis tendon that begins roughly 12 mm from the proximal end of the Mc1 and measures 12.3 mm in PD length. The flange for the dorsal interosseous tendon is also distally positioned and measures 11.0 mm in PD length. The distal articular facet is strongly asymmetric with a more palmarly projecting, but less proximally extended, lateral portion compared with the medial portion. UW101-1321 does not have prominent "beak" at the sagittal midline of the head's palmar surface. 3 mm in ML breadth and 6.7 mm in DP height at midshaft, and has a prominent crest running that sagittal midline for the attachment of the palmar interossesous muscles. The lateral side of the distal half of the shaft also flares slightly. There is no noticeable torsion to the shaft. The distal head measures 10.6 mm in maximum ML breadth and 11.3 mm in DP height. In palmar view, the distal articular surface is strongly asymmetrical, with the lateral portion of the articular surface extending much further proximally than the medial portion.

UW 101-1319 right third metacarpal
Preservation This specimen is complete and well-preserved, apart from the erosion of cortex from the dorsal surface of distal articulation and the proximal edge of medial and dorsal surface of the base. Morphology This specimen measures 49.0 mm and 48.6 mm in total and interarticular length, respectively. The shaft is robust, measuring 6.7 mm in ML breadth and 6.5 mm in DP height, and there is slight crest running along the sagittal midline of the palmar surface, ending at the medial side of the shaft-base junction, suggesting well-developed palmar interosseous muscles. The proximal base measures an estimated 12.2 mm in ML breadth and 11.6 mm in DP height. The dorsolateral corner of the base extends more proximally than the remainder of the base. This is the region of the styloid process in modern humans, though the morphology of UW101-1319 is best described as a slight extension rather than a styloid process. In proximal view, the base is triangular in shape and is dominated by the capitate facet. The capitate facet measures 9.5 mm and 4.1 mm in dorsal and palmar ML breadth, respectively and is largely mildly convex DP, except for a concave dorsolateral corner. There is a single articulation for the Mc2, which can be clearly seen in in palmar view due to its palmar and lateral orientation. The Mc2 facet measures 4.8 mm in PD length and 11.0 mm in DP height. In medial view, there is a single, small, dorsally-placed articulation for the Mc4, measuring an estimated 4.2 mm in PD length and 4.1 mm in DP height. This facet is flat and primarily medially-oriented but also slightly palmarly-oriented. The Mc4 facet is distally-placed on the proximal base, such that in articulation, the Mc4 carpometacarpal articulation is offset distally from the Mc3. This articular configuration is consistent with the morphology of the capitate and hamate as well. The head is strongly asymmetrical towards the lateral side and measures 9.9 in maximum ML breadth and 11.3 in DP height.

UW 101-033 left third metacarpal
Preservation This specimen preserves a partial proximal epiphysis and ¾ of the shaft. A large fragment is missing from the mediodorsal corner of the proximal epiphysis such that the full extent of the capitate is not preserved. Cortex is eroded from the non-articular palmar surface of the base, exposing trabeculae. A piece of the cortex is missing from the dorsomedial nonarticular surface, but trabeculae are not exposed. Morphology This left Mc3 is the likely antimere to 101-1319 right Mc3. The preserved morphology is identical to that described for UW101-1319. This bone measures 36.5 mm in total preserved PD length. DP height of the base is estimated at 12.0 mm. The midshaft measures 6.6 mm in both ML breadth and DP height.

UW 101-1318 right fourth metacarpal
Preservation This specimen is complete apart from a large portion of the lateral and dorsal portions of the distal epiphysis. The cortex is eroded from lateral border of the hamate facet (though re-articulation with hamate shows that full extent of hamate facet is retained). Morphology This specimen is short and gracile relative to the other metacarpals. It measures 42.7 mm and 42.4 mm in total and interarticular length, respectively. The midshaft measures 5.2 mm in ML breadth and 6.5 mm in DP height. Unlike the other metacarpals of Hand 1, the shaft walls do not flare laterally and a slight crest can be palpated along only the proximal half of the palmar shaft surface. The proximal base is small and square-shaped, measuring 9.2 mm in ML breadth and 9.4 mm in DP height. The hamate facet dominates the proximal surface and is generally flat, apart from a mildly convex dorsomedial corner. The hamate facet measures 7.4 mm in ML breadth and 5.3 mm in DP height and its palmar portion is slightly dorsally oriented such that it can be seen in dorsal view. Laterally, there is a single, dorsally-positioned Mc3 facet measuring 3.8 in PD length and 3.7 in DP height. Medially, the height. The proximal articulation is oval-shaped, measuring 10.8 mm in ML breadth and 8.4 mm in DP height, and strongly concave both ML and DP. The proximal base, measuring 11.7 mm in ML breadth and 9.3 mm in DP height, also is not particularly robust and the basal tubercles are present, but not distinct. The base is asymmetric, with the lateral basal tubercle being more prominent and the lateral portion extending more proximally than the medial portion. Distally, the medial trochlea is slightly more distally extended than the lateral trochlea. Together, the asymmetry of the base and trochlea are typical for right second phalanx. The trochlea measures 9.1 mm in ML breadth and 5.7 mm in DP height. There is a slight concavity just proximal to the trochlea on the palmar surface of the shaft.

UW 101-1327 right third proximal phalanx
Preservation This specimen is complete and well-preserved apart from a small fragment missing from the medial border of the trochlea.
Morphology The third proximal phalanx shows the same general morphology as that described in UW 101-1328 from the second ray. Subtle differences include slightly more well-developed flexor sheath ridges, such that there is a slight indentation along the proximal half of the medial border of the palmar surface. However, the ridges still do not extend palmarly above the remainder of the shaft surface. The palmar tubercles and distal trochlea are more symmetrical than in the other H. naledi proximal phalanges. This specimen 35.9 mm in total PD length. The base and proximal facet measure 12.0 mm and 8.9 mm in ML breadth and 10.5 mm and 8.0 mm in DP height, respectively. The midshaft and distal trochlea measure 8.9 mm and 9.2 mm in ML breadth and 6.0 mm and 6.3 mm in DP height, respectively.

UW 101-1326 right fourth proximal phalanx
Preservation This specimen is complete apart from a fragment missing from the dorsomedial boundary between the shaft and proximal articulation. The cortex is eroded from the palmar surface of the medial trochlea. Morphology This specimen shows same general morphology as the other proximal phalanges of Hand 1. The proximal base is slightly asymmetrical with the lateral part of the proximal facet extending further proximally then the medial side. There is a very slight convexity to the lateral side of the shaft for the flexor sheath ridge and only the very distal portion extends slightly beyond the remainder of the palmar shaft. The lateral trochlea extends slightly further distally than the medial side, consistent with typical asymmetry for right PP4. This PP4 measures 33.1 mm in PD length. The base measures 11.5 mm in ML breadth and an estimated 9.1 mm in DP height. The proximal facet measures 10.4 mm in ML breadth. The midshaft and distal trochlea measure 8.9 mm and 9.0 mm in ML breadth and 5.3 mm and 5.8 mm in DP height, respectively.

UW 101-1725 right fifth proximal phalanx
Preservation This specimen preserves the complete proximal ¾ of the shaft but is missing the trochlea. The cortex is eroded from the proximomedial border of the dorsal surface. Morphology This specimen is similar in overall morphology to that described for the other proximal phalanges associated with Hand 1. The proximal end is asymmetrical, with the medial side extending further proximally than the lateral side. In dorsal view, the proximal facet is asymmetrically placed such that is shifted medially. This specimen 25.5 mm in preserved PD length and is estimated to be approximately 26.5 mm if the trochlea were fully preserved. The base and proximal facet measure 9.3 mm and 7.6 mm in ML breadth and 8.4 mm and 8.3 mm in DP height, respectively. The midshaft is 8.0 mm in ML breadth and 4.7 mm in DP height.

UW 101-1311 right second intermediate phalanx Preservation
This specimen is complete apart from a small hole on the proximomedial corner of the palmar surface of the shaft, a large fragment of cortex missing from the distal half of the dorsal shaft and a smaller fragment missing from the laterodorsal surface at the proximal edge. Morphology This specimen is 17.8 mm in total PD length. The proximal base is ML broadest portion of the bone, and the walls of the shaft taper distally toward the trochlea. The shaft is ML narrowest just before the trochlea and the trochlea is nearly as ML expanded as the proximal base. The dorsal shaft is continuously DP convex, and is particularly curved at distal end (although not as pronounced as in the proximal phalanges). The proximal border of the dorsal surface is slightly flaring. The palmar shaft morphology has two convex semilunar areas in the proximal half of the shaft. The region in between the semilunar areas and the entirety of the distal half of the palmar shaft is ML flat, apart from a slight concavity at the sagittal midline just proximal to the trochlea. A well-developed palmar median bar is not present. The trochlea is more ML expanded than all but the most proximal region of the shaft. The base and proximal facet measure 9.2 mm and 8.5 mm in ML breadth and 6.8 mm and 5.1 mm in DP height, respectively. The midshaft and distal trochlea measure 5.7 mm and 7.4 mm in ML breadth and 4.5 mm and 4.3 mm in DP height, respectively.

UW 101-1310 right third intermediate phalanx
Preservation This specimen is complete apart from a small fragment missing from the centre of the proximal border of the palmar surface. The cortex is slightly eroded at the proximal border of the dorsal surface. Morphology This specimen is 22.6 mm in total PD length. The morphology is identical to that described for UW 101-1311 second intermediate phalanx, except that the proximal border of the dorsal surface is slightly more flaring. The bicondylar proximal articulation is fully preserved and each is oval-shaped, being ML broader (9.4 mm together) than they are DP tall (5.9 mm). The base is 10.5 mm in ML breadth and 7.1 mm in DP height. The midshaft and distal trochlea measure 7.6 mm and 8.1 mm in ML breadth and 4.9 mm and 5.0 mm in DP height, respectively.

UW 101-1308 right fourth intermediate phalanx
Preservation This specimen is complete apart from a small fragment missing from the centre of the proximal border of the palmar surface. Morphology This specimen is 22.1 mm in total PD length. Its morphology is identical to that described for UW101-1310, apart from a mildly ML concave, rather than flat, surface between the convex semi-lunar areas of the palmar surface and there is a slight palmar extension to the lateral border of the shaft, likely for the attachment of the flexor digitorum superficialis tendon. There is also less dorsal flaring at the proximal border than in UW101-1310. The base and proximal facet measure 10.6 mm and 9.1 mm in ML breadth and 7.1 mm and 6.3 mm in DP height, respectively. The midshaft and distal trochlea measure 7.6 mm and 7.9 mm in ML breadth and 4.4 mm and 4.4 mm in DP height, respectively.

UW 101-1325 right fifth intermediate phalanx
Preservation This specimen is complete and well-preserved. Morphology This specimen is 15.6 mm in total PD length. Its morphology is identical to that described for the other intermediate phalanges of Hand 1, with a few subtle differences. The lateral facet of the proximal articulation is DP taller than the medial facet. The semilunar convexities of the palmar shaft are ML narrower such that they appear more as ridges, separated by a more pronounced concavity than is seen in the other intermediate phalanges.
The distal half of the palmar shaft remains ML flat and there is no median bar as is typical of the other specimens. The base and proximal facet measure 8.9 mm and 8.0 mm in ML breadth and 6.4 mm and 5.5 mm in DP height, respectively. The midshaft and distal trochlea measure 6.0 mm and 7.0 mm in ML breadth and 3.6 mm and 3.7 mm in DP height, respectively.

UW 101-1351 right pollical distal phalanx
Preservation This specimen is complete and well-preserved apart from small fragments missing from the lateral and dorsal borders of the proximal base, and the erosion of the cortex along the proximal border of the base and the lateral edge of the apical tuft. The palmar surface of the apical tuft also appears slightly eroded. Morphology This pollical distal phalanx is remarkably ML broad relative to its length. It measures 15.2 mm in total PD length. The base and midshaft measure an estimated 12.4 mm and 9.9 mm in ML breadth and an estimated 6.0 mm and 4.8 mm in DP height. The base is not well-preserved but it is clear that the proximal articular surface is DP short (estimated 4.6 mm). The dorsal surface of the bone is convex in both the ML and DP dimensions. The palmar surface has a deep, large and circular proximal palmar fossa measuring 8.9 mm in ML breadth and 6.2 mm in DP height. The distal border of the palmar fossa has a roughened, palmarly-extended ridge for the attachment of the flexor pollicis longus (FPL) tendon. The ungual fossa between the FPL attachment and the apical tuft is slightly concave (though this may be accentuated by slight erosion of the palmar surface as well) and relatively short in PD length. The apical tuft is very ML expanded (10.9 mm) but DP narrow (4.4 mm), giving an overall broad and flat appearance to the bone. The medial tip of the apical tufts proximal edge (lateral side is eroded) flares medially and proximally as an ungual spine.

UW 101-1329 right second distal phalanx
Preservation This specimen is complete and well-preserved apart from erosion of cortex along the proximal border of the dorsal surface. Morphology This specimen is 12.7 mm in total PD length. The base is robust, measuring 7.5 mm in ML breadth and 4.9 mm in DP height. The palmar surface of the proximal half of the shaft has a deep and somewhat circular excavation with a prominent distal edge for the attachment of the flexor digitorum profundus tendon. In sagittal view, the dorsal shaft is DP mildly concave along the proximal half and mildly convex along the distal half. The midshaft, measuring 4.3 mm in ML breadth and 3.9 mm in DP height, is ML narrow compared with the broader base and, particularly, the very broad apical tuft, but DP tall. The apical tuft measures 6.5 mm in ML breadth and 3.3 mm in DP height. The medial proximal tip of the apical tuft expand laterally and slightly proximally as a ungula spine, but similar morphology is not seen on the lateral side.

UW 101-1722 right third distal phalanx
Preservation This specimen is missing most of its medial side, broken in the sagittal plane just medial to the sagittal midline. However the full length of the lateral ¾ of the bone is well-preserved. Morphology This specimen measures 14.0 mm in preserved PD length and an estimated 14.4 mm in total PD length. The preserved morphology is identical to that described for UW101-1329 apart from a deep palmar fossa. The base measures 5.3 mm in DP height. The midshaft and apical tuft measure 4.5 and an estimated 6.9 mm in ML breadth and 3.8 mm and 3.8 mm in DP height, respectively.

UW101-1723 right fourth distal phalanx
Preservation This specimen is complete and perfectly preserved, apart from slight erosion of the cortex on the lateral-proximal border of the palmar surface. Morphology This specimen is 14.1 mm in total PD length. The morphology is identical to that described for UW 101-1329, except that the ML breadth of the base and apical tuft are even more pronounced and both the medial and lateral proximal tips of the apical tuft project slightly proximally and palmarly as ungual spines. The base measures 9.3 mm in ML breadth and 5.2 mm in DP height. The midshaft and apical tuft measure 4.4 and 7.0 mm in ML breadth and 3.5 mm and 3.6 mm in DP height, respectively.

UW 101-1724 right fifth distal phalanx
Preservation This specimen is complete and perfectly preserved apart from cortical erosion along the proximal border of the palmar surface. Morphology This specimen is 12.4 mm in total PD length. The morphology is identical to that described for UW 101-1329, except that the palmar fossa is not as deep and the apical tuft is not as ML expanded, as would be expect for a DP5. The proximal border of the dorsal surface is more dorsally flaring than in the other non-pollical phalanges of Hand 1. The base measures 7.0 mm in ML breadth and 5.1 mm in DP height. The midshaft and apical tuft measure 3.5 and 5.4 mm in ML breadth and 3.5 mm and 3.5 mm in DP height, respectively.