Evidence of hominid-like precision grip capability in the hand of the Miocene ape Oreopithecus

Abstract

Functional and allometric analyses of the hand of the late Miocene ape Oreopithecus bambolii (Tuscany, Italy) reveal a series of features that reflect an improved grasping capability including firm pad-to-pad precision gripping that apes are unable to perform. Related features such as hand length, relative thumb length, a deep and large insertion area for the tendon of the long thumb flexor, and the form of the metacarpal 2/capitate articulation are not present in extant or fossil apes. In these features, the Oreopithecus hand closely matches the pattern of early hominids, presumably as a response to similar functional demands.

Figure 1

Size and proportions of the Oreopithecus hand. (a) Relation between hand length (carpals, metacarpals, and phalanges of the third digit) and body weight in anthropoid primates (log10; body mass, refs. and 26). Hands of orthograde anthropoids (apes) are considerably longer than those of pronograde anthropoids because a large friction surface is needed to secure a firm hold during vertical climbing and hanging from branches. Both Miocene hominoids that yielded hand remains show the expected proportions: the pronograde Proconsul (P) has relatively short hands whereas the orthograde Dryopithecus (D) has long hands, as in extant apes. Four orthograde genera, however, do not follow this rule, as they have shorter hands than expected for their body weight: in Gorilla (G), this is attributable to the great body weight and the primarily terrestrial quadrupedal locomotion (22) whereas in Homo (H) it is attributable to the shift from a mainly locomotor to a more manipulative use of the hands. The reduction of hand length in Oreopithecus (O) presumably occurred for similar reasons as in hominids (see discussion in the text). (b) Relation between hand length (carpals, metacarpals, and phalanges of the third digit) and humerus length in anthropoid primates, Oreopithecus, and other fossil anthropoids. T, Theropithecus; Pa, Papio; Pl, Pliopithecus.

Figure 2

(a) The length of the proximal phalanges of the thumb set against the length of the proximal phalanges of the index finger of anthropoids, for the two complete hands of Oreopithecus (BA#140 and IGF 11778) and other forms. In an allometric context, the platyrhine condition (solid line) is likely to be primitive for anthropoids while there are two derived characters: the relatively long first phalanx of the thumb in Papio, Theropithecus, Homo, and Oreopithecus; and the relatively short proximal thumb phalanx in colobinae, apes, and, especially, Pongo. The Proconsul specimens considered are KNM-RU 2036 and the subadult individual I from the channel deposit at Kaswanga Primate Site (27). (Lph 1/2, length of the first phalange of the second ray; Lph 1/1, length of the first phalange of the thumb.) (b) Relation between thumb (pollex) length (metacarpal 1 and phalanges) and index finger length (metacarpal 2 and phalanges; log10). Colobines, hylobatids, chimps, and gorillas show an allometric relationship. Only Pongo falls above the regression line, with a relatively very long index finger in relation to its short thumb. Oreopithecus (BA#140), however, shows the same proportions as A. afarensis (A:AL#333) and Homo sapiens. The length of the A. afarenis thumb is reconstructed from the known lengths of metacarpal 1 and proximal phalanx and with a length for the distal phalanx estimated at 20.1 mm (based on the relationship between the lengths of the mc 1 and distal phalanx in modern humans).

Figure 3

The hand of O. bambolii from Baccinello. (a) Hand of a small animal (BA#140). This specimen belongs to a young adult because some epiphyses are still unfused. The thumb/index finger ratio can be considered to be definitive because intrinsic proportions of the hands remain unchanged during growth (28, 28). (b) Hand of the Florence skeleton (IGF 11778). (c) Hand of Dryopithecus laietanus (skeleton from Can Llobateres). Note that the hand of Oreopithecus (IGF specimen) and that of Dryopithecus (CLL-18800) differ greatly in size despite the comparable estimated body mass and the similar arm lengths of both individuals (30).

Figure 4

The distal phalanx of the thumb. (a) Pan (right). (b and c) Oreopithecus (b, BA#130; c, IGF 11778, left). (d) Homo (right). Upper, palmar view; lower, proximal view. Note the strong impression for the flexor pollicis longus tendon.

Figure 5

(a–e) Right third metacarpal and proximal phalanx. (a) Papio. (b) Dryopithecus. (c). Pan. (d) Oreopithecus (proximal phalanx undetermined). (e) Homo. For comments, see text. (f–m) Capitate/metacarpal 2 morphology in apes, hominids, and Oreopithecus. (i) Proconsul. (f and j) Pan troglodytes. (g, k, and l) O. bambolii (BA#151 capitate; BA#165 Mc2; and BA#208 Mc2). (h and m) H. sapiens. Arrows indicate the orientation of the facets for the metacarpal 2 on the capitate and for the capitate on the mc2. Note the human-like (proximal) orientation of the facet for the capitate in Oreopithecus and in Homo and the ape (medial) orientation in Pan. Note the lack of waisting on the Oreopithecus capitate and the Homo capitate and its presence in Pan.