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, 111 (1), 121-4

Early Pleistocene Third Metacarpal From Kenya and the Evolution of Modern Human-Like Hand Morphology

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Early Pleistocene Third Metacarpal From Kenya and the Evolution of Modern Human-Like Hand Morphology

Carol V Ward et al. Proc Natl Acad Sci U S A.

Abstract

Despite discoveries of relatively complete hands from two early hominin species (Ardipithecus ramidus and Australopithecus sediba) and partial hands from another (Australopithecus afarensis), fundamental questions remain about the evolution of human-like hand anatomy and function. These questions are driven by the paucity of hand fossils in the hominin fossil record between 800,000 and 1.8 My old, a time interval well documented for the emergence and subsequent proliferation of Acheulian technology (shaped bifacial stone tools). Modern and Middle to Late Pleistocene humans share a suite of derived features in the thumb, wrist, and radial carpometacarpal joints that is noticeably absent in early hominins. Here we show that one of the most distinctive features of this suite in the Middle Pleistocene to recent human hand, the third metacarpal styloid process, was present ∼1.42 Mya in an East African hominin from Kaitio, West Turkana, Kenya. This fossil thus provides the earliest unambiguous evidence for the evolution of a key shared derived characteristic of modern human and Neandertal hand morphology and suggests that the distinctive complex of radial carpometacarpal joint features in the human hand arose early in the evolution of the genus Homo and probably in Homo erectus sensu lato.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
(A) 3D scan images, from left to right, in radial (Upper) and dorsal view (Lower) of third metacarpals of a common chimpanzee, Australopithecus afarensis (A.L. 438-1d), Australopithecus sediba (MH 2), a Neandertal (Shanidar 4), and a modern human (USNM 380447). All are shown as from the right hand and scaled to approximately the same articular length. Note the distinctive styloid process (black arrows) is present only in the modern human and Neandertal specimens. (B) Photograph of KNM-WT 51260 in radial, ulnar, dorsal, and volar views (Left) and proximal and distal views (Right). Note the presence of a distinctly human-like styloid process.
Fig. 2.
Fig. 2.
Third metacarpal dimensions in modern human males (n = 41) and females (n = 38), Late Pleistocene H. sapiens (n = 18), Neandertals (n = 10), and KNM-WT 51260. (A) Articular length measured between center of proximal and distal joint surfaces. (B) Styloid process length measured as proximal projection from center of proximal articular surface. (C) Styloid process length expressed as a ratio of articular length. Neandertal and Upper Paleolithic H. sapiens have relatively shorter styloid processes than modern humans, although their ranges overlap. KNM-WT 51260 falls within the ranges of both modern and fossil specimens (Fig. S2, Table S3 and Table S4).

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