Evidence in hand: recent discoveries and the early evolution of human manual manipulation

Abstract

For several decades, it was largely assumed that stone tool use and production were abilities limited to the genus Homo. However, growing palaeontological and archaeological evidence, comparative extant primate studies, as well as results from methodological advancements in biomechanics and morphological analyses, have been gradually accumulating and now provide strong support for more advanced manual manipulative abilities and tool-related behaviours in pre-Homo hominins than has been traditionally recognized. Here, I review the fossil evidence related to early hominin dexterity, including the recent discoveries of relatively complete early hominin hand skeletons, and new methodologies that are providing a more holistic interpretation of hand function, and insight into how our early ancestors may have balanced the functional requirements of both arboreal locomotion and tool-related behaviours.

Keywords: Australopithecus; Homo; arboreal locomotion; dexterity; tool use.

Figure 1.

Bony and soft tissue morphology of the human hand considered advantageous for the manipulative precision and power squeeze grips used during stone tool use and production. (a) The suite of bony features typically considered distinct in the human hand (although some specific features are found in other primates, such as broad apical tufts in baboons) that reflect the ability to forcefully oppose the pads of the thumb and fingers, the well-developed musculature to the thumb and fifth digit, the high external loading of the thumb and distribution of that load across the wrist and palm, and the broad fingertips for control and manipulation of objects, especially within one hand. (b) Human precision grip (shown, a five-jaw chuck precision grip) in which the pads of the thumb and fingers grasp and manipulate the object and (c) a power squeeze grip, in which the fingers grasp the object diagonally and thumb is in line with the forearm. In stone tool behaviours, the thumb and fifth fingers are important for the manipulating and stabilizing objects in both the dominant and non-dominant hands. Muscles of the thumb and fifth digit that are strongly activated during use of these grips in tool making are noted [34]. FPL, flexor pollicis longus; FCU, flexor carpi ulnaris; FDM, flexor digiti minimi; ADM, abductor digiti minimi; OP, opponens pollicis; DI, dorsal interosseus; AP, oblique adductor pollicis; FPB, flexor pollicis brevis; EPB, extensor pollicis brevis; Mc, metacarpal. *The styloid process found at dorsoradial corner of the third metacarpal and thus cannot been seen in palmar view. **Marzke et al. [35] show that the FPL is not strongly active in either the dominant or non-dominant hand, particularly during precision pinch grips, in experienced knappers (but see [36]).

Figure 2.

Fragmentary or unassociated early hominin hand remains. (a) The composite hand of Au. afarensis from various sites in Hadar, Ethiopia. Only metacarpals 2–5 articulate well together and are presumed to be from the same individual. (b) A sample of isolated hand bones from (i) Sterkfontein, generally attributed to Au. africanus and (ii) Swartkrans, associated with either Au. robustus or early Homo. None of the fossils can be associated with the same individual and several elements are not represented in the fossil record. (c) The associated juvenile hand fossils of H. habilis OH 7. Although the phalanges are well represented in the OH 7 hand, little of the thumb and palm is preserved making functional inferences challenging. All known wrist bones shown at bottom and numbers indicate digits 1–5. Note that the phalanges of rays 2–5 cannot be attributed to any particular ray with certainty. All bones are shown in palmar view (apart from the wrist bones) and to the same scale although siding varies. (Online version in colour.)

Figure 3.

The associated hand remains of early hominins. (a) The left hand of Au. prometheus StW 573 [78] in situ; (b) the reconstructed right hand of MH2 (palmar view), with inset image of hand bones in situ; (c) the reconstructed right hand of Hand 1 H. naledi (palmar view) with inset image of hand bones in situ. All hands are shown to the same scale. Photo credit: R. J. Clarke (a) and P. Schmid (b,c). (Online version in colour.)