Estimating thumb-index finger precision grip and manipulation potential in extant and fossil primates

Abstract

Primates, and particularly humans, are characterized by superior manual dexterity compared with other mammals. However, drawing the biomechanical link between hand morphology/behaviour and functional capabilities in non-human primates and fossil taxa has been challenging. We present a kinematic model of thumb-index precision grip and manipulative movement based on bony hand morphology in a broad sample of extant primates and fossil hominins. The model reveals that both joint mobility and digit proportions (scaled to hand size) are critical for determining precision grip and manipulation potential, but that having either a long thumb or great joint mobility alone does not necessarily yield high precision manipulation. The results suggest even the oldest available fossil hominins may have shared comparable precision grip manipulation with modern humans. In particular, the predicted human-like precision manipulation of Australopithecus afarensis, approximately one million years before the first stone tools, supports controversial archaeological evidence of tool-use in this taxon.

Keywords: Australopithecus; Neanderthal; grasping; kinematic model; manipulation; primates.

Figure 1.

The thumb and index finger kinematic model. The model is based on three thumb segments (first metacarpal (Mc1), proximal phalanx (PP1) and distal phalanx (DP1)) and four index finger segments (Mc2, PP2, second intermediate phalanx (IP2) and DP2) of associated hand specimens. The digits touch a circular object of varying size (R, radius) and rotation (α). The relative orientation of the fingertips, touching two opposing points at the object, is β₁ (thumb) and β₂ (index finger). The area in which the object can be manipulated is then calculated. The three rightmost pictures show an example workspace (WS) of one representative human for three object sizes. In those pictures, one feasible configuration between the digits and object for each object size is shown. The dots represent the object positions for which a feasible configuration exists and therefore indicate the WS area. The shaded areas represent the positions the tip of thumb (green) and index (red) finger. (Online version in colour.)

Figure 2.

Relative segment lengths of the thumb and index finger in extant primates. The sum of all segments within each specimen is scaled to 1. The Pan sample includes both chimpanzees and bonobos combined, as digit proportions were very similar between the two species. The H. sapiens sample includes African, European and small-bodied Khoisan individuals. (Online version in colour.)

Figure 3.

Workspace (WS) values relative to object size in a sample of extant primates. The bold lines or symbols represent the mean value for each taxon and the lightly shaded areas represent the standard deviation. The maximal WS line represents the highest achievable WS for each object size and was determined by finding the combination of segment lengths that resulted in the highest possible WS. H. sapiens have a much larger WS for smaller objects while Pongo has the smallest WS for most of the object sizes. All taxa generally have a similar WS for objects greater than R = 0.2. (Online version in colour.)

Figure 4.

Thumb–index finger proportion and workspace (WS) values for three objects sizes across the comparative extant and fossil primate sample. Left plot shows a ratio of thumb length to index finger length and the right plot shows the WS values for object sizes R = 0 (in which thumb and index finger are touching), R = 0.1 and R = 0.2. The vertical line represents the mean, and the symbols represent the results for each individual for the respective object sizes. The ‘mob’ column indicates mobility of the trapeziometacarpal joint according to Rose [45], where a = 37.6, b = 32.8, c = 21.4, d = 12.5 and e = 10.6°.

Figure 5.

Workspace (WS) values relative to object size in fossil hominins compared with extant Pan and H. sapiens. The bold lines and symbols represent the mean value for each taxon and the lightly shaded areas for Pan and H. sapiens represent the standard deviation. The maximal WS line represents the highest achievable WS for each object size and was determined by finding the combination of segment lengths that resulted in the highest possible WS. All fossil hominins have a larger WS for smaller objects (R < 0.15) compared with Pan. Using Pan-like trapeziometacarpal mobility, Au. afarensis composite hand and Au. sediba have a lower WS than fossil and extant Homo, but with human-like mobility are similar to these taxa. All taxa generally have a similar WS for objects greater than R = 0.15. (Online version in colour.)