precision holding in humans, non-human primates, and Plio-Pleistocene hominids

Early origin for human-like precision grasping: a comparative study of pollical distal phalanges in fossil hominins

BACKGROUND

The morphology of human pollical distal phalanges (PDP) closely reflects the adaptation of human hands for refined precision grip with pad-to-pad contact. The presence of these precision grip-related traits in the PDP of fossil hominins has been related to human-like hand proportions (i.e. short hands with a long thumb) enabling the thumb and finger pads to contact. Although this has been traditionally linked to the appearance of stone tool-making, the alternative hypothesis of an earlier origin--related to the freeing of the hands thanks to the advent of terrestrial bipedalism--is also possible given the human-like intrinsic hand proportion found in australopiths.

METHODOLOGY/PRINCIPAL FINDINGS

We perform morphofunctional and morphometric (bivariate and multivariate) analyses of most available hominin pollical distal phalanges, including Orrorin, Australopithecus, Paranthropous and fossil Homo, in order to investigate their morphological affinities. Our results indicate that the thumb morphology of the early biped Orrorin is more human-like than that of australopiths, in spite of its ancient chronology (ca. 6 Ma). Moreover, Orrorin already displays typical human-like features related to precision grasping.

CONCLUSIONS

These results reinforce previous hypotheses relating the origin of refined manipulation of natural objects--not stone tool-making--with the relaxation of locomotor selection pressures on the forelimbs. This suggests that human hand length proportions are largely plesiomorphic, in the sense that they more closely resemble the relatively short-handed Miocene apes than the elongated hand pattern of extant hominoids. With the advent of terrestrial bipedalism, these hand proportions may have been co-opted by early hominins for enhanced manipulative capabilities that, in turn, would have been later co-opted for stone tool-making in the genus Homo, more encephalized than the previous australopiths. This hypothesis remains may be further tested by the finding of more complete hands of unequivocally biped early hominins.

A clarification of Pouydebat et al., 2008, evolution of grasping among anthropoids

Several statements by Pouydebat et al. (2008) do not adequately represent views of authors cited, in part because they reflect confusion in the literature about terminology regarding precision gripping. We address these problems, by tracing definitions of precision grips through the literature on manipulative behaviour and identifying the grip that is central to the Pouydebat et al. (2008) study. This allows us to offer a clarification of the statements by Pouydebat et al. (2008) regarding the sequence of appearance of human grip capabilities and possible morphological correlates to these capabilities in extant species.

Comparative morphometrics of the primate apical tuft

The relationship between the structure and function of the primate apical tuft is poorly understood. This study addresses several hypotheses about apical tuft morphology using a large modern primate comparative sample. Two indices of tuft size are employed: expansion and robusticity. First, comparisons of relative apical tuft size were drawn among extant nonhuman primate groups in terms of locomotion and phylogenetic category. Both of these factors appear to play a role in apical tuft size among nonhuman primates. Suspensory primates and all platyrrhines had the smallest apical tufts, while terrestrial quadrupeds and all strepsirrhines (regardless of locomotor category) had the largest tufts. Similarly, hypotheses regarding the apical tufts of hominins, especially the large tufts of Neandertals were addressed using a comparison of modern warm- and cold-adapted humans. The results showed that cold-adapted populations possessed smaller apical tufts than did warm-adapted groups. Therefore, the cold-adaptation hypothesis for Neandertal distal phalangeal morphology is not supported. Also, early modern and Early Upper Paleolithic humans had apical tufts that were significantly less expanded and less robust than those of Neandertals. The hypothesis that a large apical tuft serves as support for an expanded digital pulp is supported by radiographic analysis of modern humans in that a significant correlation was discovered between the width of the apical tuft and the width of the pulp. The implications of these findings for hypotheses about the association of apical tuft size and tool making in the hominin fossil record are discussed.

Neural pattern in the human pollical distal phalanx

A morphological study of the neural pattern in the human ungual region of the pollical distal phalanx was carried out on eight male cadavers. The dissections showed a palmar neural arrangement consisting of four designated ungual nerves, two proximal and two distal, for each of the ulnar and radial palmar digital nerves at the lateral sides of the thumb. This neural configuration was associated with the compartmentalization of its ungual pulp, the difference between the type of sensory receptors within the ungual pulp, the overlapping of the tactile composition at the thumb tip, seen clinically after laceration of one of the palmar digital nerves, and the sensory supply to the nail bed in the dorsal portion of the thumb. The proximal ungual pulp compartment had a single proximal medial ungual nerve, which did not appear, as far as visually possible, to overlap at the midline of the proximal ungual pulp. In contrast, the distal ungual pulp compartment was supplied by a medial and a lateral ungual nerve, both of which did appear to overlap to their contralateral sides in the thumb tip. A single proximal dorsal sensory nerve branched dorsally from each of ulnar and radial palmar digital nerves at the level of the proximal ungual pulp to supply the nail bed on the dorsum of the thumb.

Shape variation of the human pollical distal phalanx and metacarpal

Human distal pollical phalanx form has been associated with tool manufacture, and the broad tuft of this bone in Neanderthals has been suggested to be a climatic adaptation and/or an aid to a tremendously powerful grip. A wide first metacarpal head has also been proposed to be useful in distinguishing tool-dependent hominids from those less reliant on tools. In order to contribute to an evaluation of these hypotheses variation in first metacarpal and distal phalanx shape is explored among samples of modern humans and compared to that of fossil hominids. Modern humans are from the Terry Collection, Larsen Bay, a Chinese-Alaskan cemetery, Egypt, and Sully and Mobridge. Hominid fossils include AL 333w-39, SKX 5016, SK 84, Stw 294, OH 7, several Neanderthals, Skhūl 4 and 5, and Predmostí 3. Analysis involves length-width ratios, regressions of distal phalanx tuft width on base width and of metacarpal head width on length, and pattern profiles based on Z-scores with reference to the Larsen Bay sample. Larsen Bay individuals are robust, while Terry "blacks," Egyptians, and Chinese-Alaskan males tend to be gracile. Fossil hominids are most distinctive for distal phalanx radioulnar tuft and mid-shaft widths relative to length. Security of grip is one plausible explanation. While most modern samples are positively allometric for tuft width relative to base width, the Larsen Bay and fossil hominid samples are not; thus caution is advised in accepting a base-tuft width comparison as a tool-dependence marker. Separation from modern humans is not easily achieved with metacarpal measures, but the Hadar metacarpal has distinctively narrow radioulnar head width ratios. While first metacarpal head expansion among hominids may plausibly be related to tool manufacture, other activities that place stress on the metacarpophalangeal joint should also be considered.

Hand function and tool behavior in early hominids

Prompted by the discovery of the Olduvai Hominid 7 hand (Homo habilis) in 1960, studies of primate hands were conducted in order to understand better the functional morphology of the earliest hominids and the evolution of tool behavior. Since Napier's work in the early 1960's, the hand bones of a number of other species have been recovered in East and South Africa. New hominid finds and a prolific archaeological record have broadened our perspective on the evolution of tool behavior in Australopithecus, Paranthropus, and early Homo. A debate is underway at this time on the questions of what exactly we can glean from the fossil record regarding the evolution of tool behavior and what are the best approaches to interpreting the evidence. I support the model of Napier that identified morphological correlates of precision and power grasping in the hands of extant primates and in early hominid hand bones. At the same time, I question both the underlying rationale and attempts to identify more subtle aspects of precision grasping, based on present evidence.

Precision grips, hand morphology, and tools

This study asks whether there are discernable links between precision gripping, tool behaviors, and hand morphology in modern hominoids, which may guide functional interpretation of early hominid hand morphology. Findings from a three-pronged investigation answer this question in the affirmative, as follows: (1) Experimental manufacture of early prehistoric tools provides evidence of connections between distinctive human precision grips and effective tool making. (A connection is not found between the "fine" thumb/index finger pad precision grip and early tool making.) (2) Manipulative behavior studies of chimpanzees, hamadryas baboons, and human show that human precision grips are distinguished by the greater force with which objects may be secured by the thumb and fingers of one hand (precision pinching) and the ability to adjust the orientation of gripped objects through movements at joints distal to the wrist (precision handling). (3) Morphological studies reveal eight featured distinctive of modern humans which facilitate use of these grips. Among these features are substantially larger moment arms for intrinsic muscles that stabilize the proximal thumb joints. Examination of evidence for these reveals that three of the eight features occur in Australopithecus afarensis, but limited thumb mobility would have compromised tool making. Also, Olduvai hand morphology strongly suggests a capacity for stone tool making. However, functional and behavioral implications of Sterkfontein and Swartkrans hand morphology are less clear. At present, no single skeletal feature can be safely relied upon as an indicator of distinctively human capabilities for precision gripping or tool making in fossil hominids.

Kto pierwszy przemówił? Uwarunkowania morfologiczne mowy ludzkiej

The evolution of human speech was probably involved into development of brain structures responsible for cultural behaviour, specially tool-making. Moreover, it has been sugested that only Homo erectus species had the speech apparatus formed the way which allowed to articulate some sounds.