Abstract

BackgroundThe 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 FindingsWe 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.ConclusionsThese 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.

Highlights

  • One of the hallmarks of humankind is the possession of a complex repertoire of manual grips [1,2]

  • 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 capability is reflected in the morphology of the distal phalanges, especially in the pollical distal phalanx (PDP), which shows specific features related to the soft tissues involved in precision grasping

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Summary

Introduction

One of the hallmarks of humankind is the possession of a complex repertoire of manual grips [1,2]. The most refined expression of human manipulation is attained during pad-to-pad precision grasping, which consists in the opposition of the proximal pulp of the thumb against that of one or more fingers ([3]; see Figure 1) This capability is reflected in the morphology of the distal phalanges, especially in the pollical distal phalanx (PDP), which shows specific features related to the soft tissues involved in precision grasping [3,4]. The presence in humans of ungual spines and ungual fossa are indicative of a fully compartmentalized digital pulp, with a fatty and mobile proximal portion (related to the ungual fossa) as well as a large and more or less static distal part (related to the distal tuberosity; [3,4]) The presence of these two different pulps, each with distinctive properties, ensures an adequate friction and accommodation of the object between the pulp of the thumb and those of the fingers during precision grasping (Figure 1). 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 possible given the human-like intrinsic hand proportion found in australopiths

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