Comment on “Human-like hand use in Australopithecus africanus ”

Abstract

, based on metacarpal trabecular bone structure, argue that Australopithecus africanus employed human-like dexterity for stone tool making and use 3 million years ago. However, their evolutionary and biological assumptions are misinformed, failing to refute the previously existing hypothesis that human-like manipulation preceded systematized stone tool manufacture, as indicated by the fossil record. S kinner et al.(1) analyze metacarpal trabecular bone structure in the 3-million-yearold hominin Australopithecus africanus and infer based on these data that this taxon "was capable of habitual and forceful humanlike opposition of the thumb and fingers during [...] tool-related behaviors, providing morphological evidence of committed tool use in a hominin hitherto considered not to be capable of these behaviors" (1). Specifically, they found that the trabecular organization of the pollical metacarpal of A. africanus is, in some respects, more humanlike than chimpanzee-like, which they interpret as "morphological evidence…that can be linked to behavior and hand use during life" (1), with clear allusions to stone tool making and use throughout the article. Although we appreciate their effort to investigate a novel aspect of fossil morphology like trabecular structure, we note here limitations in the proposed evolutionary importance of their findings, which together with misinformed assumptions about bone mechanobiology make many of their conclusions unwarranted. We hope that the concerns that we raise will help promote a constructive discussion dealing with the complex topic of the relationships between form and function and redirect future research studies in human evolution.It is well known that, like humans, all living great apes make and use tools, and some chimpanzees and capuchin monkeys even engage in regular stone tool use (2). Humans, however, display especially advanced manual dexterity facilitated by a unique (among modern hominoids) type of interaction between the proximal pulps of our thumb and fingers (3). The primary anatomical requirement for this pad-to-pad precision grasping is having a long thumb relative to finger lengths (3-5). Previous analyses of hand bone morphology in australopiths (4, 6-8) and the Miocene hominin Orrorin (5) have provided compelling evidence for pad-to-pad precision grasping before the widespread occurrence of flaked stone tools, which indicates that a basic human-like hand structure is plesiomorphic for modern humans (Fig. 1). The emerging evolutionary picture is that increased reliance on bipedalism and enhanced manipulation were very early apomorphies of the hominin lineage, as a part of our original adaptive shift from apes in relation to new foraging strategies (5, 7). Thus, the eventual application of human-like hand structure to stone tool flaking and use was almost certainly an exaptation, not an adaptation (4). In this light, the inference by Skinner et al. of human-like hand use among australopiths is neither unprecedented nor unexpected.Skinner et al. downplay pre...

Response to Comment on “Human-like hand use in Australopithecus africanus ”

“…We do not claim that our results "refute the previously existing hypothesis that human-like manipulation preceded systematized stone tool manufacture" (1); to the contrary, we state that our results are consistent with the use of "forceful hand grips for any number of manipulative behaviors" (2). Almécija et al (1) refer to previous studies (mainly their own) based on the relative lengths and external morphology of hand bones that "have provided compelling evidence for padto-pad precision grasping before the widespread occurrence of flaked stone tools" (1) and that australopiths and earlier hominins "were likely capable of human-like manipulation" (1).…”

“…The asymmetric distribution of trabecular bone within the base of the thumb and metacarpal heads in humans, Neandertals, and A. africanus (and the absence of this pattern in apes) provides evidence for habitual, forceful opposition of the thumb to the fingers. We agree that inferences for humanlike hand use among australopiths referred to by Almécija et al are neither "unprecedented nor unexpected" (1). However, the inferences we can now make, based on the trabecular bone distribution and the well-accepted concept that trabeculae remodel in response to habitual load during an individual's lifetime (4,5), are much stronger than they and others have been able to make based on external morphology of hand bones alone.…”

“…Finally, Almécija et al (1) suggest that a lack of differences in pairwise comparisons in BV/TV in the third metacarpal (Mc3) between humans and Asian apes weakens our argument for a link between form and function. Unfortunately, they are conflating two sets of results: those for average BV/TV throughout the epiphysis and the distribution of trabecular bone throughout the epiphysis.…”

“…Almécija et al (1) refer to previous studies (mainly their own) based on the relative lengths and external morphology of hand bones that "have provided compelling evidence for padto-pad precision grasping before the widespread occurrence of flaked stone tools" (1) and that australopiths and earlier hominins "were likely capable of human-like manipulation" (1). We agree; however, Almécija et al (1) fail to acknowledge that our results are novel because they reflect (i) actual behavior, rather than the capability for particular behaviors and (ii) forceful pad-to-pad precision grasping, which is what separates human dexterity from that of other primates (3). The asymmetric distribution of trabecular bone within the base of the thumb and metacarpal heads in humans, Neandertals, and A. africanus (and the absence of this pattern in apes) provides evidence for habitual, forceful opposition of the thumb to the fingers.…”

“…First, they note that trabecular bone remodels under dynamic loads and not static loads, such as when one is "tool grasping" (1). Not only does experimental evidence show that gripping force varies during static holding, and thus that loads experienced by the hand are dynamic (9), but when one uses an object, gripping force will fluctuate throughout the duration of the task (10, 11).…”