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Journal of Experimental Biology partnership with Dryad

Response to ‘Human fist evolution: a critique’
David R. Carrier, Michael H. Morgan

Nickle and Goncharoff (Nickle and Goncharoff, 2013) raise an interesting and, we believe, important challenge to the protective buttressing hypothesis of hominin hand evolution. If the hand of hominins evolved to be a dangerous weapon, it is reasonable to predict that the primary targets of fist strikes would have undergone coevolution resulting in increased robusticity and protective buttressing. However, Nickle and Goncharoff are wrong in their assumption that human-like hand proportions appear for the first time in the genus Homo. The proportions of the hand that allow modern humans to form a buttressed fist were present in the earliest hominins, at approximately the same time our lineage became habitually bipedal (reviewed in Morgan and Carrier, 2013). Because human-like hand proportions first appeared in basal hominins, it is in these species, not late archaic or modern humans, that we must look for evidence to test the hypothesis of coevolution. We believe the fossil record of basal hominins is consistent with the evolution of increased robusticity and protective buttressing of the face.

Nickle and Goncharoff are correct in stating that we ‘provide no compelling evidence to reject the null hypothesis that this fortuitous structure [i.e. the hominin hand] was the result of either random genetic drift or exaptation’. In truth, hypotheses about ancient genetic drift or exaptation cannot be rejected, and Nickle and Goncharoff join us in Panglossian violation of rigorous comparative methods to test adaptive hypotheses when they propose that the proportions of the human hand are ‘the result of natural selection on traits that impart fine motor skills, strong grip, the ability to use tools, etc.’. Functional hypotheses about the original selective value of a trait, in contrast, can effectively be rejected if the trait can be shown not to provide a performance advantage for the relevant behavior. In the case of the proportions of the human hand, performance advantages clearly exist for manual dexterity and, as indicated by the results of our study, for fighting by striking with fists. We suspect that selection for improved manual dexterity and fighting were both important. However, the protective buttressing hypothesis may provide a more compelling explanation for the specific proportions of the hand skeleton than does the manual dexterity hypothesis.