In vitrostrain in human metacarpal bones during striking: testing the pugilism hypothesis of hominin hand evolution

Abstract

The hands of hominins (i.e. bipedal apes) are distinguished by skeletal proportions that are known to enhance manual dexterity but also allow the formation of a clenched fist. Because male-male physical competition is important in the mating systems of most species of great apes, including humans, we tested the hypothesis that a clenched fist protects the metacarpal bones from injury by reducing the level of strain during striking. We used cadaver arms to measure in vitro strain in metacarpals during forward strikes with buttressed and unbuttressed fist postures and during side slaps with an open palm. If the protective buttressing hypothesis is correct, the clenched fist posture should substantially reduce strain in the metacarpal bones during striking and therefore reduce the risk of fracture. Recorded strains were significantly higher in strikes in which the hand was secured in unbuttressed and slapping postures than in the fully buttressed posture. Our results suggest that humans can safely strike with 55% more force with a fully buttressed fist than with an unbuttressed fist and with twofold more force with a buttressed fist than with an open-hand slap. Thus, the evolutionary significance of the proportions of the hominin hand may be that these are the proportions that improved manual dexterity while at the same time making it possible for the hand to be used as a club during fighting.

Muscle activation during maximal effort tasks: evidence of the selective forces that shaped the musculoskeletal system of humans

“…Although the extent to which the muscles of extinct hominin species were specialized for locomotor or fighting performance cannot be directly demonstrated, the fossil record does provide relevant clues. Several of the skeletal characters most diagnostic of the hominin lineage have been suggested to be functionally consistent with anatomical specialization for aggressive behavior, including habitual bipedal posture ( Darwin, 1871 ; Livingstone, 1962 ; Wescott, 1963 ; Jablonski and Chaplin, 1993 ; Carrier, 2011 ), proportions of the hand that allow formation of a fist ( Morgan and Carrier, 2012 ; Horns et al, 2016 ), robusticity of the facial skeleton ( Carrier and Morgan, 2015 ), sexual dimorphism of the upper body and arms ( Frontera et al, 1991 ; Puts, 2010 ; Carrier, 2004 , 2011 ), and sexual dimorphism of the facial skeleton ( Carrier and Morgan, 2015 ). These anatomical correlates combined with the importance of physical aggression in the male-male and intergroup competition of great apes, including humans (see references cited above), suggests that patterns of muscle activation we observed in modern humans are likely to be applicable to species of early Homo .…”

“…Given the impact of male-male and intergroup aggression on reproductive fitness in the Hominidae, it is possible that selection on aggressive behavior was one of the many factors that has influenced the musculoskeletal system of hominins. Anatomical characters that have been suggested to be associated with selection on male-male and intergroup competition include: sexual dimorphism in muscle mass, upper body strength, stature and facial robustness ( Frontera et al, 1991 ; Puts, 2010 ; Carrier, 2004 , 2011 ; Carrier and Morgan, 2015 ); habitual bipedalism ( Darwin, 1871 ; Livingstone, 1962 ; Wescott, 1963 ; Jablonski and Chaplin, 1993 ; Carrier, 2011 ); and hand proportions that allow the formation of a clenched fist ( Morgan and Carrier, 2012 ; Horns et al, 2016 ).…”

Sexual selection on skeletal shape in Carnivora

“…), kangaroos (Jarman , ), and humans (Morgan and Carrier ; Carrier and Morgan ; Horns et al. ). These differences may also reflect constraints on trait evolution imposed by other behaviors (Maynard Smith et al.…”

A Geometric Morphometric Examination of Hominoid Third Metacarpal Shape and Its Implications for Inferring the Precursor to Terrestrial Bipedalism

“…Skeletal elements of the manus have provided valuable insight into the evolutionary history of the Hominoidea (e.g., Lewis, 1977;Susman, 1979;Marzke, 1987, 2000;Ricklan, 1987;Stern Jr et al, 1995;Tocheri et al, 2008;Lovejoy et al, 2009;Kivell et al, 2011;Rein and Harvati, 2013;Ward et al, 2014;Drapeau, 2015;Almécija et al, 2015b). The shapes of these bones are the result of a complex interplay between adaptation to hand function, phylogeny, and developmental constraints.…”