Humeral torsion and throwing proficiency in early human evolution.

BackgroundIn their recent paper, “Clavicle length, throwing performanceand the reconstruction of the Homo erectus shoulder,” Roach andRichmond (2015) test the hypothesis that relative clavicularlength predicts scapular position and explore whether or notvariation in relative clavicular length in a sample of modernhumans from northwestern Kenya is correlated to throwing per-formance. Since the impetus to undertake these analyses is in partbased on supposed claims I have made in my research on homininshoulder evolution, I feel compelled to correct some mis-representations they make of my work.The shoulder of H. erectusThe references I have made to relative clavicular length in myinvestigation of hominin shoulder evolution (Larson et al., 2007;Larson, 2007, 2009, 2013) concern a potential means of function-ally interpreting the very low degree of humeral torsion typifyingearly hominins and early Homo. Contrary to the claim of Roach andRichmond (2015:108). I have not suggested that a more lateralposition for the scapula in early Homo and an anterior facingshoulder joint was “necessary to allow early H. erectus to overcomedeﬁcits in internal shoulder rotation caused by very low humeraltorsion.” What I haveargued is that the verylowdegree of humeraltorsion that has been documented for all early hominins and earlyHomo would result in a reduced range of internal shoulder rotationifthesetaxahadadorsalscapularpositionsimilartothatofmodernhumans. The difference here concerns what is cause and what iseffect.Icontend thatgivenourcurrentunderstandingof thefactorsinﬂuencing humeral torsion, the verylowdegree of torsion in earlyH. erectus is surprising, and to explain why I believe this to be thecase I will digress brieﬂy with the following overview.Humeral torsionHumeral torsion refers to the orientation of the proximalarticular surface relative to the distal end of the humerus (Fig. 1).The use of the distal humerus as a reference is not simply a con-venience. The elbow in virtually all mammals is a unidirectionalhinge joint with little if any rotational mobility. What ﬂexibility inhand positioning that exists in a taxon depends on the range ofmotion available at the shoulder coupled with forearm pronation/supination mobility. Therefore, the functional plane of elbowﬂexion/extension depends on the conﬁguration of the gleno-humeral joint. Intuitively, an elevated degree of humeral torsion(humeral head facing more inward) in humans and apes is relatedto the dorsal position of their scapulae since this results in morelaterally facing glenoid fossae. The shift from a laterally positionedscapula on a dorsoventrally deep thorax typical of monkeys to adorsal position on a broad thorax in hominoids is generally un-derstood to be related to a greatly expanded range of motion at theshoulder due to the lateral reorientation of the shoulder joint. It isnot actually known, however, whether selection for this increasedrange of motion caused the change in thoracic shape and scapularposition, or if an enhanced range of motion was the fortuitousresult of thoracic shape change bringing about scapularrepositioning.Modern humans and extant African apes are characterized by ahighdegreeofhumeraltorsioncomparedtootherprimates(Fig.2),and for many years this was counted as a shared derived feature ofhominoids (e.g., Le Gros Clark,1969; Andrews,1985; Martin,1986;Harrison,1987). However, as Fig. 2 shows, Asian apes have a muchlower degree of torsion, though all apes have dorsally positionedscapulae. In 1988 I reported that the very low humeral torsiondisplayed by gibbons was likely related to the extreme degree of