Humeral torsion and throwing proficiency in early human evolution

Abstract

BackgroundIn their recent paper, "Clavicle length, throwing performance and the reconstruction of the Homo erectus shoulder," Roach and Richmond (2015) test the hypothesis that relative clavicular length predicts scapular position and explore whether or not variation in relative clavicular length in a sample of modern humans from northwestern Kenya is correlated to throwing performance. Since the impetus to undertake these analyses is in part based on supposed claims I have made in my research on hominin shoulder evolution, I feel compelled to correct some misrepresentations they make of my work. The shoulder of H. erectusThe references I have made to relative clavicular length in my investigation of hominin shoulder evolution (Larson et al.

Humeral torsion does not dictate shoulder position, but does influence throwing speed

“…We further concluded that the data better support the presence of modern human-like, laterally facing shoulders dating back to H. erectus or earlier. Larson (2015) does not agree with our conclusions or with the way we present her research.…”

“…Given that relative clavicle length is both the only data support for the anterior shoulder model and the primary logic behind it, understanding the clavicle is clearly the key to evaluating Larson's hypothesis. Larson (2015) suggests that our summary of her work conflates what anatomical factors are causes and what are effects. We have not proposed any hypotheses about cause and effect.…”

“…to overcome deficits in internal rotation at the shoulder due to low humeral torsion (Larson, 2015). Larson (2015: 199) states that very low torsion in early hominins "(does not) make sense if they all possessed a human-like scapular position."…”

“…With the glenoid fossae now facing anteriorly, Larson (2007Larson ( , 2009Larson ( , 2015 argues that low humeral torsion does not provide any impediment to tool use as shoulder rotational range of motion (ROM) easily allows the hands to overlap. Based on this logic, and an observation that clavicle lengths in KNM-WT 15000 and LB1 appear to be short relative to humeral length, Larson (2007Larson ( , 2009 proposed the anterior shoulder model for early Homo.…”

“…In her reply to our paper, Larson (2015: 200) defines her anterior shoulder model as "essentially a neutral by-product of other evolutionary changes, and low torsion (as) an accommodation to the resulting more anteriorly facing glenoid fossae." While it is difficult to reconcile this "neutrality" with her statements about tool use (Larson, 2007(Larson, , 2015, here we focus instead on what the evolutionary changes Larson alludes to might be. If this is meant to reference a more caudally rotated glenoid, modern humans living today have caudally rotated glenoids and do not have anteriorly facing shoulders, demonstrating that these features are not necessarily linked.…”