Morphology and structure of humeri from Zhoukoudian, Locality 1.

Background: Regional diversity in the morphology of the postcranium is not broadly documented, in part, because of the paucity of Asian sites preserving postcranial fossils. Yet, such an understanding of the initial hominin taxon to spread throughout multiple regions of the world is fundamental to documenting the adaptive responses to selective forces operating during this period of human evolution.

Methods: The current study reports the first humeral rigidity and strength properties of East Asian and places its diaphyseal robusticity into broader regional and temporal contexts. We estimate true cross-sectional properties of Zhoukoudian Humerus II and quantify new diaphyseal properties of Humerus III using high resolution computed tomography. Comparative data for African and Eurasian Late Pleistocene were assembled, and new data were generated from two modern Chinese populations.

Results: Differences between East Asian and African were inconsistently expressed in humeral cortical thickness. In contrast, East Asian appears to exhibit greater humeral robusticity compared to African when standardizing diaphyseal properties by the product of estimated body mass and humeral length. East Asian humeri typically differed less in standardized properties from those of side-matched Late Pleistocene hominins (e.g., Neanderthals and more recent Upper Paleolithic modern humans) than did African , and often fell in the lower range of Late Pleistocene humeral rigidity or strength properties.

Discussion: Quantitative comparisons indicate that regional variability in humeral midshaft robusticity may characterize to a greater extent than presently recognized. This may suggest a temporal difference within , or possibly different ecogeographical trends and/or upper limb loading patterns across the taxon. Both discovery and analysis of more adult humeri are critical to further evaluating and potentially distinguishing between these possibilities.