Trabecular bone structure of the proximal capitate in extant hominids and fossil hominins with implications for midcarpal joint loading and the dart-thrower's motion

OBJECTIVES

This research examines whether the distribution of trabecular bone in the proximal capitates of extant hominids, as well as several fossil hominin taxa, is associated with the oblique path of the midcarpal joint known as the dart-thrower's motion (DTM).

MATERIALS AND METHODS

We analyzed proximal capitates from extant (Pongo n = 12; Gorilla n = 11; Pan n = 10; fossil and recent Homo sapiens n = 29) and extinct (Australopithecus sediba n = 2; Homo naledi n = 1; Homo floresiensis n = 2; Neandertals n = 3) hominids using a new canonical holistic morphometric analysis, which quantifies and visualizes the distribution of trabecular bone using relative bone volume as a fraction of total volume (rBV/TV).

RESULTS

Homo sapiens and Neandertals had a continuous band of high rBV/TV that extended across the scaphoid, lunate, and hamate subarticular regions, but other fossil hominins and extant great apes did not. A. sediba expressed a distinct combination of human-like and Pan-like rBV/TV distribution. Both H. floresiensis and H. naledi had high rBV/TV on the ulnar-side of the capitate but low rBV/TV on the radial-side.

CONCLUSION

The proximal capitates of H. sapiens and Neandertals share a distinctive distribution of trabecular bone that suggests that these two species of Homo regularly load(ed) their midcarpal joints along the full extent of the oblique path of the DTM. The observed pattern in A. sediba suggests that human-like stress at the capito-scaphoid articular surface was combined with Pan-like wrist postures, whereas the patterns in H. floresiensis and H. naledi suggest their midcarpal joints were loaded differently from that of H. sapiens and Neandertals.

The ontogeny of nasal floor shape variation in extant humans

Variation in nasal floor topography has generated both neontological and paleontological interest. Three categories of nasal floor shape (Franciscus: J Hum Evol 44 (2003) 699-727) have been used when analyzing this trait in extant humans and fossil Homo: flat, sloped, and depressed (or "bi-level"). Variation in the frequency of these configurations within and among extant and fossil humans has been well-documented (Franciscus: J Hum Evol 44 (2003) 699-727; Wu et al.: Anthropol Sci 120 (2012) 217-226). However, variation in this trait in Homo has been observed primarily in adults, with comparatively small subadult sample sizes and/or large age gradients that may not sufficiently track key ontogenetic changes. In this study, we investigate the ontogeny of nasal floor shape in a relatively large cross-sectional age sample of extant humans (n = 382) ranging from 4.0 months fetal to 21 years post-natal. Results indicate that no fetal or young infant individuals possess a depressed nasal floor, and that a depressed nasal floor, when present (ca. 21% of the sample), does not occur until 3.0 years postnatal. A canonical variates analysis of maxillary shape revealed that individuals with depressed nasal floors were also characterized by relatively taller anterior alveolar regions. This suggests that palate remodeling at about 3.0-3.5 years after birth, under the influence of tooth development, strongly influences nasal floor variation, and that various aspects of dental development, including larger crown/root size, may contribute to the development of a depressed nasal floor. These results in extant humans may help explain the high frequency of this trait found in Neandertal and other archaic Homo maxillae.