Postcranial robusticity in Homo. III: Ontogeny

Abstract

The influence of developmental factors on long‐bone crosssectional geometry and articular size in modern humans is investigated using two approaches: (1) an analysis of the effects of increased mechanical loading on long‐bone structure when applied during different developmental periods, using data collected for a study of upper limb bone bilateral asymmetry in professional tennis players; and (2) an analysis of the relative timing of age changes in femoral dimensions among juveniles from the Pecos Pueblo Amerindian archaeological sample. Results of these analyses are used to interpret the femoral morphology of three pre‐Recent Homo juveniles—the H. erectus KNM‐WT 15000 and the archaic H. sapiens La Ferrassie 6 and Teshik‐Tash 1—as well as observed differences in postcranial morphology between adult Recent and earlier Homo (Ruff et al., 1993). Our findings indicate the following: (1) There are age‐related changes in long‐bone diaphyseal envelope sensitivity to increased mechanical loading, with the periosteal envelope more responsive prior to mid‐adolescence, and the endosteal envelope more responsive thereafter. The periosteal expansion and endosteal contraction of the diaphysis documented earlier for adult pre‐Recent Homo relative to Recent humans (Ruff et al., 1993) is thus consistent with a developmental response to increased mechanical loading applied throughout life. The relatively large medullary cavity in the 11–12‐year‐old KNM‐WT 15000 femur is also consistent with this model. However, the two archaic H. sapiens juveniles show relatively small medullary cavities, possibly indicating a modified developmental pattern in this group. (2) Articulations follow a growth pattern similar to that of long‐bone length (and stature), while cross‐sectional diaphyseal dimensions (cortical area, second moments of area) show a contrasting growth pattern, with slower initial growth from childhood through mid‐adolescence, followed by a “catch‐up” period that continues through early adulthood. This latter pattern is more similar to the growth curve for body weight, and may in fact partially reflect adaptation of the diaphysis to increased weight bearing. Because of these different growth patterns, articulations appear relatively large, and diaphyseal breadths relatively small during late childhood to mid‐adolescence (i.e., about 9–13 years), when compared to adults from the same population. KNM‐WT 15000 shows this same proportional difference from adult early Homo specimens, which is therefore interpreted as simply a developmental consequence of his age at death. (3) When standardized for differences in body size and shape, midshaft femoral cross‐sectional areas for the three pre‐Recent juveniles in our sample, which span an age range from about 4 to 12 years, all fall in the upper part of the general data scatter for modern juveniles of a similar age. This is very similar to the pattern we found earlier for adult Homo specimens (Ruff et al., 1993). Thus, increased diaphyseal robusticity relative to modern humans, and the mechanicalhehavioral factors that produced this structural difference, were apparently as characteristic of immature as of adult pre‐Recent Homo. © 1994 Wiley‐Liss, Inc.