Shape Variation in the Distal Femur of Modern Humans and Fossil Hominins
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Date
2016-08-10
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Johns Hopkins University
Abstract
The distal femur of modern humans is uniquely shaped as a result of bipedality. There is debate, however, on how to interpret differences in femoral morphology and therefore locomotor behavior between early hominins and modern humans. One difficulty is that relatively little is known about the range and causes of variation in distal femoral shape within humans. This project therefore seeks to describe the variation in hominin distal femoral shape, and test several mechanically-based hypotheses about the effects of sex, body size, and body proportions on this morphology.
Distal femora were analyzed in a sample of 290 modern humans from six populations, 39 chimpanzees, and ten fossil hominins (australopiths, early Homo and Neanderthals). Modern human populations were chosen to encompass a wide range of body sizes and proportions. 3-D surface scans of each femur were aligned and linear measurements were taken in extension and at multiple angles of simulated knee flexion. Surface sliding semi-landmarks were placed on the distal articular surface and a 3D geometric morphometric (GM) analysis was carried out using Principal Components Analysis (PCA) to examine shape differences. PCA in form space was also used to test for allometric shape effects.
Sex had a strong influence on distal femoral shape in modern humans, primarily affecting its overall proportions. Additionally, smaller humans had more curved condyles and a deeper patellar groove than larger individuals. Body breadth appears to have a slight effect on patellar groove morphology, but body proportions have little overall influence on distal femoral shape.
Distal femora of fossil Homo are similar to those of modern humans. Australopiths, despite clearly being bipedal, have rounder condyles, a shorter patellar surface on the anteroposterior axis, a less projecting lateral lip of the patellar groove and a shallower patellar groove in slight knee flexion, and a higher ratio of medial/lateral condyle size than modern humans. The length of the patellar surface, and to some extent the greater curvature of the condyles, may be explained by the small body size of australopiths. The other unique features, however, likely indicate minor differences in gait and/or arboreal behavior between australopiths and later Homo.
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Keywords
hominin locomotion, australopithecus, femur