Simulating Hip Growth in Early Humans
Megan Reyes ’21, Caroline Klureza ’22 and Professor Zachary D. Cofran (Anthropology)
The human pelvis is uniquely adapted for bipedal walking, and the shape of the ilium plays an important role in locomotion. Shape changes to the human ilium during individual growth and development reflect biomechanical adaptations. Australopithecus africanus from South Africa walked bipedally around 3 million years ago, but their ilia have some shape attributes similar to both humans and non-human apes. It has previously been proposed that A. africanus ilia followed a growth trajectory more similar to apes than to humans. However, that research was based on methods that do not provide high-resolution shape information. We revisited the issue of ilium growth using advanced geometric morphometric methods, to test the hypothesis that A. africanus ilia follow the same shape growth pattern as humans.
We established the human growth trajectory using 40 individuals in various stages of development from infancy to adulthood. We quantified ilium shape with 148 three-dimensional landmark coordinates for each individual. We obtained a 3D growth trajectory with a principal component analysis, characterizing shape differences among human ilia that were caused by growth. Using this human growth projection, we “grew” two immature A. africanus ilium fossils into simulated adults, which we then compared to an actual adult A. africanus fossil. To test our hypothesis, we also simulated development of immature human ilia and compared the simulated versus actual adult shape.
Results were conflicting: one simulated A. africanus adult suggested the species may follow a human growth pattern, whereas the other simulated adult suggested a different pattern. This discrepancy could be due to many factors. On the one hand, A. africanus ilia may have grown differently from humans, as previously suggested. Alternatively, the limited fossil sample may not be ideal representatives of the species, perhaps due to distortion or incorrect reconstruction.