Dissertation Research

Ecogeographical influences on trunk modularity in recent humans
Advisor: Susan Antón


The shift to obligate bipedality entailed numerous morphological changes, including major reorganization of the ribcage, vertebral column, and pelvis, and additional changes in the trunk reflect climatic, obstetric, and energetic demands. What remains unexplored is how this complex suite of selective pressures influenced the degree to which these elements relate and track one anotherthe pattern of morphological integration.
Here I focus on morphological relationships within the trunk skeleton—the ribcage, vertebral column, and pelvis—to provide new insights into the evolution of body form. In hominins, the bipedal locomotor need for trunk stability, the obstetrical pressures that influence pelvic morphology, and the ecogeographic pressures that affect core body shape combine to create a complex series of interactions on trunk form. Examining the extent to which the skeletal regions of the trunk covary has the potential to provide information about how the combined and potentially competitive effects of multiple selective pressures influence the evolution of anatomical complexes.
This project confirmed untested assumptions about strong trunk integration in chimpanzees while demonstrating that trunk integration in humans is weaker. The more weakly integrated human trunk is accompanied by only minor differences in the patterns of phenotypic covariation among thorax, spinal, and pelvic elements relative to chimpanzees, contrary to expectations that bipedality may have caused a drastic reorganization of such patterns.
The results support the idea that the chimpanzee trunk skeleton responds as more of a unit to selection than does the trunk of recent humans. In humans, trunk integration is weaker, and what integration there is seems to be driven mainly by rough correspondence in breadths, which relates well to climatic variables, particularly minimum annual temperature. The relatively stronger integration of the true pelvis in humans compared to chimpanzees, and in human females compared to males, suggests that the novel pattern of integration of the os coxa related to the adoption of bipedalism facilitated later adaptations to difficult obstetrics in the genus Homo.
This project also reinforced support for strong correlations of bi-iliac breadth with latitude in Old World populations of recent humans but demonstrated that New World populations do not follow this trend, and suggests that the retention of wider body forms in the New World relates to a combination of reduced evolutionary flexibility from colonization coupled with relaxed thermoregulatory selection on body breadth in non-cold environments. Overall, the results suggest that ecogeographic effects, in the form of novel climatic selective pressures and neutral genetic evolution, reduce trunk modularity in recent humans.
            As a whole, these results contribute to our growing understanding of the selective trade-offs and integration of the hominin trunk skeleton while affirming the utility of a comparative approach focused on investigating phenotypic covariation across a large skeletal region to address questions of evolutionary change. On this basis further inquiry into other hominoid and ecogeographically-diverse taxa are warranted. The results of this project lay the foundation for consideration of phenotypic integration of skeletal and soft tissue morphology, which will refine our understanding of the mechanisms underlying the evolution of body form.

This research was supported by the Wenner-Gren Foundation, the Leakey Foundation, and NYU GRI.

Digitizing chimpanzee ribs at the Powell Cotton Museum

2016  Middleton, E.R. 2D versus 3D shape signals of climatic adaptations in the trunk skeleton 
         of recent humans. Fed. Am. Soc. Exp. Biol. J. 30:11.3.

2016 Middleton, E.R. Obstetric pressures as a driving force of differences in trunk modularity 
         between recent humans and chimpanzees? Am. J. Phys. Anthropol. 159 (S162), 228-229.

2015 Middleton, E.R. Insights into trunk modularity: the relationship between lumbar vertebral 
         dimensions and pelvic shape in recent humans and chimpanzees. Am. J. Phys. Anthropol. 
         156 (S60), 225.

2014 Middleton, E.R. Phenotypic patterns of rib and pelvis covariation in Old vs. New World 
         human populations. Am. J. Phys. Anthropol. 153 (S58), 185.

2013 Middleton, E.R. Trunk modularity in recent human populations: a preliminary look at rib 
         and pelvis covariation. Am. J. Phys. Anthropol. 150 (S56), 198.