Advisor: Susan Antón
Abstract:
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
another—the
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.
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.
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| 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.
To read about my dissertation data collection travel adventures spanning multiple continents, you can visit my travel blog: The Wandering Osteologist
To read about my dissertation data collection travel adventures spanning multiple continents, you can visit my travel blog: The Wandering Osteologist