Spondylolysis and spinal adaptations for bipedalism

Abstract

Background and objectives The study reported here focused on the aetiology of spondylolysis, a vertebral pathology usually caused by a fatigue fracture. The goal was to test the Overshoot Hypothesis, which proposes that people develop spondylolysis because their vertebral shape is at the highly derived end of the range of variation within Homo sapiens. Methodology We recorded 3D data on the final lumbar vertebrae of H. sapiens and three great ape species, and performed three analyses. First, we compared H. sapiens vertebrae with and without spondylolysis. Second, we compared H. sapiens vertebrae with and without spondylolysis to great ape vertebrae. Lastly, we compared H. sapiens vertebrae with and without spondylolysis to great ape vertebrae and to vertebrae of H. sapiens with Schmorl’s nodes, which previous studies have shown tend to be located at the ancestral end of the range of H. sapiens shape variation. Results We found that H. sapiens vertebrae with spondylolysis are significantly different in shape from healthy H. sapiens vertebrae. We also found that H. sapiens vertebrae with spondylolysis are more distant from great ape vertebrae than are healthy H. sapiens vertebrae. Lastly, we found that H. sapiens vertebrae with spondylolysis are at the opposite end of the range of shape variation than vertebrae with Schmorl’s nodes. Conclusions Our findings indicate that H. sapiens vertebrae with spondylolysis tend to exhibit highly derived traits and therefore support the Overshoot Hypothesis. Spondylolysis, it appears, is linked to our lineage’s evolutionary history, especially its shift from quadrupedalism to bipedalism. Lay summary: Spondylolysis is a relatively common vertebral pathology usually caused by a fatigue fracture. There is reason to think that it might be connected with our lineage’s evolutionary shift from walking on all fours to walking on two legs. We tested this idea by comparing human vertebrae with and without spondylolysis to the vertebrae of great apes. Our results support the hypothesis. They suggest that people who experience spondylolysis have vertebrae with what are effectively exaggerated adaptations for bipedalism.

The earliest Pleistocene record of a large-bodied hominin from the Levant supports two out-of-Africa dispersal events

“…Its morphologic features indicate that UB 10749 is a lower lumbar vertebra of an early Pleistocene Homo. The dorsal wedging of the vertebral body, the widening of the inferior plate compared with the superior plate, the concavity of the inferior plate, and the ventrodorsally elongated vertebral body compared with vertebral height are all familiar lower lumbar human traits 13,[20][21][22] .…”

“…The size, the large vertebral plate, and the relatively short vertebral body of UB 10749 indicates that it belongs to hominoidea. The lordotic wedging and the concavity of the inferior plate further suggests that this is a hominin vertebra 20,21 .…”

The changing shape of palaeopathology: The contribution of skeletal shape analyses to investigations of pathological conditions

“…On top of illustrating the growth of interest in shape analyses in palaeopathology, our meta‐analysis also revealed key trends that summarize how shape analyses have so far been used in palaeopathology. Overall, we identified four such themes: Describe pathological conditions and support/refute diagnoses (e.g., Berezina & Buzhilova, 2021; Berner et al, 2021; Colombo et al, 2018; Darton et al, 2015; DeSilva & Papakyrikos, 2011; Gunz et al, 2004; Hershkovitz et al, 1997; Milella et al, 2015; Rmoutilová et al, 2020; Slon et al, 2013; Thompson, 2014; Trinkaus, 2018; Vairamuthu & Pfeiffer, 2018; Viciano et al, 2017; Zhao et al, 2020) Understand how specific traits and shape characteristics predispose individuals to pathological conditions (e.g., Bédécarrats et al, 2021; Calce et al, 2018; Mansukoski & Sparacello, 2018; Mays, 2006; Plomp et al, 2012; Plomp et al, 2019; Plomp et al, 2020; Plomp, Viðarsdóttir, Weston, et al, 2015; Shepstone, 2001; Shepstone et al, 1999; Ward et al, 2010; Weiss, 2005) Identify and interpret adaptations and alterations in functional anatomy in response to pathological conditions (e.g., Berner et al, 2021; Churchill & Formicola, 1997; Gilmour et al, 2019; Lazenby & Pfeiffer, 1993; Licata et al, 2019; Micarelli et al, 2018; Roberts et al, 2004; Sparacello et al, 2016; Thompson, 2014; Trinkaus, 2018; Viciano et al, 2017; Viva et al, 2021; Zhao et al, 2020) Provide insight into aspects of physical and social experience, care practices, therapeutic interventions, and social organization (e.g., Bédécarrats et al, 2021; Chevalier, 2019; Cowgill et al, 2015; Formicola & Buzhilova, 2004; Gilmour et al, …”

“…Situating palaeopathological data into established theoretical frameworks has and will continue to enable hypothesis‐driven research that can answer questions that are important for understanding the past, a key aim of palaeopathology, and also for understanding humans as animals that have undergone millions of years of evolution. For example, the vertebral shape traits that both Ward et al (2010) and Plomp et al (2020) identified as increasing an individual's propensity to develop spondylolysis can be explained as evolutionary adaptations for bipedalism. Both groups of researchers situated their interpretations within an evolutionary and biomechanical framework to explain why and how some humans are prone to spondylolysis.…”

“…The why is based on an understanding of the functional anatomy of a bipedal spine. Specifically, the increased dorsal wedging found on people with spondylolysis acts to increase the contact of the pars interarticularis (e.g., the fracture site in spondylolysis) with the inferior zygapophyseal facets of the superior vertebra, causing repetitive stress on the pars interarticularis of the inferior vertebrae and eventually leading to the fracture that is spondylolysis (Plomp et al, 2020; Ward et al, 2010). Thus, interpreting comparative and functional anatomy of the spine with an evolutionary perspective provided invaluable insight into why humans, and only humans, develop this condition.…”