Objectives: To clarify the potential risk factors and etiology of low back pain (LBP)-related disability, including structural changes of the spine (spinal scoliosis) and body composition components in a population with a high prevalence of LBP. Methods: In this cross-sectional study, two self-reported validated questionnaires were used to collect back pain and disability data in an ethnically homogeneous family-based population sample (N = 1078). The scoliosis angle of trunk rotation was measured by a scoliometer on three spinal levels while the patient was bent forward. Body composition parameters, including relative to weight (WT), fat, relative skeletal muscle mass (SMM/WT), and total body water were determined by bioelectrical impedance analysis. Statistical analysis was conducted, accounting for the familial composition of the sample. Results: The mixed multiple regression analyses with several LBP-related phenotypes as dependent variables consistently showed significant independent associations with scoliosis and SMM/WT, irrespective of other covariates. The odds ratios (OR)/95% CI for scoliosis ranged between 1.40 (1.19–1.64) and 1.51 (1.27–1.80), and from 0.61(0.51–0.72), to 0.71(0.58–0.87) for SMM/WT, depending on the LBP phenotype. The genetic components of the respective correlations between the LBP-phenotypes and scoliosis or SMM/WT were negligible. Conclusions: The associations between LBP-related conditions and postured scoliosis and SMM/WT were consistent and significant and therefore may serve as markers in predicting the development of LBP-related disability. We interpret the origin of these correlations as the evolutionary event due to the imperfect spine anatomy adaptation to a vertical posture resulting from a quick transition to bipedalism from a quadrupedal ancestor.