A bioarchaeological examination of the impact of early-life stress on later-life outcomes using a Procrustean assessment of dental fluctuating asymmetry

Early life is a time of heightened vulnerability to stress which, due to the high phenotypic plasticity which characterises the period, shapes life-course trajectories. For dependent offspring, mothers play a crucial role in mediating stress (e.g., by contributing to immunity and nutritional provisioning in utero and in later breastfeeding) and the effects of their influences during development impact later-life outcomes. Investigating early-life stress and the mother-child nexus bioarchaeologically has proven challenging. However, as first permanent molars (M1s) form during early life without remodelling, stress-induced deviations to symmetry, known as fluctuating asymmetry (FA), can be evaluated to explore this critical time.

In this thesis, FA was quantified through geometric morphometric (GM) techniques so that early-life experience could be investigated and relationships with later-life outcomes identified. It was found that skeletally immature remains were associated with significantly higher FA than mature individuals, and within the immature cohort there was a significant positive correlation between FA and age-at-death. Higher FA was linked to markers of active and systemic infection and a proinflammatory physiology, while childhood stress was associated with growth deficits. Thus, stress experience at different periods was connected to specific outcomes. Elevated early-life stress appears to have increased frailty and decreased resilience, contributing to mortality risk and delayed somatic development, but may also have been associated with phenotypic programming that promoted short-term survival, supporting the Thrifty Phenotype Hypothesis. Despite significant between-site differences in childhood and later-life stress, site-based differences in FA were largely insignificant, suggesting that mothers successfully mediated contextual stressors for offspring in early life and that within-group differences reflect variance in maternal health.

These findings are the first to demonstrate the viability of GM assessed M1 FA as a proxy for early-life stress and successfully reveal connections between formative experiences, maternal influences and life-course trade-offs in past lives.