Why was life expectancy in early cities shorter than among nomadic hunter-gatherer peoples?

Life expectancy in early cities was shorter than among nomadic hunter-gatherer peoples due to higher population densities in cities leading to increased spread of disease, poor sanitation, limited access to diverse food sources, and greater exposure to pollution and environmental hazards. Nomadic hunter-gatherer societies had a more varied diet, lower disease transmission rates, and fewer environmental stressors, contributing to longer life expectancy.