Complex sentence: Due to the primitive and relatively stagnant state of agriculture throughout this period the ratio of rural to urban population remained at a fixed equilibrium, though a significant increase in the percentage of the global urban population can still be traced in the 1st millennium BCE.
Split into simpler sentences: Due to the primitive and relatively stagnant state of agriculture throughout this period, the ratio of rural to urban population remained at a fixed equilibrium. However, a significant increase in the percentage of the global urban population can be traced in the 1st millennium BCE.