I remember doing a heat transfer simulation in a modeling and simulation class -- seems that the typical adobe wall was just about thick enough to induce a 12 hour phase shift in heat transfer (so that the heat of the day would reach the inside of the dwelling during the coldest part of the night, and vice versa). Thus cool in the heat of the day, and warm at night (this works well in a desert climate where it's very hot in the daytime and quite cold at night).

It probably took generations to arrive at the optimum wall thickness.