I grew up in New Mexico. Adobe construction ("mud huts", if you want to get all ethnocentric about it) was ubiquitous there in 1915 and is still common there today; it's substantially more comfortable than most of the modern alternatives, but also more expensive. Some of my relatives there had outhouses when I was growing up in the 1980s.
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.
We have surviving adobe walls from four hundred and fifty generations ago at Çatalhöyük, but it seems likely that they've been in use for much longer than that, and just haven't survived. So we've had several generations to experiment. :)
